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Tagus Floods '06 Workshop
Project POCTI/CTA/39427/2001
ABSTRACTS BOOK
19-21 July, 2006 Lisbon Portugal
www.workshoptejo.fc.ul.pt
Abstracts
Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
ORGANIZED BY Centre of Geology of the Faculty of Sciences of the University of Lisbon Department of Geology of the Faculty of Sciences of the University of Lisbon Centre of Geographical Studies of the University of Lisbon
ORGANIZING COMMITTEE (University of Lisbon)
M. Teresa M. Azevêdo
Catarina RamosElisabete Nunes
Cátia Sousa
Rita Caldas Pedro Fonseca André Fernandes
SCIENTIFIC COMMITTEE
Ana Ramos Pereira (Lisbon, Portugal)
M. Conceição Freitas (Lisbon, Portugal) César F. Andrade (Lisbon, Portugal)
Pedro P. e Cunha (Coimbra, Portugal) Diamantino Pereira (Braga, Portugal)
João Cabral (Lisbon, Portugal) João Rocha (Lisbon, Portugal)
M. Eugénia M. Palomares (Madrid, Spain) Gerardo Benito (Madrid, Spain) Suzanne Daveau (Lisbon, Portugal) Rui Taborda (Lisbon, Portugal) Mini Garzón (Madrid, Spain) Nuno Pimentel (Lisbon, Portugal) Paolo Mozzi (Padova, Italy)
SUPPORTED BY
Centro de Geologia Universidade de Lisboa
Fundação para a Ciência e a Tecnologia – Ministério da Ciência e do Ensino Superior
SPONSORED BY Câmara Municipal de Lisboa Caixa Geral dos Depósitos
Flor do Tejo Construções, Lda. Front cover – the postcard dates back to 1910 and the background photo was taken by J. Benoliel, in 1912.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Contents
CONFERENCE TIMETABLE 3
GEOTARIF PROJECT PRESENTATION 9
The Geotarif Project AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &
PEREIRA, D. I. 11
KEY NOTE PRESENTATIONS 15
A hydrometeorological approach to the floods of Tajo River. Llasat, C. 17
Apprendre à vivre dans des zones inondables FAYE, J. 21As cheias fluviais em leitos aluvionares com variabilidade natural e alterada. Evidências no
Rio Tejo. ROCHA, J. S. & FERNANDES J. N. 25Radiotracers in flooding processes. SANCHEZ-CABEZA, J.A. 29O Paleolítico no Médio e Baixo Tejo. RAPOSO, L. 33
Paleoflood and historical flood records along the middle and upper Tagus river catchment: Climatic and flood hazard implications.
BENITO, G. 35
PRESENTATIONS 37 Scale invariant study of river discharges from the Tagus Basin LIMA, M.I.P., LIMA, J.L.M.P., RODRIGUES, R.R., BRANDÃO, C. & ROSÁRIO, A.S.S. 39The role of NAO extreme phases on the monthly discharges, and the volume and date of the
annual maxima flows (1950-2003) in the Tagus River. LÓPEZ-MORENO, J.I., VICENTE-SERRANO, S.M., BEGUERÍA, S. & GARCÍA-RUIZ, J.M. 42Usefulness of Paleo-flood information in flood quantile estimation PARIDA, B.P. 44The impact of the North Atlantic Oscillation on precipitation and water resources of the Iberian
Peninsula TRIGO, R. M. 48A Medição dos Caudais de Cheia no Rio Tejo. ROCHA, J.S., VAN ZELLER, M.E. & FERNANDES, J.N. 51
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Active tectonic structures in the Lower Tagus Valley: state of the art. CABRAL, J. 53Acções antrópicas na Bacia Hidrográfica do Tejo na época contemporânea: florestação,
correcção torrencial e controlo de cheias. FREITAS, J.G. 56As cheias históricas na região de Abrantes. ALMEIDA, M.C., AZEVÊDO, T.M. & PIMENTEL, N.L. 59Floods in the Czech Republic during the past millennium: data, fluctuations, impacts. BRÁZDIL, R. 62The Tagus River and its historical floods (Santarém, Portugal). AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &
PEREIRA, D. I. 64Palaeoenvironments of the Tagus valley during the last 15 ka: sedimentological, palynological
and micromorphological evidence of the Entrevalas SEV coring (Santarem, Portugal). AZEVÊDO, M.T., FAVARETTO, S., MIOLA, A., MOZZI, P., NICOSIA, C., NUNES, E. & SOSTIZZO, I. 68Sistemas meteorológicos de precipitação e as cheias nas bacias hidrográficas: Rio Tejo. SIMÕES, I.M.S.N.V. 71Possibilidades de aplicação da luminescência opticamente estimulada (OSL) na datação de
sedimentos do rio Tejo. MARTINS, A.A. & CUNHA, P.P. 72Geo-archaeological evidence of destructive floods during the settlement of Faial (Azores). MADEIRA J. & SILVEIRA, A.B. 76Geomorphological assessment of the Middle Tagus alluvial plain. PEREIRA, A. RAMOS; RAMOS, C.; AZEVÊDO, T. M.; NUNES, E.; FREITAS, M. C.; ANDRADE, C. &
PEREIRA, D.I. 79Characterization of the Tagus floodplain sediments: their importance in the search of channel
variations and human impacts over time. AZEVÊDO, T. M., NUNES, E., RAMOS, C., PEREIRA, A. RAMOS, FREITAS, C., ANDRADE, C. &
PEREIRA, D. I. 82Understanding lateglacial and Holocene environmental changes through the sedimentological
study of a core (Santarém region) FREITAS,, C., ANDRADE,, C. , AZEVEDO, T.M., PEREIRA, A. RAMOS, RAMOS, C., NUNES, E.&
PEREIRA, D.I. 85
POSTERS 91
Estimation of recent alluvial sedimentation rates, Upper Odra, Poland. CZAJKA, A. 93Peligros e impactos producidos por las modificaciones de origen antrópico durante la avenida
del arroyo Rivillas en 1997 (SW de España). MOYA-PALOMARES, M.E. & CENTENO, J.D. 95Desarrollo espacial y evolución de una flashflood a través de la información aportada por
depósitos efímeros ORTEGA, J.A. & GARZÓN, G. 98
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Conference Timetable
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ROOM 6.2.56
WEDNESDAY, 19 JULY, 2006
9.30H - 10.00H RECEPTION AND REGISTRATION
10.00H - 11.00H OPENING SESSION
11.00H - 11.30H COFFEE BREAK
11.30H - 12.00H THE GEOTARIF PROJECT
Teresa M. Azevedo, Catarina Ramos, Ana Ramos Pereira, Elisabete Nunes, Conceição Freitas, César Andrade & Diamantino I. Pereira
12.00H – 12.30H
KEYNOTE PRESENTATION A HYDROMETEOROLOGICAL APPROACH TO THE FLOODS OF TAGUS RIVER Maria-Carmen Llasat & Concepción Marcuello
12.30H – 12.40H DISCUSSION
12.40H – 14.00H LUNCH
14.00H – 14.30H KEYNOTE PRESENTATION APPRENDRE A VIVRE DANS DES ZONES INONDABLES Jacques Faye
14.30H – 14.40H DISCUSSION
ORAL PRESENTATIONS
14.40H – 14.55H SCALE INVARIANT STUDY OF RIVER DISCHARGES FROM THE TAGUS BASIN Maria Isabel P.de Lima, J.L.M.P. de Lima, R.R. Rodrigues, C. Brandão, A.S.S. Rosáriz
14.55H – 15.10H THE ROLE OF NAO EXTREME PHASES ON THE MONTHLY DISCHARGES, AND THE VOLUME AND DATE OF THE ANNUAL MAXIMA FLOWS (1950-2003) IN THE TAGUS RIVER Juan I. López-Moreno, Sergio M. Vicente-Serrano, Santiago Beguería, José M. García-Ruiz
15.10H – 15.25H USEFULNESS OF PALEO-FLOOD INFORMATION IN FLOOD QUANTILE ESTIMATION Bhagabat P. Parida
15.25H – 15.40H THE IMPACT OF THE NORTH ATLANTIC OSCILLATION ON PRECIPITATION AND WATER RESOURCES OF THE IBERIAN PENINSULARicardo M. Trigo
15.40H – 16.00H DISCUSSION
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16.00H - 16.20H COFFEE BREAK
16.20H - 16.50H KEYNOTE PRESENTATIONL AS CHEIAS FLUVIAIS EM LEITOS ALUVIONARES COM VARIABILIDADE NATURAL E ALTERADA. EVIDÊNCIAS NO RIO TEJO
João Soromenho Rocha & João Nuno Fernandes
16.50H - 17.00H DISCUSSION
ORAL PRESENTATIONS
17.00H - 17.15H A MEDIÇÃO DE CAUDAIS DE CHEIA NO RIO TEJO João Soromenho Rocha, Maria Emília van Zeller e João Nuno Fernandes
17.15H - 17.30H ACTIVE TECTONIC STRUCTURES IN THE LOWER TAGUS VALLEY: STATE OF THE ART João Cabral
17.30H - 17.45H DISCUSSION
17.45H - 18.15H POSTER SESSION ESTIMATION OF RECENT ALLUVIAL SEDIMENTATION RATES, UPPER ODRA, POLAND Agnieszka Czajka PELIGROS E IMPACTOS PRODUCIDOS POR LAS MODIFICACIONES DE ORIGEN ANTRÓPICO DURANTE LA AVENIDA DEL ARROYO RIVILLAS EN 1997 (SW DE ESPAÑA) Maria Eugénia Moya-Palomares & Juan D. Centeno DESARROLLO ESPACIAL Y EVOLUCIÓN DE UNA FLASHFLOOD A TRAVÉS DE LA INFORMACIÓN APORTADA POR DEPÓSITOS EFÍMEROS José A. Ortega & Guilhermina Garzón GEOTARIF PROJECT POSTERS
18.30H ICE-BREAKER
THURSDAY, 20 JULY, 2006
8.30H - 19.00H FIELD TRIP TO THE MIDDLE AND LOWER TAGUS
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ROOM 6.2.56
FRIDAY, 21 JULY, 2006
9.30H - 10.00H KEYNOTE PRESENTATION RADIOTRACERS IN FLOODING PROCESSES Joan Albert Sanchez-Cabeza
10.00H - 10.15H DISCUSSION
ORAL PRESENTATIONS
10.15H - 10.30H ACÇÕES ANTRÓPICAS NA BACIA HIDROGRÁFICA DO TEJO NA ÉPOCA CONTEMPORÂNEA: FLORESTAÇÃO, CORRECÇÃO TORRENCIAL E CONTROLO DE CHEIAS Joana Gaspar de Freitas
10.30H - 10.45H AS CHEIAS HISTÓRICAS NA REGIÃO DE ABRANTES Mónica C. Almeida, Teresa M. Azevêdo & Nuno L. Pimentel
10.45H - 11.00H FLOODS IN THE CZECH REPUBLIC DURING THE PAST MILLENNIUM: DATA, FLUCTUATIONS, IMPACTS Rudolf Brázdil
11.00H - 11.15H THE TAGUS RIVER AND ITS HISTORICAL FLOODS (SANTARÉM, PORTUGAL) Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira
11.15 - 11.35H DISCUSSION
11.35H - 11.55H COFFEE BREAK
11.55H - 12.25H KEYNOTE PRESENTATION O PALEOLÍTICO NO MÉDIO E BAIXO TEJO Luís Raposo
12.25H - 12.35H DISCUSSION
12.35H - 14.00H LUNCH
14.00H - 14.30H KEYNOTE PRESENTATION PALAEOFLOOD AND HISTORICAL FLOOD RECORDS ALONG THE MIDDLE TAGUS RIVER CATCHMENT: CLIMATIC AND FLOOD HAZARD IMPLICATIONS Gerardo Benito
14.30H - 14.40H DISCUSSION
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ORAL PRESENTATIONS
14.40H - 14.55 PALAEOENVIRONMENTS OF THE TAGUS VALLEY DURING THE LAST 15 KA: SEDIMENTOLOGICAL, PALYNOLOGICAL AND MICROMORPHOLOGICAL EVIDENCE OF THE ENTRE-VALAS SEV CORING (SANTAREM, PORTUGAL) M. Teresa Azevêdo, S. Favaretto, A. Miola, Paolo Mozzi, C. Nicosia, Elisabete Nunes & I. Sostizzo
14.55H - 15.10 SISTEMAS METEOROLÓGICOS DE PRECIPITAÇÃO E AS CHEIAS NAS BACIAS HIDROGRÁFICAS: RIO TEJO Ilda Mª Sanfins Novo Villa Simões
15.10H - 15.25 POSSIBILIDADES DE APLICAÇÃO DA LUMINESCÊNCIA OPTICAMENTE ESTIMULADA (OSL) NA DATAÇÃO DE SEDIMENTOS DO RIO TEJO António Antunes Martins & Pedro Proença e Cunha
15.25H - 15.40 GEO-ARCHAEOLOGICAL EVIDENCE OF DESTRUCTIVE FLOODS DURING THE SETTLEMENT OF FAIAL (AZORES) José Madeira & António Brum da Silveira
15.40H - 16.00H DISCUSSION
16.00H - 16.20H COFFEE BREAK
16.20H - 16.35H HUMAN INTERVENTIONS IN TAGUS RIVER CHANGES Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira
16.35H - 16.50 GEOMORPHOLOGICAL ASSESSMENT OF THE MIDDLE TAGUS ALLUVIAL PLAIN Ana Ramos Pereira, Catarina Ramos, Teresa M. Azevêdo, Elisabete Nunes, Conceição Freitas, César Andrade & Diamantino I. Pereira
16.50H - 17.05H CHARACTERIZATION OF THE TAGUS FLOODPLAIN SEDIMENTS: THEIR IMPORTANCE IN THE SEARCH OF CHANNEL VARIATIONS AND HUMAN IMPACTS OVER TIME Teresa M. Azevêdo, Elisabete Nunes, Catarina Ramos, Ana Ramos Pereira, Conceição Freitas, César Andrade & Diamantino I. Pereira
17.05H - 17.20H UNDERSTANDING LATEGLACIAL AND HOLOCENE ENVIRONMENTAL CHANGES THROUGH THE SEDIMENTOLOGICAL STUDY OF A CORE (SANTARÉM REGION) Maria da Conceição Freitas, César Andrade , Teresa M. Azevedo, Ana Ramos Pereira, Catarina Ramos, Elisabete Nunes & Diamantino I. Pereira
17.20H - 17.45H DISCUSSION
20.00H CLOSING DINNER
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GEOTARIF Project
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THE GEOTARIF PROJECT
TERESA M. AZEVÊDO1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, ELISABETE NUNES1,CONCEIÇÃO FREITAS1,
CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3
1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016
Lisboa, Portugal
e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal
e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
e-mail: [email protected]
Middle age authors described the Tagus as a second Nile, due to the extraordinary fertility of
its “lezíria” (from al-jazira, which means “the island” in Arabic language). There are several
references of the human occupation of Tagus Valley before the Portuguese nationality,
especially Arabic and Roman occupations. Archaeological excavations in this region confirm the
great importance of all this area in proto-historic times. However Tagus floods were since
always, the most catastrophic events, killing cattle and people, destroying homes and
agricultural lands. Even today, floods isolate populations for weeks, cause roads and railway
cuttings and great economic losses. They are referred since the beginning of the Portuguese
nationality and an extensive literature review, including works by archaeologists, historians and
military civil engineers, shows how they have always been a main concern of Portuguese
governors, since the XII century. Since 70’s these events were highly diminished by the
construction of dams in the Spanish and Portuguese Tagus Basin.
Following a first very grateful experience with the Praxis XXI Project "An Interdisciplinary
Approach to Flood Risk Evaluation - Hydrology, Geomorphology and Sedimentology of the
Douro, Tejo and Sado Basins" in which were developed for the first time in Portugal, at
academic level, techniques and methodologies not previously used, the research team focused
on the effectiveness of a follow-up Project that would use hydrological, geomorphological,
sedimentological and historical study of the Tagus floods.
Main Goals of the GEOTARIF Project
– Historical research of the Portuguese Tagus flood, namely relationship between the
several historic flood episodes and the artificialization of the river and comparative analyses of
maps since 1760.
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– Geomorphological research based on XX century charts and field survey in order to
evaluate alluvial plain mesomorphology, the old positions of the channels and other previous
morphological units as well as the last 500 years dynamics.
– Sedimentological research for the evaluation of the textural parameters of cored and
present-day analogues to evaluate hydrological energy changes in the flows.
– Geochemical analysis to evaluate, through the variation of peak concentrations, the
degree of pollution in the floodplain sediments.
– Dating of samples using 14C, 210Pb and 137Cs and evaluation of sedimentation rates.
To collect all the historical data about the Tagus river valley the following tasks were carried
out: (i) historical research in the main Archives (Distrital de Santarém, Torre do Tombo, MOP,
National Library) on engineering works performed on the Tagus in historic times, and in
Eclesiastic Archives about past floods; (ii) compilation and analysis of historical cartography of
Middle Tagus basin with comparison between all the available maps since the oldest till the
present ones to evaluate the changes in width and direction of the channels and migration of
the bars.
Morphological elements of the alluvial plain were defined based on 5m contour lines and
numerous elevation points of several charts of the Portuguese topographical charts. A model of
georeferenced elevation points and every meter contour lines of the alluvial plain was built,
allowing the construction of a DEM model. This methodology allowed to select in the field, the
best sites for manual and mechanical drilling and to accomplished the geomorphological
boreholes framework as well as to collect the most significant superficial samples in order to
compare than with core samples.
Several drills were opened to obtain undisturbed sedimentary sequences for
sedimentological, geochemical studies and dating, in order to reconstruct the past
environmental changes of the study area, focusing on the identification, characterization and
frequency assessment of floods
This work included test coring with light equipment (gauge and Edelman augers, Van der
Staay suction corer) completed by opening of small trenches for macroscopic evaluation of the
local lithostratigraphy. This consists essentially in boreholing the soft sedimentary column using
heavy pressure driven coring equipment and casing to obtain an undisturbed and continuous
set of cores at each target location. All coring locations were coordinated and connected to the
Portuguese geodetical Datum by the same group using adequate tacheometer procedures.
The sedimentological analysis enables to define the transport and deposition of sediments
during the Holocene in order to evaluate the fluvial dynamics changes of the Tagus River.
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Sediments were described for colour, texture and mineralogy (granulometric, morphoscopic
and heavy mineral study of the sandy fraction, and the mineralogical study of clays), fossil
contents, structures and facies variation. The correlation between statistic parameters of the
present analogues and the core samples allowed identify the correspondent environments,
avulsion episodes and progressive changes of the channel.
One of the most important components was the geochemical study of the variation of the
heavy metal concentration, only studied in Portugal for the river waters or for the estuarine
sediments but very few for the floodplain ones. This study, which started in the prior project,
continued due the given precious information about the influence of human activity on the heavy
metal contamination through times. In the floodplain fine sediments, heavy metals supplied by
mineral exploitation, industrial development and agricultural fertilizers can concentrate in high
levels. The evaluation of the heavy metal concentration along the drill profiles allowed
evaluate the degree of pollution over time and the 14C, isotopic and polinic datings of the main
variation levels allowed to interpret them in terms of historical events.
Radionuclides are also disseminated in the floodplain sediments. The radionuclides 137CS
and 210Pb evaluation for the first meters of the same drill profiles of the Tagus floodplain
sediments also contributed to this original scope in the sense that it was possible to get
knowledge of the contamination of soil profiles of Tagus floodplain and evaluate the recent
sedimentation rate. The dating of these levels allowed establishes the most probable origin for
the peaks and the eventual degree of contamination by the radionuclides.
The the palynological results of fourteen samples from the Entrevales sequence enabled to
consider the existence of three zones completely sterile in pollen content, suggesting to divide
the record into two main parts: for the first half of the Holocene (from bottom till 8m deep),
pollen, spore and NPP records suggest the existence, under mediterranean climatic conditions,
of a freshwater shallow basin which dried out in summer. Locally the aquatic community,
dominated by Isoetes, indicates an acidic and oligotrophic environment, but some foraminifers
could reveal that, at least sporadicly, marine or brackish water could reached the basin. In the
upper part of the sequence palynological results seem to indicate deforestation and heathland
expansion starting during the Bronze Age.
So, this Project aims at a new perspective, in the sense it correlates all the geological and
morphological data with historical and archaeological events having positive impacts in ethic,
social and environmental terms in the sense it allowed support adequate policies of
environmental management and risk/hazard assessment. The results achieve within the Project
was possible only by the interdisciplinary team work
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The diffusion of results was undertaken by:
-Technical progress reports and final report to the FCT;
-Oral and poster presentations at national and international scientific meetings;
-A workshop in the 3rd year for the presentation, diffusion and discussion of the results with
other researchers of the same scientific areas.
-Publication of the results, models and conclusions on scientific (refereed) journals or revues;
-Reports to local authorities, of the most relevant conclusions on flood risk, soil contamination
by high heavy metal levels and by radionuclides pollution.
The implementation of the “Núcleo de Estudos do Tejo” (NET) in the Department of Geology
of the FCUL, including a Tagus Database, which allows to insert all the works concerning the
Tagus river, in so different scientific areas as history, geology, physical geography,
archaeology, cartography, pedology, agriculture, palinology, geochemistry, geochronology,
hydraulic engineer, literature etc.
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Key Notes
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A HYDROMETEOROLOGICAL APPROACH TO THE FLOODS OF TAGUS RIVER
MARIA-CARMEN LLASAT1 AND CONCEPCIÓN MARCUELLO2
(1) Department of Astronomy and Meteorology, University of Barcelona, Avda. Diagonal 647, 08028-Barcelona, Spain
e-mail: [email protected](2) Centro de Estudios Hidrográficos del CEDEX, Paseo Bajo Virgen del Puerto, 3, 28005-Madrid, Spain
e-mail: [email protected]
Introduction Between 1950 and 1999 there were a total of over 2,200 deaths in Spain (of which 1,400
were in Catalonia) and material damage amounting to over €301,000,000 a year as a
consequence of floods. Usually the most affected area is the Mediterranean one, where heavy
rainfalls and floods are frequent during the autumn season. For instance, in Catalonia
(approximately 35000 km2), the highest number of floods corresponds to the river Ter in Girona,
with a figure of 177 for the period 1322-2002, while the river Llobregat shows 171 floods at its
mouth, for the period 1315-2002, and the river Segre, only 51 floods for the period 1306-2002
(Llasat et al, 2005). However, although in the centre and west part of the Iberian Peninsula
floods are not so important, their role and impact are not insignificant. Then, more than 150
floods have been recorded in the Tagus basin (approximately 55800 km2) between the years
849 and 1985 A.D. Some places, like Aranjuez, have been affected by more than 50 floods in
the last 500 years, 60% of them were recorded between the months of January and March,
while Talavera de la Reina has recorded for the same period, 19 floods with a 50% of them
between December and January. Besides the different season of the year during which floods
are produced, Mediterranean floods have different features that those ones of the Tagus basin,
being one of them the size of the catchments, that is only comparable to this one of the Ebro
River.
The objective of this contribution is to analyse the main hydrometeorological features of the
floods recorded in the Tagus basin and comparing them with these ones of the Mediterranean
catchments, for which more meteorological analysis and information is available. Besides this,
and taking into account the climate variability, a reflexion on the rainfall temporal distribution
and its effect in floods evolution is done.
A floods classification A distinction between four kinds of floods in the Iberian Peninsula can be made:
1) Short-lived events (less than 3 hours) of very intense precipitation (peaks of rainfall rate
above 3 mm.min-1) but for which the total amount of rainfall is not usually very high (less than
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100 mm). They appear during summer and early autumn and produce “local flash-floods” in
short water courses (5-50km2) with considerable slope and characterised by torrential regime
and, sometimes, non-permanent flows.
2) Episodes of heavy rain sustained for several hours that can produce catastrophic floods
due to the daily rainfall amount (usually more than 100 mm) or widespread extension (usually
more than 2000 km2) very high above-normal levels. It is possible to distinguish between two
subclasses. Type 2a usually lasts less than 24 hours and the maximum precipitation is usually
recorded in less than 6 hours, with accumulated rainfall of nearly 200 mm. They can produce
catastrophic flash floods in rivers of the previous type or rivers with modest basins and flows
(50-2000 km2). Type 2b lasts more than 24 hours but generally less than 5 days. Although
accumulated rainfall usually has values between 200 and 400 mm, values of more than 800 mm
are possible in some Spanish regions.
3. Episodes of long duration with weak average pluviometric intensity values, while there
may be peaks of high intensity. Total precipitation can be above 200 mm. If floods occur, they
are usually in rivers of high-mountain (>2000 m) and pluvial-snowmelt origin with large basins
and perennial flows (>2000 km2).
4. Flood events mainly produced by snow melting. It should be noted that the melting of
snow around the mountainous headwaters of the rivers only brings regular and foreseeable
rises during the spring, and only occasionally gives rise to overbank flows when accompanied
by heavy rainfall or moderate but continuous rainfall.
Other kinds of floods not considered in the above classification are those produced by the
rising levels of phreatic water, ice breaking, dam’s bursts and sea waves.
Main features of floods recorded in the Tagus basin They are usually produced in winter (mainly during December and January months) by long
rainfalls over the entire region with some convection embedded in stratiform systems, and
would be considered of type 3. This convection can be responsible of daily values above 140
mm, like the event recorded in January 1996. These precipitation systems are mainly developed
as a consequence of the pass of Atlantic fronts, usually associated to deep depressions. But
information about recent events shows that heavy rainfalls are also produced in summer and
autumn and that can have similar features to those events related with catastrophic floods
produced in Mediterranean region (Rigo and Llasat, 2004). Summer floods are usually
produced by convective events that can affect an extended area and that usually are produced
by perturbations that affect the region between 1 and 3 days (i.e. August 1987), but that can last
more than 5 days (i.e. July 1987). In these cases daily rainfall is usually less than 80 mm. They
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would be of type 1. On the contrary, floods produced in autumn are related with convective
events developed in the framework of generalized stratiform rainfalls that last more than 10
days and affect the entire region (i.e. November 1989). Sometimes they can be considered like
floods of type 2. Cumulated daily rainfalls can overpass 100 mm and total rainfalls more than
200 mm, although their never achieve so high values than in the Mediterranean region.
Rainfall and floods evolution
The analysis of areal and punctual rainfall for the Duero and Tagus basins do not show any
common significant trend for the last century (Barrera and Llasat, 2004; Llasat and Quintas,
2003), not for annual values neither for winter values. Trends on flood frequency are not clear,
but a high sensitivity to the climatic variability is founded when the last millennium has been
analysed (Benito et al, 2003) and some anomalies common to the Mediterranean rivers have
been founded. Then, the better comprehension of the meteorological situations responsible of
those floods and its evolution in future scenarios could add to diagnose possible future
changes.
References Barrera, A. and M.C. Llasat, 2004. Evolución regional de la precipitación en España en los últimos 100
años. Ingeniería Civil, 135, 105-114
Benito, G., A. Díez-Herrero and M. Fernández de Villalta, 2003. Magnitude and frequency of flooding in
the basin (Central Spain) over the last millenium. Climatic Change, 58, 171-192.
Llasat, M. C., and Quintas, L., 2004. Stationarity of monthly rainfall series, since the middle of the 19th
century. Application to the case of Peninsular Spain, Natural Hazards, 31, 613-622, 2004.
Llasat, M.C., Barriendos, M., Barrera, A. & Rigo, T. 2005. Floods in Catalonia (NE Spain) since the 14th
century. Dans Benito, G, Ouarda, T.B.M.J., Bárdossy, A (Eds.) Palaeofloods, hystorical data & climate
variability: Applications in flood risk assessment, Journal of Hydrology 313 (1-2), 32-47.
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APPRENDRE A VIVRE DANS DES ZONES INONDABLES
JACQUES FAYE Direction de la prévention des pollutions et des risques. Ministère de l'Écologie et du Développement Durable,
France
e-mail: [email protected]
Apprendre sauve Pour une éducation à la prévention des risques majeurs
Depuis juin 2003, la Stratégie nationale pour le développement durable intègre l’éducation
aux risques majeurs dans l’éducation à l’environnement pour un développement durable.
Des concepts précis Chaque jour, la vulnérabilité globale augmente progressivement et se différencie en fonction
du niveau de développement des territoires. Or, on ne peut empêcher les séismes ni contrôler
les tempêtes.
Aux images de catastrophes, au sentiment de fatalité ou de peur, nous voulons répondre par
une éducation à la prévention qui permette à chacun de prendre des précautions, d’anticiper les
évènements, d’être plus lucide et confiant en lui et ses proches. Cette éducation est à la croisée
de l’éducation à l’environnement et de l’éducation à la sécurité. Avec l’éducation à la
citoyenneté, à la santé, à la sécurité sur la route, à la maison ou au travail…. Elle contribue à
une éducation à la responsabilité individuelle. Elle permet de comprendre les interactions
existantes entre l’homme et son milieu environnant, car, s’il est nécessaire de protéger
l’environnement, il nous faut aussi le prendre en compte pour nous protéger. L’objectif est de
susciter des comportements réfléchis, adaptés et solidaires dans la perspective d’un
développement durable des territoires et des sociétés.
Il convient de décrire l’aléa d’origine naturelle ou anthropique et de l’analyser à différentes
échelles d’espace et de temps, par rapport à des enjeux humains, économiques,
environnementaux et patrimoniaux afin de réduire leur vulnérabilité, ce qu’on nomme mitigation
ou résilience.
Pour une éducation préventive Aujourd’hui, la société française refuse de considérer les catastrophes comme une fatalité et
devient de plus en plus exigeante à l’égard de ceux qui ont mission d’assurer notre sécurité. On
constate cependant une évolution du citoyen de ne pas s’en remettre uniquement à l’État et de
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participer à sa propre sauvegarde, à celle de ses proches et de ses biens, dans la mesure où il
a connaissance des mesures et des comportements d’urgence appropriés. C’est dans cette
optique que l’éducation à la prévention des risques majeurs est affirmée, depuis plusieurs
années, comme l’une des finalités de l’école. Comme l’éducation à la sécurité routière, les
programmes scolaires intègrent les risques majeurs dès le premier degré, avant que l’essentiel
ne soit abordé au collège en classes de 5ème et de 4ème, puis de nouveau au lycée, en 2nde, en
croisant les disciplines et favorisant des dispositifs pédagogiques comme les Travaux
Personnels Encadrés pour ne citer qu’un seul exemple.
À ce titre, la Direction de la prévention des pollutions et des risques du Ministère de
l’écologie et du développement durable [MEDD] anime depuis une douzaine d’années, un
réseau de formateurs Risques Majeurs éducation [RMé] pour inciter, ou venir en appui, à des
projets éducatifs dans le cadre scolaire ou périscolaire. Plus de quatre cents agents de
l’Éducation nationale, mais aussi d’autres ministères comme l’Équipement, l’Intérieur ou
l’Agriculture, interviennent auprès des enseignants, des chefs d’établissement ou des directeurs
de centres de loisirs qui les sollicitent. À la demande du MEDD, ils se sont rassemblés en 1998
au sein d’un Institut français des formateurs risques majeurs et protection de l’environnement
(IFFO-RME). D’autres structures comme la Gendarmerie nationale avec ses formateurs relais
enquête environnement (FREE), les secrétariats permanents pour la prévention des pollutions
industrielles (SPPPI), les Centres Permanents d’Initiatives pour l’Environnement ou des
établissements publics interviennent aussi dans le cadre de cette éducation préventive.
Pédagogie et exemplarité Si l’École doit être pour l’élève, le lieu privilégié de l’apprentissage à cette prévention,
l’établissement qui l’abrite doit éviter d’être particulièrement vulnérable. Or, si la tempête de
Noël 1999 a révélé la fragilité de certains bâtiments, comme le souligne l’Observatoire National
de la Sécurité des Etablissements scolaires et d’Enseignement Supérieur, c’est surtout la
grande impréparation de la communauté scolaire à ce type de situation qui suscite des
inquiétudes. Que doit faire un chef d’établissement ou un directeur d’école en cas de danger ou
d’alerte? Comment éviter toute panique due à l’ignorance des enseignants et des élèves alors
que les consignes demandent aux parents de ne pas aller chercher leurs enfants ?
La mise en place du Plan Particulier de Mise en Sûreté se conçoit dans un cadre élargi d’éco
responsabilité des établissements d’enseignement. Son élaboration implique aussi la
communauté gestionnaire : commune, conseil général ou régional, pour la mise en œuvre des
règles de mise en sûreté au niveau de l’aménagement, de l’équipement et de la signalétique
comme elle le fait pour l’incendie et l’accessibilité. En décembre 2000, à Sofia (Bulgarie), à
l’initiative de l’Accord Partiel Ouvert « risques majeurs » du Conseil de l’Europe, les
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représentants des différentes délégations avaient déjà retenus dix points essentiels : identifier
les risques prévisibles, évaluer la vulnérabilité des bâtiments et des installations, engager les
travaux indispensables à la sécurité des personnes, former les différentes personnes
responsables, sensibiliser les personnels, les élèves et les parents aux conduites à tenir,
organiser la communauté éducative, répondre aux besoins des minorités ou des personnes
handicapées de l’établissement, afficher les consignes et les évaluer régulièrement par des
simulations et intégrer l’éducation préventive dans le projet pédagogique de l’établissement.
Une pédagogie ludique et différenciée L’éducation à la prévention des risques majeurs se conçoit au sein de l’école mais
également en dehors, d’autant que le temps périscolaire tend à devenir aussi important que le
temps scolaire et que l’École a d’autres missions. C’est pourquoi il convient de développer des
outils adaptés à une éducation associée aux sports et aux loisirs : le jeu est ainsi un vecteur
essentiel du message. Plusieurs prototypes ont été élaborés : du simple jeu comme Cata’stop à
des jeux de rôle plus complexes requérant l’intervention d’un éducateur comme Rivermed,
consacré aux inondations en régions méditerranéennes, Les activités sportives ou de pleine
nature sont également l’occasion de former la jeunesse aux risques et à l’apprentissage des
moyens de prévention et d’autoprotection comme le montre la démarche ballon vert.
Une culture de prévention généralisée et cohérente Des réflexions sont menées par le Ministère de l’Intérieur, Direction de la défense et de la
sécurité civiles [DDSC], sur la sensibilisation aux exigences de sécurité civile ainsi qu’à celles
menées dans le cadre de l’Accord partiel ouvert sur les risques majeurs du Conseil de l’Europe
et celles de la Direction Générale Environnement [DG XI] de l’Union Européenne sur la sécurité
des enfants en Europe.
L’éducation préventive intégrant tous les risques (routier, accidents domestiques, incendie…)
a sa place dans la formation citoyenne. Elle doit être généralisée mais aussi se structurer,
s’organiser de manière cohérente, se donner des éléments de référence et concevoir des outils
d’évaluation pour être relayée par l’institution scolaire, les collectivités territoriales, la société
civile.
L’atout Internet et des ressources pédagogiques L’émergence des nouvelles technologies de communication et d’information est un atout
extraordinaire car elles permettent de répondre aux exigences de transparence et de
démocratie en facilitant l’accès à l’information environnementale. A ce titre, la Direction de la
prévention des pollutions et des risques [DPPR] a créé www.prim.net comme site portail pour
l’accès à l’information préventive, qu’elle ait pour origine l’État, les collectivités territoriales, les
établissements publics ou privés. Actualisé en permanence, il permet d’avoir un panorama
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complet des sources d’information possibles, de profiter d’une veille sur les sites dédiés .
L’usage d’Internet correspond de plus à la mondialisation des compétences et des actions
nécessaires à la prévention des risques. Il permet d’offrir à chacun le contenu pédagogique
nécessaire à son autoprotection : sorte de cartable virtuel très utile pour l’éducation à distance
et à la Francophonie.
Une nouvelle stratégie internationale La Stratégie Internationale de Prévention des Catastrophes a pour objet de développer la
résistance des communautés face aux catastrophes en les sensibilisant à l’importance de la
gestion des risques comme volet à part entière au service du développement durable-,
atténuant ainsi les pertes au plan humain, socio-économique, environnemental et culturel
causées par les catastrophes naturelles ou technologiques. L’éducation a un rôle essentiel à
jouer pour que cet objectif soit atteint. En janvier 2005, 168 gouvernements, réunis à Kobe,
Hyogo (Japon) à l’occasion de la Conférence mondiale sur la prévention des catastrophes, ont
adopté un plan décennal pour rendre le monde plus sûr face aux catastrophes naturelles. Le
« Cadre d’action de Hyogo pour 2005-2015: Pour des nations et des collectivités résilientes
face aux catastrophes » définit les lignes d'action, fixe les priorités et détermine les moyens
pratiques permettant de réduire considérablement, d’ici 2015, les sinistres liés aux
catastrophes. Dans ce contexte l’UNESCO mobilise une alliance mondiale susceptible de
promouvoir coopération et partenariats pour informer, sensibiliser et éduquer la jeunesse afin
que les collectivités soient mieux préparées et plus fortes. L’Accord partiel ouvert Risques
Majeurs du Conseil de l’Europe participe à cette même dynamique qui doit être visible par
l’inscription de l’autoprotection dans les droits de l’enfant, la mise en place d’un label de
reconnaissance pour les établissements et sa promotion à l’échelle internationale.
Pour un monde plus sûr Sans sous-estimer l’évolution climatique, la grande vulnérabilité de nos sociétés reste la
raison principale des désastres. Les années passées montrent l’intérêt et l’efficacité de la
prévention pour nous protéger et poser les bases d’un aménagement durable des territoires.
L’éducation à la prévention des risques majeurs doit nous apprendre à anticiper les
phénomènes extrêmes ou exceptionnels, à ne plus les baptiser catastrophes, à ne plus les tenir
pour des manifestations de la fatalité. Chacun pourra ainsi apprendre à les observer avec
vigilance et à réduire leurs conséquences.
Les orientations de l’Education nationale, le code de l’environnement, la loi de modernisation
de la sécurité civile par son article 5 et la volonté du Gouvernement français, exprimée par une
mission confiée au professeur Michel RICARD, constituent, pour la France, un cadre
dynamique pour l’éducation à la prévention des risques majeurs.
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AS CHEIAS FLUVIAIS EM LEITOS ALUVIONARES COM VARIABILIDADE NATURAL E ALTERADA. EVIDÊNCIAS NO RIO TEJO.
JOÃO SOROMENHO ROCHA & JOÃO NUNO FERNANDES Laboratório Nacional de Engenharia Civil, Av. Brasil, 101, 1700-066 Lisboa, Portugal
e-mail: [email protected], [email protected]
Os estudos hidráulicos de regolfo, que permitem a definição das zonas inundáveis têm
definir adequadamente a grande complexidade dos leitos de cheia, onde se destacam as obras
de defesa, diques. Conhecidos os dados de base, devem ser calibrados e validados os
cálculos efectuados por modelos numéricos, os quais incluem entre outros os coeficientes de
rugosidade dos leitos (Brunner, 2002).
Na determinação das zonas inundáveis é importante a definição da geometria dos perfis do
rio. A topografia fora do leito principal pode ser bem avaliada utilizando as Cartas Militares de
Portugal à escala 1:25 000. Num leito aluvionar é importante avaliar a variação do leito
principal. Essa variação não é perceptível facilmente em levantamentos com pequenos
intervalos de tempo mas já é perceptível quando se comparam levantamentos com intervalos
de vários anos (HP, 1977).
Na análise da evolução morfológica do leito principal do rio Tejo foram também verificadas
as variações dos talvegues que ocorreram nas três secções das estações hidrométricas. Desta
forma, verificou-se que a variação total da tendência do talvegue ocorrida em 15 anos em
Almourol foi de 1 m, a mesma que em Ómnias em 13 anos. No Tramagal não houve uma
tendência para a variação do talvegue. Esta análise permitiu também verificar as grandes
variações sazonais e interanuais da cota do talvegue, bem como a rápida redução da cota do
talvegue para caudais elevados, e a “recuperação” dessa cota mais lenta com caudais mais
reduzidos (Rocha et al, 2005). Verificou-se que as variações anuais, que podem atingir valores
até 3 m, são superiores à variação das tendências a médio prazo.
A avaliação dos erros de cálculo numérico pode ser feito com o levantamento de marcas de
cheia. No rio Tejo foi feito esse levantamento com equipamento de GPS. Por recurso aos
dados disponíveis de caudais foi possível, para algumas datas, conhecer o caudal escoado.
Tendo em conta a localização da marca, o caudal e a altura de água fez-se a comparação com
o perfil de cálculo mais próximo. Desta forma, foram levantadas 64 marcas de cheia ao longo
do troço, das quais é conhecido o caudal em 26.
Relativamente a esta análise, é necessário ressalvar que são comparadas as alturas de
água calculadas, com a batimetria de 1998, com as alturas de água de diferentes datas.
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Existem diversos factores que deverão ser tidos em conta nesta comparação, como a distância
da marca de cheia ao leito principal do rio, a dificuldade de obtenção de caudais exactos para
os anos ou datas presentes nas marcas de cheia e a exactidão de algumas das marcas de
cheia. Além disso, importa referir que a comparação é feita com o perfil de cálculo mais
próximo, havendo que considerar os erros associados.
A análise realizada permitiu concluir que do cálculo hidráulico resultam, maioritariamente,
valores superiores às marcas de cheia. Concretamente, das 26 marcas de cheia, em 21 o
estudo hidráulico forneceu valores superiores e em 5 forneceu valores inferiores. As diferenças
entre o cálculo e a marca de cheia são maiores quanto maior o caudal escoado. A média das
diferenças encontradas é de 0,95 m. Existe a possibilidade de o coeficiente de Manning, n,
estar estimado por excesso no leito de cheia e de não terem sido tidas em conta as variações
do leito aluvionar.
A análise teórica da morfologia fluvial deve considerar uma grande variedade de escalas de
tempo e de espaço (de Vriend, 1997). O movimento de um grão de areia deve ser analisado
com uma escala de tempo correspondente à da turbulência e com uma escala espacial da
ordem dos milímetros.
A análise da forma da secção transversal exige uma escala temporal dos dias (em ocasiões
de cheia) ou dos anos (para a estiagem) e uma escala espacial das centenas de metros.
Por sua vez, para a análise da evolução do perfil longitudinal, com uma escala espacial das
centenas de quilómetros, deve ser considerada uma escala temporal dos anos até aos séculos.
No limite a escala geológica também controla a evolução do perfil longitudinal. Para a análise
da evolução do leito do rio Tejo e da sua influência nas cheias e inundações, as duas escalas
intermédias são as mais importantes.
Foi também determinada a relação entre os caudais sólidos presentes no rio Tejo. Nesta
análise foram tidas em conta as parcelas provenientes de montante e dos afluentes, bem como
a capacidade de transporte ao longo do rio. Assim, sempre que a capacidade de transporte
exceder a estimativa da afluência de material sólido existe uma tendência para a erosão e
consequente escavação do fundo do rio. No caso contrário se o material afluente for superior à
quantidade de material que possa ser transportado haverá lugar a assoreamento.
Para além da variação natural do leito aluvionar do rio Tejo, há também variações que
derivam da actividade humana. Estas são a construção de grandes albufeiras (Castelo do
Bode, Alcântara, …), a diminuição de zonas florestais e a extracção de areias (DRA-LVT,
2000).
Esta última actividade é a que tem maior impacto na variação do leito aluvionar do rio Tejo.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
A avaliação teórica do material sólido afluente ao vale do rio Tejo está sujeita a elevada
incerteza, muito maior do que a da avaliação dos volumes de extracção. A menor incerteza na
avaliação é da variação do fundo aluvionar. Por esse motivo, o parâmetro dominante na
avaliação do estado da linha de água é a variação do fundo aluvionar.
A análise dos valores de extracção e de variação do fundo permite concluir da existência de
uma actividade de desassoreamento que, considerando o balanço de extracção da ordem dos
– 1,6.106 m3, equivale a uma descida anual de cerca de 5 cm por ano. No entanto, o
desassoreamento não é uniforme, sendo por esse motivo importante verificar a sua distribuição
ao longo do rio Tejo.
Verifica-se que o desassoreamento do rio Tejo ainda se encontra, na generalidade, numa
gama de valores que é menor do que a variação natural do fundo aluvionar do rio Tejo, na
relação de cerca de 1 para 2, isto é, a descida média do leito do rio de 1 m é inferior à variação
média interanual do leito médio, da ordem dos 2 m.
Referências Bibliográficas Brunner, G.W., 2002. HEC-RAS River Analysis System Hydraulic Reference Manual (Version 3.1 ed.) –
http://www.hec.usace.army.mil/software/hec-ras/ USACE, Davis, EUA
DRA-LVT, 2000. Extracção de inertes no rio Tejo. Estudo de Incidências Ambientais. Relatório Síntese.
Direcção Regional do Ambiente de Lisboa e Vale do Tejo. Ministério do Ambiente e do Ordenamento
do Território. Lisboa. 48 páginas e Anexos.
Hidrotécnica Portuguesa, 1977. Regularização do rio Tejo – Plano Geral. Anexo 1 - Estudo hidráulico do
rio Tejo. Ministério das Obras Públicas, Direcção-Geral dos Recursos e Aproveitamentos Hidráulicos,
Lisboa.
Rocha, J. Fernandes, J., Ferreira, G., Ferreira, T., 2005. Plano Específico de gestão de extracção de
Inertes no Domínio Hídrico do rio Tejo. Relatório 396/05-NRE, Laboratório Nacional de Engenharia
Civil.
de Vriend, H.J., 1997. River morphology: A manifestation of nonlinear dynamics. In Managing water:
Coping with scarcity and abundance. Proceedings of Theme A, Water for a changing global
community, XXVII IAHR Congress and ASCE, San Francisco, pp. 10-15, August.
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RADIOTRACERS FOR DATING FLOOD PROCESSES
JOAN-ALBERT SANCHEZ-CABEZA IAEA Marine Environment Laboratories, 4 Quai Antoine 1er, MC-98000 Monaco
e-mail: [email protected]
Environmental radiotracers have been proved to be a very useful tool with a large variety of
applications such as the study of the distribution and migration of pollution in the environment,
and radionuclide inputs and budgets in the terrestrial, freshwater and marine environments. In
particular, atmospheric radiotracers such as 137Cs have become common tools for the
quantification of soil erosion processes (Zapata, 2002). 137Cs is a man-made radionuclide injected into the atmosphere by nuclear weapons tests
which occurred mainly in the late 1950s and early 1960s and subsequently deposited on the
Earth’s surface. The 137Cs levels changed dramatically in Europe due to the Chernobyl accident,
when large amounts of 137Cs were released, dispersed and deposited in the environment in a
very heterogeneous pattern, closely following rainfall during the passage of the polluted plume.
The atmospheric concentrations of 137Cs follow a well-known time trend, with an offset in 1954,
a maximum in 1963 and, in Europe, a second maximum in 1986 (UNSCEAR, 2000). These
signatures can be used as environmental radio-chronometers in a variety of ways. Dried and
homogenised samples of a few tens of grams are measured by low-background high-resolution
gamma spectrometry. 210Pb is a member of the 238U decay series. 222Rn (T1/2= 3.8 d) exhales from the continental
crust to the lower troposphere and is dispersed until it decays into 210Pb, which attaches to
aerosols and is deposited on the Earth’s surface (excess 210Pb). On the other hand, 210Pb
formed in situ in soils and in equilibrium with its parent radionuclide 226Ra (T1/2 = 1601 yr) is
referred to as base (or supported) 210Pb. In general, the governing mechanism in the deposition
of atmospheric substances to soils is wet deposition. Although 210Pb can be measured by
gamma spectrometry, when the sample size is small (100s of mg) or the calibration of gamma
spectrometers is difficult, it can also be measured through its daughter radionuclide 210Po by
low-background high-resolution alpha spectrometry after total digestion using an analytical
microwave oven (Sanchez-Cabeza et al., 1998). 14C (T1/2 = 5730 yr) is also used in many environmental studies because absolute
chronologies can be obtained when proper samples are collected. Nowadays it is commonly
determined in very small samples (10s of mg of C) by graphite synthesis and subsequent
Accelerator Mass Spectrometry. Its use as an absolute radio-chronometer is limited by bomb-
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fallout in the XX century, although its use can be complementary for shorter lived radionuclides
such as 137Cs and 210Pb.
The depth distributions of 137Cs and excess 210Pb (210Pbex) in undisturbed soil cores usually
decrease almost exponentially. Some may also exhibit sub-surface concentration maxima of
varying widths. The depth distribution of a fallout radionuclide is the result of many factors, such
as soil properties, overlying vegetation and hydrology. Processes such as sorption, leaching or
mass transport through macropores or by bioturbation, can also drastically affect the radiotracer
profile. It has been shown that soil cores show 210Pb atmospheric fluxes which correlate very
well with mean annual rainfall in large regions such as N Spain, and this can also be the case
for 137Cs but only at the basin scale.
Undisturbed sediments with constant sediment accumulation show an exponential decrease
of 210Pbex concentrations due to radioactive decay that can be used to date the sediment
horizons. When sediment accumulation is not constant and/or sediment mixing occurs, more
complex models can be used to determine accumulation rates. 137Cs profiles can also be used
to confirm the dates obtained from 210Pb dating. If the sediment core is long enough, it is also
useful to use 14C to confirm mean accumulation rates or to determine changes of accumulation
over longer time periods.
However, during flood events, a large amount of matter (and therefore radionuclides
attached to particles) is remobilised and the dating hypotheses are no longer valid. In
radionuclide profiles it is not unusual to find lower activities due to signal dilution with older
catchment soils and, if the sampling and analytical resolution is good enough, a layer of
homogenous concentration can be observed and dated from the upper and lower profile
sections. Intensity of the flood can also be deduced from the width of the flood layer. This
technique has been used in some environments such as reservoirs, floodplains and river
prodeltas.
Various strategies are used to date flood events. First, 137Cs is an excellent time marker of
floods after 1954. 210Pb high resolution sedimentary records, together with grain size, and
possibly some characteristic metals, can be a powerful technique to date floods which occurred
over the last 100 years. 14C signals during older floods should not be dissimilar to that of other
radiotracers and absolute dating of events can be obtained from a high-resolution age model.
Finally, it is suggested that a full characterisation of flood events would need an extensive
knowledge of radiotracer distributions in a particular catchment, such as the studies carried out
in modern erosion research.
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References Sanchez-Cabeza, J.A., Masqué, P., Ani-Ragolta, I., 1998. 210Pb and 210Po analysis in sediments and soils
by microwave acid digestion. J. Radioanal. Nucl. Chem., 227/1-2, 19-22.
UNSCEAR, 2000. Sources and Effects of Ionizing Radiation, United Nations Publications, New York.
Zapata, F., 2002. Handbook for the Assessment of Soil Erosion and Sedimentation Using Environmental
Radionuclides, Kluwer.
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32
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O PALEOLÍTICO NO MÉDIO E BAIXO TEJO
LUÍS RAPOSO Director do Museu Nacional de Arqueologia. Professor convidado do Departamento de História da Faculdade de
Letras da Universidade de Lisboa
e-mail: [email protected]
Apresentam-se os principais locais e zonas de ocupação humana no vale do Tejo, em
território português, de acordo com três critérios de observação, que se procuram cruzar entre
si: (a) história das investigações; (b) situação geográfica; (c) cronologia e inserção cultural.
Relativamente ao primeiro aspecto, salientam-se as descobertas feitas no início do século
XX nos arredores de Lisboa (especialmente na chamada “várzea de Loures” e zonas
limítrofes); os importantes trabalhos de Georges Zbyszewski e Henri Breuil realizados
principalmente na região de Alpiarça; e as investigações conduzidas nas últimas décadas tanto
no vale do rio principal (região de Ródão), como em vales subsidiários (Zêzere, Trancão, Rio
Maior, Almonda).
Quanto ao segundo aspecto, estabelecem-se os contrastes existentes entre o Médio Tejo e
o Baixo Tejo, e, dentro deste, entre o Tejo aluvial e o Tejo estuarino.
Finalmente, quanto ao terceiro aspecto, apresentar-se-ão os principais locais do Paleolítico
do Vale do Tejo, nos seus sucessivos momentos evolutivos, com referência especial à
economia das matérias-primas, às práticas de subsistência e às territorialidades. Serão ainda
sugeridas e discutidas hipóteses de trabalho que sugerem a forte interacção existente entre
cultura e ambiente na caracterização da ocupação humana paleolítica do Vale do Tejo.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
PALAEOFLOOD AND HISTORICAL FLOOD RECORDS ALONG THE MIDDLE TAGUS RIVER CATCHMENT: CLIMATIC AND FLOOD HAZARD IMPLICATIONS
GERARDO BENITO CSIC, Centro de Ciencias Medioambientales, Serrano 115 bis, 28006 Madrid, Espanha
e-mail: [email protected]
Information on hydrologic variability and extreme floods of the Tagus River have been
completed using both, palaeoflood hydrology (a new and developing branch of hydrology and
geomorphology based on geologic indicators) and historical information (based on documents
and chronicles). Slackwater flood deposits along the Tagus River (at Puente del Arzobispo and
Alcántara) provide an excellent record of flood events for the last 10,000 years. For this period,
more than 80 individual flood events have been recorded and their minimum peak discharge
have been estimated. The palaeoflood record shows that extreme floods are not randomly
spaced in time but there are clusters of flood events at during specific periods. High-magnitude
floods occurred on the Tagus river from 9440 to 9210 14C yr BP (8540-8110 BC), 8500 to 8000 14C yr BP (7500-7000 BC), ∼6750 14C yr BP (∼5000 BC), 1200 to 950 14C yr BP (AD 785-1205),
∼410 14C yr BP (AD 1450-1500), and 170 to 80 14C yr BP (AD 1670-1950). The palaeoflood
record over the last 1000 years was supported with documentary flood data collected at
Aranjuez, Toledo, Talavera de la Reina and Alcántara. High flood frequencies were registered
in the periods: AD 1160-1210 (3%), 1540-1640 (11%; peak at 1590-1610), 1730-1760 (5%),
1780-1810 (4%), 1870-1900 (19%), 1930-1950 (17%) and 1960-1980 (12%). According to the
present flood-producing mechanisms, these periods with increased flood magnitude and/or
frequency in the Tagus River are strongly related to increased moisture influx and winter
precipitation in the Iberian Peninsula, especially in the western zone. The sedimentary and
documentary record of water surface elevation reached by past floods enabled the calculation
of robust palaeodischarge estimates for floods that occurred during recent centuries or
millennia. The frequency and magnitude of these past flood records are discussed on the
context of flood risk assessment and on long-term flood-climate relationships.
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Presentations
37
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
SCALE INVARIANT STUDY OF RIVER DISCHARGES FROM THE TAGUS BASIN
MARIA ISABEL PEDROSO DE LIMA1,2, J.L.M.P. DE LIMA1,3, R.R. RODRIGUES4, C. BRANDÃO4, A.S.S. ROSÁRIO4, 5
1Institute of Marine Research - Coimbra Interdisciplinary Centre, Portugal 2Forestry Department, ESAC/Polytechnic Institute of Coimbra, ESAC/IPC, Bencanta, 3040-316 Coimbra, Portugal
e-mail: [email protected] Department of Civil Engineering, University of Coimbra, Portugal
4Institute for Water, Portugal 5Research fellow
River discharge is known to be dependent on climatic regimes (namely the rainfall input) and
complex rainfall-runoff processes occurring at a variety of scales in time and across the
catchment’s basin. As a consequence, this highly non-linear hydrologic process exhibits wide
variability over a broad range of time and space scales. This variability is an important issue in
many studies and areas of research (e.g. hydrology, hydraulics, water resources, land planning,
and urban development). It involves a large dynamic range, which in certain cases leads to
catastrophic events related, for example, to both flood and drought situations.
It is thus important to analyse runoff with methods that have the potential to assess the full
range of river flow fluctuations. Multifractal theory (e.g. Schertzer and Lovejoy, 1989, 1991) can
play an important role in such studies. Moreover, multifractal methods are innovative for the
analysis of extremes, offering practical tools for assessing their probability of occurrence.
Multifractal theory predicts the existence of heavy tails (i.e., power-law tails) in the flow rate
probability distributions. This statistical behaviour may indicate that, in some cases, the
probability of exceeding certain events is greater than the probability predicted by more
"conventional" models (e.g. Gumbel distribution). Runoff models based on non-scaling type
processes (e.g. Auto Regressive Moving Average type processes) usually involve only weak
variability (e.g. exponential probability tails). In this framework, two or more different
distributions are often necessary to fit different regimes such as the “regular” and the “extreme”
events. Multifractal theory offers a single framework to deal with these different regimes.
The approach to the study of river flows based on multifractal theory has been reported in
only a few studies, e.g. Ladoy et al. (1991), Tessier et al. (1996), Pandey et al. (1998).
Therefore, the applicability of multifractal theory to runoff processes has still not been fully
explored.
This work reports results of scale invariant and multifractal analyses of river discharges from
the Tagus basin, concentrating on the statistics of extreme events. Monthly and daily data are
analysed. The time span of the records is up to several decades. The longest river flow
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
discharge time series studied is from Vila Velha de Ródão, in the river Tagus, for a period of 74
years; the corresponding drainage basin is 59247 km2.
The data are from different hydrometric stations located in the Tagus drainage basin; these
stations are located in the Tagus River and in some of the tributaries. The drainage basins have
different geometric, geological and climatic characteristics, as well as land use. The areas of the
basins are comprehended between less than 100 km2 and almost 65000 km2. The Tagus
drainage basin covers an area of about 80629 km2, being 24800 km2 (29.8%) in Portugal. The
basin has a general orientation ENE to WSW; its length is approximately 700 km and the
average width is around 120 km. The Tagus River is 1100 km long; 230 km run in Portugal and
43 km in the border with Spain. In Portugal, the main tributaries are, in the left bank, the river
Sorraia, with a drainage basin of 7555 km2, and in the right bank (northern sector) the river
Zêzere, with 5080 km2. The annual average flow discharge from the river Zêzere is about 3290
hm3/year, whereas in the river Sorraia is about 1185 hm3/year.
Results confirm that the temporal structure of river discharge exhibits scale invariant
behaviour. Thus, discharges Qλ observed at scales of resolution λ-1 respect the multiple scaling
behaviour of the probability distributions: , where c(γ) is the codimension
function and γ is the order of singularity of the discharge (e.g. Schertzer and Lovejoy, 1987).
Moreover, the statistics of extreme events can be described by a power-law, ,
where s is a sufficiently large discharge-threshold; this behaviour indicates divergence of
statistical moments above a certain critical order q
)()Pr( γγλ λλ cQ −≈≥
DqssQ −≈≥ )Pr( λ
D (fig. 1). This value is rather low in certain
cases, indicating violent river regimes. The magnitude of this moment was observed to vary with
the river course analysed within the Tagus basin.
Fig. 1 - Empirical probability distributions of daily river
discharge, for the data from Vila Velha de Ródão.
Discharge is expressed in m3/s.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
This study will pursue investigating the relations between the statistical behaviour of
extremes and the physical characteristics of the drainage basins as well as the statistics of the
rainfall input.
This work has been carried out under the research project POCTI/ECM/48619/2002, funded
by the Portuguese Foundation for Science and Technology and FEDER.
References Ladoy, P., S. Lovejoy and D. Schertzer, 1991. Extreme variability of climatological data: scaling and
intermittency. In: Schertzer, D., and S. Lovejoy (eds.), Non-linear variability in Geophysics: scaling and
fractals. Kluwer Academic Publ., 241-250.
Pandey, G., S. Lovejoy and D. Schertzer, 1998. Multifractal analysis of daily river flows including extremes for
basins of five to two million square kilometres, one day to 75 years. J. Hydrology, 208(1-2): 62-81.
Schertzer, D., and S. Lovejoy, 1987. Physical modeling and analysis of rain and clouds by anisotropic scaling
multiplicative processes. Journal of Geophysical Research, 92(D8), 9693-9714.
Schertzer, D., and S. Lovejoy, 1989. Nonlinear variability in Geophysics: multifractal simulations and analysis.
In: Pietronero, L. (ed.), Fractals’ physical origin and properties. Plenum Press, 49-79.
Schertzer, D., and S. Lovejoy (eds.), 1991. Non-linear variability in Geophysics: scaling and fractals. Kluwer
Academic Publ., 318 p.
Tessier, Y., S. Lovejoy, P. Hubert, D. Schertzer and S. Pecknold, 1996. Multifractal analysis and modeling of
rainfall and river flows and scaling, causal transfer functions. J. Geophysical Res., 101: 26427-26440.
41
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
THE ROLE OF NAO EXTREME PHASES ON THE MONTHLY DISCHARGES, AND THE VOLUME AND DATE OF THE ANNUAL MAXIMA FLOWS (1950-2003) IN THE TAGUS
RIVER
JUAN I. LÓPEZ-MORENO1,2,*, SERGIO M. VICENTE-SERRANO2, SANTIAGO BEGUERÍA2, JOSÉ M. GARCÍA-RUIZ2,*
1Dpt. Geosciences. University of Fribourg. Perolles, CH-1700 Switzerland *e-mail: [email protected]
2Instituto Pirenaico de Ecología, CSIC (Spanish Research Council), Campus de Aula Dei, P.O. Box 202, Zaragoza,
Spain *e-mail: [email protected]
The North Atlantic Oscillation (NAO) is one of the most important atmospheric circulation
patterns in Europe, which determines noticeably the climate in the Tagus basin (Trigo et al.,
2004). Positive phases of the NAO are related to high Sea Level Pressures (SLP) and low
precipitation. Negative phases show the opposite pattern. The NAO is better recognised during
winter, season in which its climate effects are more important.
On the other hand, water resources in the Tagus basin are characterised by a high temporal
variability, being the drought periods and floods highly frequent (Benito et al., 2003). In the
Tagus basin, drought periods usually cause important conflicts between users; whereas
frequent floods may be considered as a risk and cause important economic losses (Ramos and
Reis, 2001).
Using river discharges (monthly and annual maxima flows) and reservoir storages, from the
Confederación Hidrográfica del Tajo (Spain) (http://www.chtajo.es/redes/cantidad.htm) and the
Sistema Nacional de Informaçao de Recursos Hidricos (Portugal) (http://snirh.inag.pt/), we have
analysed the role of the extreme phases of the winter NAO on the water resources and flows in
the Tajo’s basin between 1950 and 2003.
We show that river discharges are highly determined by the winter NAO extreme phases.
Negative NAO phases direct positive anomalies in the river discharges whereas the positive
NAO phases produce negative anomalies. Moreover, these anomalies are even identified
several months after the winter, during the summer and the fall. Also some differences exist in
this response between areas as a function of the location and the river characteristics
(regulation, average flows, etc.). There is a delay in the response of the river discharges to the
NAO extreme phases; nevertheless the impacts are better recognised towards the East. Thus,
in the headwaters, significant anomalies are even identified seven months after the NAO
extreme phase.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
There is not a lineal relationship between the NAO and the river discharges of the Tagus
basin since the response to the negative phases is slower than positive phases but the former
shows anomalies maintained during a higher number of months.
Annual maxima flows are also highly affected by the NAO extreme phases. During the
negative phases, the volume of the maxima flows is higher and the annual maxima flows are
mainly recorded in winter. On the contrary, during the positive phases, the volume of the annual
maxima is lower and the maximum annual flow is dominantly recorded in autumn.
References Benito, G., Diez-Herrero, A. and de Villalta, M.F., 2003. Magnitude and frequency of flooding in the
Tagus basin (Central Spain) over the last millennium. Climatic Change. 58: 171-192.
Ramos, C. and Reis, E., 2001. The floods in the south of Portugal in different kinds of drainage basins.
Finisterra, 71: 61-82.
Trigo, R.M., Pozo-Vázquez, D., Osborn, T.J., Castro-Díez, Y., Gámiz-Fortis, S., Esteban-Parra, M.J.,
2004. North Atlantic Oscillation influenceon precipitation, river flor and water resources in the Iberian
Peninsula. International Journal of Climatology, 24: 925-944.
43
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
USEFULNESS OF PALEO-FLOOD INFORMATION IN FLOOD QUANTILE ESTIMATION
BHAGABAT .P. PARIDA Dept. of Environmental Science, University of Botswana, P/Bag UB 00704, Gaborone, Botswana.
e-mail: [email protected]
Collapse of the climate negotiations in the recent past have freed the industrialised nations
from any restrictions in their greenhouse gas emissions, as a result of which the world is likely
to see a substantial increase in CO2 emissions, the major contributor to the risk of global climate
change including rise in temperature (Raskin and Kemp-Benedict, 2004). It is well known that
resulting general rise in global temperature will accelerate the hydrologic cycle involving
changes in precipitation hence in the runoff, which may ultimately result in increased frequency
of many weather related disasters including floods and have been estimated at some 12000
disasters since 1980. As reported by Office of Disaster Assistance of the USAID, floods alone
had the highest profile affecting some 275 million people between the decade of 1980-1990 and
attained highest number of declarations (165) in the subsequent decade i.e. 1990-2000, which
in 2002 alone resulted in 66% of the total losses with some 600 events from out of a total of 700
natural disasters. Even recently, in 2004, weather related disasters caused nearly $105 billion
losses (in 2003 dollars) – which was almost twice the total in 2003. Even looking at the
disasters due to floods, it not only had the highest numbers of events but also has been
responsible in affecting the highest number people compared to any other disaster. For
example, the number of flood events rose from 107 in 2004 affecting about 74 million people to
168 in 2005 affecting nearly 117 million people (EM-DAT, 2006).
Frederick and Major (1997) in yet another study have reported that not only the timing and
magnitude of the global temperature changes are uncertain, but less is known about the climate
change and their impacts at basin and watershed levels. General Circulation Models (GCMs)
the principal tools relating changes in atmospheric chemistry to changes in climatic variables
such as temperature and precipitation; though project that a 1.5 to 4.5 ºC rise in global mean
temperature would increase the global mean precipitation about 3 to 15%, do not provide the
requisite degree of region specific information. It is projected that, in areas with increased
precipitation, higher evaporation rates may lead to reduced runoff. So, hydrological
uncertainties attributed to changing atmospheric chemistry are likely to persist in the
foreseeable future.
Again, it has been observed that climate change predictions are primarily based on
greenhouse gas emissions while the impact of land use has been a neglected factor in such
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
studies. In a recent study Feddema et al., (2005) indicated that consideration of the land use
land cover changes have shown significant changes with opposite signs in the A2 scenario
compared to the ones issued by Inter Governmental Panel on Climate Change (IPCC,2001),
and may lead to far reaching consequences. This is particularly important looking at the world
wide rapid rate of urbanisation leading to significant changes in land use. For example, during
the year 2000, the global population changed from less than 50% urban to more than 50%
urban. It is estimated that by 2010, urban population will account to more than 60% of the world
population, threatening a significant change in the land use in terms of human settlements and
industries. It is therefore quite likely that one may experience larger and more frequent floods.
Managing such events though may remain more human oriented, yet providing tangible
solutions still remain scientific.
In flood estimation procedure, generally emphasis on two aspects is given viz. on the
appropriate choice of a statistical model and a robust method of parameter estimation, such that
both descriptive and predictive aspects are well covered (Cunnane, 1987). While this is true, the
limitation of the historic data is often overlooked and any peculiarity in the data are analysed by
hypothesising an elaborate statistical model. For example, the present-day analysis a priori
assumes the data to be perfectly correct and free from any measurement error. Related studies
by Potter and Walker (1981) have shown that the effect of modelling error because of this is far
more damaging than the sampling error. Even, another related study by Hosking and Wallis
(1986) have shown that measurement error can result in yielding less accurate design flood
estimates even with the use of Probability Weighted Moments (PWM) method of parameter
estimation. This particularly is more relevant when it is well accepted that the method of PWM
(Greenwood et. al., 1979; Hosking, 1986) or even the method of L-Moments (Hosking, 1990)
produce efficient and robust estimators while dealing with small samples or samples containing
outliers or even in a case where a wrong choice of the parent distribution has been made
(Wallis and Wood, 1985). It is also in general agreement that a three-parameter distribution by
and large is acceptable for modelling the flood data as it is capable of taking into account the
skew properties of the flood data adequately and yield quantiles with least mean squared error.
Studies by Cong and Xu (1987) and Parida (2002) showed that measurement error can change
the skew properties of the sample considerably and may suggest a different distribution other
than the parent. To overcome this, inclusion of paleoflood information wherever available has
been highly recommended (Hosking and Wallis, 1986; Stedinger and Cohn, 1986; Parida,
2000). Even they emphasize on its usefulness when the number of parameters to be estimated
are more as in case of three parameter distribution compared to a two parameter one.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Yet another dimension to the problem of flood estimation is to obtain reliable design flood
estimates when the catchments are ungauged or when one needs to develop a nation-wide
uniform flood estimation procedure. For these, Regional Flood Frequency Analysis (RFFA)
methods in various forms have been in advocated, which besides being able to provide
meaningful solutions to the above problems, have been found to improve the at-site flood
estimates within the region (Cunnane, 1988). It is particularly so, as the regional estimators are
least affected by most of the data related problems mentioned earlier. It can be therefore be
said that reliable estimates of flood quantiles can be obtained through RFFA together with
consideration of Paleoflood information, as it allows the series to get the experience of what the
stream has experienced. In a simulation study Parida (2000) has shown that incorporation of
paleoflood information infested with as high as 30% measurement error to a series of
observations infested with 10% yield more reliable quantiles than from the same sample having
no measurement error. But what is not clearly known is the extent of usefulness in inclusion of
such paleo flood events on the at-site and regional quantile estimates, particularly when a
region is delineated using critical attributes such as the climate and land use responsible for
generation of such extreme floods. Therefore, a case study using real life data from the 3-d
hydro-meteorological region in India has been undertaken, where at two out of some 17 gauged
sites evidence of such extraordinary floods events had been observed. Quantiles using PWM
procedure with the inclusion of such extra-ordinary data with recent observations (i.e. the case
of censored sample) were undertaken. It was found that the quantile estimates though did
improve considerably particularly at sites where these extraordinary events were available, did
not change the regional growth curve characteristics for the study region.
References: Cong, S., and Xu, Y., 1987. Effects of Discharge Measurement Error on the Results of Flood Frequency
Analysis, Application of Frequency and Risk in Water Resources, V.P. Singh, Ed, 175 – 190.
Cunnane, C., 1987. Review of Statistical Models for Flood Frequency Estimation, Hydrologic Frequency
Modeling, D. Reidel Pub. Co., Holland, V.P. Singh, Ed., 49 – 95.
Cunnane, C., 1988. Methods and Merits of Regional Flood Frequency Analysis, J. Hydrology, 100, 269 –
290.
EM-DAT, 2006. The OFDA/CRED International Disaster Database, University Ctholique de Louvain,
Brussels, Belgium.
Feddema, J.J., Oleson, K.W., Bonan, G.B., Mearns, L.O., Buja, L.E., Meehl, G.A. and Washington, W.M.,
2005. The importance of Land-Cover in Simulating Future Climates, Science, 310 (5754), 1674-1678.
Frederick, K.D. and Major, D.C., 1997. Climate Change and Water Resources, J. Climate Change, 37,
7-23.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Greenwood, J.A., Landwehr, J.M., Matalas, N.C., and Wallis, J.R., 1979. Probability Weighted Moments:
Definitions and Relation to Parameters of Several Distributions Expressible in Inverse Form, Water
Resour. Res., 15(5), 1049 – 1054.
Hosking, J.R.M., 1990. L-Moments: Analysis and Estimation of Distribution Using Linear Combination of
Order Statistics, J. Royal Statistical Society –B, 52 (1), 105 –124.
Hosking, J.R.M., and Wallis, J.R., 1986. The Value of Historical Flood Data in Flood Frequency Analysis,
Water Resour., Res., 22(4), 543 – 550.
IPCC , 2001. Climate Change: Impacts, Adaptations and Vulnerability, Inter governmental Panel on
Climate Change, Cambridge University Press, p 1031.
Parida, B.P., 2000. Worth of Extreme Flood Events in Site Specific Flood Frequency Analysis,
Proceedings International Conference on Integrated Water Resources Management for Sustainable
Development, New Delhi, India, 823 – 832.
Parida, B.P., 2002. Effect of Measurement Error on Flood Estimation, Proceedings International
Conference on Flood Estimation, Berne, Switzerland , CHR Report II-17, 775-778.
Potter, K.W., and Walker, J.F., 1981. A Model of Discontinuous Measurement Error and its Effect on the
Probability Distribution of Flood Discharge Measurement, Water Resour. Res., 17(5), 1505 – 1509.
Raskin, P. and Kemp-Benedict, E., 2004. Background Paper for UNEP’s Third Global Environment
Outlook Report (GEO-3), UNEP, Kenya.
Stedinger, J.R., and Cohn, T.A., 1986. Flood Frequency Analysis with Historical and Paleoflood
Information, Water Resour. Res., 22(5), 785 – 793.
Wallis, J.R., and Wood, E.F., 1985. Relative Accuracy of L-P III Procedures, J. Hydraul. Div. ASCE,
111(7), 1043 – 1056. (see also discussion and reply 113(9), 1205 – 1214.)
47
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
THE IMPACT OF THE NORTH ATLANTIC OSCILLATION ON PRECIPITATION AND WATER RESOURCES OF THE IBERIAN PENINSULA
RICARDO M. TRIGO Centro de Geografia da Universidade de Lisboa, University of Lisbon, Portugal
e-mail: [email protected]
The Iberian Peninsula area is characterised by large values of inter-annual variability of
precipitation. As a consequence river flow is also characterised by large disparities between wet
and dry years, especially in southern Iberia. This situation portrays a major problem in water
resources management in general, and production of hydroelectricity in particular. In Portugal
the hydroelectric production represents (on an average year of precipitation) one third of the total
Portuguese electricity production but its absolute value can vary by a factor of almost three
between wet (16 TWh) and dry (6 TWh) years.
Only recently it has been recognized that the strong control exerted by NAO on precipitation
over the Mediterranean basin could be directly reflected on the seasonal streamflow of rivers
across the region (Trigo et al., 2004). In fact, recent works have proved that this is the case for
the Middle East rivers Tigris and Euphrates (Cullen and deMenocal, 2000) and the large central
European river Danube (Stanev and Peneva, 2002; Rimbu et al., 2002). Here we intend to show
that such influence on river flow regimes is extensive to the Iberian rivers, and that the
magnitude of such influence is even larger for Iberian rivers than it is the case with the Danube,
Tigris or Euphrates.
River flow data from both Douro (at Pocinho) and Tejo (at Fratel) spans between 1922 and
1997. The impact on the hydrological cycle (from Oct. to Sep.) of years characterized by winters
with large positive and negative NAO index anomalies is shown in Fig. 1a for Tejo River. For
river Douro (not shown) these differences are significant (at the 5% significance level) only
between January and April while for river Tejo they are consistently significant between January
and September. This reflects the fact that river Tejo basin is located in central Iberia, a region
more affected by the impact of the NAO in the precipitation field (Trigo et al., 2002, 2004).
We have computed the correlation coefficients between winter river flow (DJFM) and
contemporaneous winter NAO index. We have also computed the lagged correlation between
the NAO index for DJF and the river flow for JFM. Furthermore, we have computed correlation
coefficients for shorter periods, namely between 1922 and 1971 and between 1972 and 1997.
Two important conclusions can be drawn:
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1) The magnitude of all lagged correlation coefficients is consistently higher than the
corresponding non-lagged ones. This fact is particularly relevant because it increases the
potential for using these empirical relationships in effective forecasting tools.
2) There is a major increment in the magnitude of correlation coefficient values between the
first sub-period and the second sub-period. This is particularly impressive for the Douro River
where it increases between -0.28 and -0.62. Fig. 1b shows the inter-annual variability of river
Tejo against the lagged winter NAO index (multiplied by –1 to facilitate analysis).
These results show that the large inter-annual variability of these two rivers flow is largely
modulated by the NAO phenomena. Such modulation, associated with the recent positive trend
of the NAO index, might implicate a significant decrease of the available flow. This reduction can
represent an important hazard for the two Iberian economies due to its negative impact in
agricultural yield and hydroelectric power production.
Fig. 1 - (a) Monthly river flow of river Tejo at Fratel for winters with NAO index > 0.5 (dotted curve), for
winters with NAO index > -0.5 (dashed curve) and for the average winter (solid line), (b) Interannual
variability of the mean winter (JFM) river flow (solid curve) and the lagged winter (DJF) NAO index,
multiplied by –1 to facilitate analysis (dashed curve); both curves have been normalised and so are
dimensionless.
References Cullen, H. M. and P.B. de Menocal, 2000. North Atlantic Influence on Tigris-Euphrates Streamflow. Int. J.
Climatol., 20, 853-863.
Rimbu, N., C. Boroneant, B. Carmen and D. Mihai, 2002. Decadal variability of the Danube river flow in
the lower basin and its relation with the North Atlantic Oscillation. Int. J. Climatol., 22, 1169-1179.
Stanev, E.V. and L.P. Elissaveta, 2002. Regional sea level response to global climatic change: Black sea
examples. Global and Planetary Changes, 32, 33-47.
Trigo, R.M., T.J. Osborn and J. Corte-Real, 2001. The North Atlantic Oscillation influence on Europe:
49
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
climate impacts and associated physical mechasims. Climate Research, 20, 9-17. 10.
Trigo R.M., Pozo-Vazquez D., Osborn T.J, Castro-Diez Y., Gámis-Fortis S., Esteban-Parra M.J., 2003.
North Atlantic Oscillation influence on precipitation, river flow and water resources in the Iberian
Peninsula. International Journal of Climatology, 24, 925-944.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
A MEDIÇÃO DE CAUDAIS DE CHEIA NO RIO TEJO
JOÃO SOROMENHO ROCHA1, MARIA EMÍLIA VAN ZELLER2 E JOÃO NUNO FERNANDES1
1Laboratório Nacional de Engenharia Civil
e-mail: [email protected], [email protected]ão de Coordenação e Desenvolvimento Regional de Lisboa e Vale do Tejo
Av. Brasil, 101, 1700-066 Lisboa, Portugal
e-mail: [email protected]
A medição de caudais no rio Tejo é feita em3 estações hidrométricas, Tramagal, Almourol e
Ómnias.
A estação do Tramagal fica situada próxima da localidade com o mesmo nome, a cerca de
100 km de Vila Franca de Xira. Para montante, o afluente mais próximo é Rio Torto.
A estação de Almourol fica situada a montante do castelo de Almourol, a cerca de 85 km de
Vila Franca de Xira. Para montante, o afluente mais próximo é o Rio Zêzere.
A estação de Ómnias fica situada a jusante de Santarém, a cerca de 45 km de Vila Franca
de Xira. O afluente mais próximo desta estação, para montante, é o Rio Alviela.
De forma a determinar as curvas de vazão destas estações, ao longo do tempo, foram
sendo efectuados levantamentos batimétricos pela DRAOTLVT permitindo, ao mesmo tempo, o
conhecimento da evolução morfológica nestas secções. Além disso, dadas estarem disponíveis
as séries de caudais para estas estações é possível verificar a variação do talvegue e do leito
médio com o regime de caudais.
As séries de caudais disponíveis com interesse para o estudo das cheias são:
i) séries de medições do caudal instantâneo efectuadas na altura do levantamento;
ii) séries de caudais médios diários disponíveis no SNIRH,
iii) nível hidrométrico instantâneo da rede hidrométrica automática (que, com o
conhecimento das curvas de vazão permite o conhecimento do caudal instantâneo).
Com os dados disponíveis foi possível determinar as variações do talvegue ao longo dos
períodos de tempo disponíveis para cada estação (desde 1990/91 a 2004/05 para as estações
de Tramagal e Almourol e 1986/87 a 1999/00 para a estação de Ómnias). Nas séries de
caudais disponíveis são conhecidos os caudais instantâneos medidos na altura do
levantamento para todas as estações. A rede automática foi iniciada em 200l. Os caudais
médios diários publicados pelo SNIRH terminam em 1990, cobrindo assim uma parte da série
de batimetrias da estação de Ómnias (com períodos coincidentes).
Relativamente à variação do talvegue nas estações hidrométricas verifica-se que:
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− Existe uma grande variabilidade sazonal e inter-anual da cota do talvegue, chegando a
verificar-se variações de 2,5 m em curtos períodos de tempo;
− A relação indirecta entre a cota do talvegue e do caudal, um caudal elevado provoca
uma redução acentuada da cota do talvegue, a “recuperação” para a cota original é mais lenta,
− No Tramagal não houve uma tendência de variação significativa no período de
observação (cerca de 15 anos);
− Em Almourol observou-se uma tendência para o abaixamento, traduzido em cerca de
1 m no período de observação (cerca de 15 anos);
− Em Ómnias observou-se uma tendência para o abaixamento do talvegue de cerca de
1 m no período de observação (cerca de 13 anos).
A análise da variação do leito médio encontra-se dependente da digitalização dos dados. No
futuro poder-se-á ainda tentar recuperar dados mais antigos para estender para datas mais
recuadas a avaliação da variação do fundo aluvionar do rio Tejo.
Referências Bibliográficas Hidrotécnica Portuguesa, 1977. Regularização do rio Tejo – Plano Geral. Anexo 1 - Estudo hidráulico do
rio Tejo. Ministério das Obras Públicas, Direcção-Geral dos Recursos e Aproveitamentos Hidráulicos,
Lisboa.
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ACTIVE TECTONIC STRUCTURES IN THE LOWER TAGUS VALLEY: STATE OF THE ART
JOÃO CABRAL1
1 Fac. Ciências de Lisboa, Dep. Geologia, LATTEX / IDL, Ed. C6, 2º Piso, Campo Grande, 1749–016 Lisboa,
Portugal
e-mail: [email protected]
Geological Setting The Lower Tagus Valley (LTV, central-western zone of mainland Portugal) is sited in the
Lower Tagus Basin (LTB), a NE-SW elongated tectonic depression comprising up to nearly
2000m of Cenozoic sediments, located east of the Mesozoic Lusitanian Basin. It was formed
mainly in the Neogene as a compressive foredeep related to tectonic inversion of the Lusitanian
Basin, due to a NW-SE Miocene compression (Ribeiro et al., 1990, Rasmussen et al., 1998;
Carvalho, 2003). There is structural inheritance from the underlying Mesozoic and Paleozoic
basement through the compressive reactivation of previously extensional structures of the
Lusitanian Basin, in a relatively complex fault pattern that makes up the Lower Tagus Valley
fault system.
Data from the surface geology, from oil exploration wells drilled in the LTV area and from
seismic reflection profiles that cross the LTB show that the lowermost Cenozoic deposits, of
Eocene-Oligocene age, are of relatively uniform thickness (200-400 m), whereas the
subsequent Neogene sediments are much thicker and variable. More than 800 m of
predominantly siliciclastic continental and interfingered shallow marine Miocene sediments have
been preserved in Neogene depocenters. Fluvial feldspathic sands and gravels of Pliocene age
unconformably overlie the Miocene deposits.
The culminating surface of the Pliocene sedimentation is still preserved in a few areas of the
LTB, reaching a height of 100-120 m in the LTV (Martins, 1999). Stepped Quaternary terrace
deposits of the Tagus River and its tributaries occur along their valleys providing evidence of
reversal of the regional subsidence to uplifting in most of the LTV area, which generated the
entrenchment of the fluvial network since the Lower Pleistocene (Cabral, 1995).
The distribution of the Tertiary sediments in the LTB is asymmetric. The Miocene rocks
outcrop predominantly in the western side of the Tagus valley, while the Pliocene deposits
dominate at the eastern side. This is an effect of differential subsidence between the two areas
during deposition of the Pliocene fluvial sediments, leading to a smaller thickness of these
sediments at the west, where they were mostly removed by subsequent erosion in the
Quaternary. The evolution from a drainage network flowing on a vast alluvial plain in the Upper
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Pliocene to an entrenched fluvial system in the Quaternary, as a result of relative lowering of
base level, was apparently controlled by the same NNE-SSW regional structure, which settled
the lower reach of the Tagus river and so controlled the distribution of most of the Quaternary
sediments in the area.
Regional seismicity and active faults in the LTV The LTV has been subjected to earthquakes that caused severe damage and many
casualties, making seismic hazard evaluation a major regional issue. The regional seismicity
includes large distant events, as the 1755 “Lisbon earthquake” (M ≥ 8.5), generated in the
Eurasia-Africa plate boundary zone (Martins and Mendes Victor, 1990, Zitellini et al., 1999,
2001; Baptista et al., 2003; Gracia et al., 2003), and local intraplate earthquakes with M=6-7, as
in 1344, 1531 and 1909 (Martins and Mendes Victor, 1990; Teves-Costa et al., 1999, and
others). Since 1909, seismic activity has been low and diffuse, difficult to correlate with the
surface geology.
The regional earthquake activity indicates the presence of active seismogenic faults in the
area that represent a serious threat for the densely populated region. However, major
uncertainties concerning the seismic sources persist due to the low slip rates and the presence
of a thick sedimentary cover that conceals the faults in the underlying basement. In fact, most of
the active faults in the LTV region are hidden or partially hidden under the Tertiary and
Quaternary alluvial fill of the LTB. Evidence of surface faulting in the Quaternary is scarce,
although for a few structures there is geomorphic and outcrop evidence of post- Upper Pliocene
faulting, as for the Azambuja fault (Cabral et al., 2004). The predominance of hidden faults
requires the search for sub-surface data, mainly through geophysical methods (seismic
reflection, gravimetric, geoelectric) and from boreholes (mostly drilled for the oil industry or
water exploitation).
Not withstanding the existence of a macro-scale, NNE-SSW trending, wide fault zone in the
basement, no evidence was found for the presence of such a continuous structure at the upper
crustal levels that might correspond to a so-called Lower Tagus Valley Fault, often referenced in
the bibliography. The sub-surface data, together with the surface geology, reveal structural
complexity characterized by a segmented regional fault system, with NNE-SSW en echelon
faults presenting Neogene reverse offsets, linked by WNW-ESE faults that accommodate large
vertical offsets, probably working as transfer zones.
Seismogenic potential of the active faults Evidence of Quaternary tectonic activity was found for some of these structures. A few
estimates of low slip rates for regional active structures have been obtained (0.1-0.05mm/yr).
The faults range in length from 10 to 40 km, being capable of individually generating maximum
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earthquakes of magnitude 6.2 to 6.9. These fall in the magnitude range estimated for the
historical seismicity, suggesting that the historical earthquakes represent “maximum
earthquakes”. However, the historical earthquakes in LTV deviate from the Gutenberg-Richter
recurrence law obtained for the instrumental seismicity, suggesting the interaction between
adjacent faults which produces clustering of earthquakes and reduces recurrence time.
References Cabral, J., 1995, Neotectónica em Portugal Continental. Memórias do Instituto Geológico e Mineiro,
Memória 31, Lisbon, 265 pp.
Cabral, J., Ribeiro P., Figueiredo P., Pimentel N., Martins A. (2004). The Azambuja fault: An active
structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal).
Journal of Seismology, 8, pp. 347–362.
Carvalho, J., 2003. Sísmica de alta resolução aplicada à prospecção, geotecnia e risco sísmico. PhD
Thesis, University of Lisbon.
Gràcia, E., Dañobeitia, J., Vergés, J., the PARSIFAL Team (N. Zitellini, M. Rovere, D. Accetella, A.
Ribeiro, J. Cabral, L. Matias, R. Bartolomé, M. Farrán, D. Casas, A. Maldonado, A. Pazos, D.
Córdoba, and X. Roset), 2003. Mapping active faults offshore Portugal (36ºN-38ºN): Implications for
seismic hazard assessment along the southwest Iberian margin. Geology 31, 83-86.
Martins, A. A. (1999): Caracterização Morfotectónica e Morfosedimentar da Bacia do Baixo Tejo
(Pliocénico e Quaternário). Dissertação de doutoramento, Universidade de Évora, Évora, 500 pp.
Martins, I., Mendes-Víctor, L. A., 1990. Contribuição para o Estudo da Sismicidade de Portugal
Continental. Instituto Geofísico Infante D. Luís, Publicação 18, University of Lisbon.
Rasmussen, Erik S., Lomholt, S., Anderson, C., Vejbaek, O. V., 1998. Aspects of the structural evolution
of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal. Tectonophysics
300, 199-225.
Ribeiro, A., Kullberg, M. C., Kullberg, J. C., Manupella, G., Phipps, S., 1990. A review of Alpine tectonics
in Portugal: Foreland detachment in basement and cover rocks. Tectonophysics 184, 357-366.
Teves-Costa, P., Rio, I., Marreiros, C., Ribeiro, R., Borges, J. F., 1999. Source parameters of old
earthquakes: Semi-automatic digitalization of analog records and seismic moment assessment.
Natural Hazards 19, 205-220.
Zitellini, N., Chierici F., Sartori, R., Torelli, L., 1999. The tectonic source of the 1755 Lisbon earthquake
and tsunami. Annali di Geofisica 42, 49-55.
Zitellini, N., Mendes Victor, L. A., Cordoba, D., Danobeitia, J., Nicolich, R., Pellis, G., Ribeiro, A., Sartori,
R., Torelli, L., Bartolome, R., Bortoluzzi, G., Calafato, A., Carrilho, F., Casoni, L., Chierici, F., Corela,
C., Correggiari, A., Della Vedova, B., Gracia, E., Jornet, P., Landuzzi, M., Ligi, M., Magagnoli, A.,
Marozzi, G., Matias, L., Penitenti, D., Rodriguez, P., Rovere, M., Terrinha, P., Vigliotti, Ln, Zahinos
Ruiz, A., 2001. The 1755 Lisbon earthquake and Tsunami: localization and investigation of the
tectonic source. EOS, Transactions, Am. Geophys. Union 82, 290-1.
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ACÇÕES ANTRÓPICAS NA BACIA HIDROGRÁFICA DO TEJO NA ÉPOCA CONTEMPORÂNEA: FLORESTAÇÃO, CORRECÇÃO TORRENCIAL E CONTROLO DE
CHEIAS
JOANA GASPAR DE FREITAS Faculdade de Letras de Lisboa/ R. Prof. Prado Coelho, n.º 40, 6.º Dto 1600-656 Lisboa
e-mail: [email protected]
Introdução «O Tejo é como um cavalo selvagem, indómito, impetuoso, cheio de vida, destruidor.
Domesticado, pode ser, na Península Ibérica, um servidor leal, paciente, utilitário, produtor do
bem-estar das populações ribeirinhas e valioso auxiliar nas economias nacionais» (Correia,
1967).
A história das acções antrópicas na bacia hidrográfica do Tejo nos últimos dois séculos
caracteriza-se pelas tentativas do homem para dominar o rio com o intuito de mitigar a acção
destruidora das grandes cheias e de explorar o potencial económico das suas águas, através
das múltiplas actividades que ali se podem desenvolver. As relações entre o homem e o rio
remontam a tempos pré-históricos, mas é sobretudo a partir do século XIX que se assiste a
uma intensificação do labor humano sobre a bacia hidrográfica do Tejo. Numa época em que
os progressos da ciência e da técnica permitiram o aumento significativo da capacidade de
intervenção e transformação do meio, os trabalhos levados a cabo tiveram reflexos profundos
na configuração do rio.
Resultados e Discussão O interesse das autoridades estatais pelo Tejo girou sempre em torno do problema das
cheias e da utilidade económica das suas águas. Até ao século XX, a riqueza deste rio estava
nas férteis lezírias das suas margens e na sua navegabilidade. Por isso, os projectos e
trabalhos feitos nesse período visavam sobretudo proteger os campos da acção das
inundações mais violentas, promover o enxugo e drenagem de paúis e garantir a navegação
até ao interior do país, permitindo a circulação de pessoas e bens entre a capital e as Beiras
(Gaspar, 1970; Azevedo, 2004). Depois, outras funcionalidades foram encontradas para as
águas do rio: a produção de energia hidro-eléctrica, a rega e o aproveitamento industrial e
turístico. A tecnologia disponível e uma maior capacidade financeira permitiram a realização de
grandes obras de engenharia, que vieram solucionar (em parte) a velha preocupação das
cheias.
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Vários estudos técnicos foram realizados com o objectivo de identificar as causas dos danos
provocados nos campos adjacentes e de apresentar soluções para minimizar os estragos
(Cabral, 1790; Relatório acerca da arborização, 1868; Loureiro, 1911). Já, em finais do século
XVIII, se considerava que a medida mais eficaz passava pela regularização do curso do rio,
impedindo o seu assoreamento e promovendo a fixação das margens (Cabral, 1790). Na
legislação de Oitocentos está presente a preocupação com a arborização das vertentes da
bacia hidrográfica para travar a sua erosão (Diário da Câmara dos Senhores Deputados, 19-
10-1844, 18-02-1881, 28-04-1887). A partir de 1901, a correcção torrencial passou a ser uma
das incumbências dos Serviços Florestais (Diário da Câmara dos Senhores Deputados, 31-12-
1901), trabalho cuja importância foi reconhecida pelo Estado Novo, que o incrementou através
do Plano de Povoamento Florestal (1939) e dos Planos de Fomento (Diário das Sessões da
Assembleia Nacional, 21-11-1952, 13-04-1958, 07-11-1867). Assim, foram identificados os
afluentes que mais contribuíam para o aumento do caudal sólido das cheias do Tejo e parte
dos seus trechos foi submetida ao regime florestal (Diário da Câmara dos Deputados, 17-04-
1919; Plano de Povoamento Florestal, 1939; Diário das Sessões da Assembleia Nacional, 08-
09-1958, 19-07-1965; Rego, 2001). Verifica-se que o maior esforço de arborização foi feito nas
bacias dos afluentes e não no rio principal, por se considerar que o regime de águas do Tejo
seria sempre determinado pelo que se fizesse em Espanha, sendo pois preferível investir nos
cursos de água que nasciam em Portugal (Plano de Povoamento, 1939, p. 159-160). Depois
dos anos 50, mesmo com a construção de barragens na bacia do Tejo e com a prossecução
das obras de correcção torrencial, a arborização continuou a ser fundamental para consolidar a
eficácia destes trabalhos (Andrada, 1982).
O problema das cheias não foi ainda hoje ultrapassado. Durante os séculos XIX e XX são
recorrentes as queixas dos proprietários ribeirinhos e é prolixa a legislação referente à
arborização e à reparação de diques e valas (Diário da Câmara dos Senhores Deputados, 03-
02-1823, 19-10-1944, 18-02-1881, 28-04-1887, 08-03-1902; Diário das Sessões da Assembleia
Nacional, 22-04-1938), o revela não só a dificuldade em fazer aplicar a lei, mas também a
incapacidade técnica para controlar as cheias (Diário das Sessões da Assembleia Nacional,
18-01-1956, 08-01-1964). A falta de meios pecuniários e a impossibilidade de prosseguir com
os trabalhos de forma racional, metódica e continuada, recorrendo-se sobretudo a intervenções
pontuais e de emergência, ajudam a explicar parte da questão (Diário das Sessões da
Assembleia Nacional, 11-01-1940; Diário da Assembleia da República, 12-03-1981).
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Conclusão Dado que a acção humana sobre a bacia hidrográfica do Tejo tem muitos séculos de história
documentada, conhecer de que forma e com que objectivos tentou o homem modificar as
características do rio e seus afluentes pode ser significativo para entender a dinâmica actual
desses cursos fluviais e determinante no planeamento de futuras intervenções.
Fontes e Bibliografia Andrada, Eduardo Campos, 1982. 80 anos de actividade de correcção torrencial – hidráulica florestal
(1901-1974), s.l., Direcção Geral das Florestas.
Azevêdo, M.ª Teresa, 2004. As mudanças de percurso do Tejo nos tempos modernos. Causas naturais e
antrópicas. Evolução geohistórica do litoral português e fenómenos correlativos. Actas do Colóquio,
Lisboa, Universidade Aberta.
Cabral, Estevão Dias, 1790. Memória sobre os danos causados pelo Tejo nas suas ribanceiras.
Memórias Económicas para o Adiantamento da Agricultura, das Artes e da Indústria em Portugal e
suas Conquistas, tomo II, Lisboa, Oficina da Academia das Ciências.
Correia, José de Araújo, 1967. O Tejo. Lisboa, Verbo.
Diário da Assembleia da República, 1976-1990
Diário da Câmara dos Deputados, 1911-1926
Diário da Câmara dos Senhores Deputados da Nação Portuguesa, 1820-1910
Diário do Governo, 1821-1970
Diário das Sessões da Assembleia Nacional, 1935-1974
Gaspar, Jorge, 1970. Os portos fluviais do Tejo. Separata da Revista Finisterra. Revista Portuguesa de
Geografia, vol. V-10, Lisboa.
Loureiro, Adolfo, 1911. “O rio Tejo e a sua navegação”, Trabalhos da Academia das Ciências de
Portugal, 1.ª série, tomo II, 1.ª parte, Lisboa, Academia das Ciências.
Plano de Povoamento Florestal, 1939. Relatório, proposta de lei, parecer da Câmara Corporativa e lei n.º
1971, publicada no «Diário do Governo», n.º 136, 1.ª série, de 15 de Junho de 1938. Lisboa,
Imprensa Nacional.
Rego, Francisco Castro, 2001. Florestas Públicas, s.l., Direcção-Geral das Florestas.
Relatório acerca da arborização geral do país apresentado a sua Ex.ª o Ministro das Obras Públicas,
Comércio e Indústria em resposta aos quesitos do artigo 1.º do decreto de 21 de Setembro de 1867,
1868. Lisboa, Imprensa Nacional.
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AS CHEIAS HISTÓRICAS NA REGIÃO DE ABRANTES
MÓNICA C. ALMEIDA1, TERESA M. AZEVÊDO2 & NUNO L. PIMENTEL2
1Mestrado C.T.V., Fac. Ciências da Univ. de Lisboa
e-mail: [email protected] e Dep. Geologia da Fac. Ciências da Univ. de Lisboa
e-mail: [email protected], [email protected]
Contexto regional As inundações do Tejo são quase tão antigas quanto o próprio rio, sendo a principal razão
da fertilidade e importância dos terrenos da lezíria, mas também a causa de destruição de
numerosos bens. O fenómeno ocorre com alguma frequência, devido às características da
bacia hidrográfica que tornam quase impossível confinar as águas do rio no seu leito ordinário,
principalmente durante os Invernos mais chuvosos (Ramos, 2000).
Durante dias, por vezes semanas, a área de Abrantes mais próxima do rio, cobre-se de
água, mágoa e inquietação, com várias povoações isoladas, onde apenas se chega de barco.
Isto ocorre nomeadamente devido à morfologia do vale, às características das litologias
aflorantes, e devido a certas acções do Homem. A montante de Abrantes os terrenos do
Maciço Antigo, rochas resistentes e impermeáveis, favorecem o escoamento superficial e, por
consequência, as inundações. Por outro lado, a crescente urbanização dos vales inundáveis na
região de Abrantes, que sempre constituíram um pólo de atracção para o desenvolvimento da
habitação, têm contribuído para a impermeabilização crescente do solo, que durante a
ocorrência de inundações leva a água a atingir níveis elevados. A inundação de povoações
como Rossio ao Sul do Tejo, na margem esquerda, e Rio de Moinhos, na margem direita, está
sobretudo relacionada com a baixa cota a que estas povoações se encontram, o que em
conjugação com a morfologia do vale, vai facultar o espraiamento das águas e a consequente
inundação. Outro aspecto de igual relevância para a ocorrência de cheias, é o assoreamento
do rio Tejo, que na região de Abrantes, ocorre devido, em grande parte, à desflorestação e/ou
alteração do coberto vegetal existente (Madeira, 2001).
O registo histórico documental das cheias na Região de Abrantes
É a partir do século XIX que começam a surgir com maior frequência os relatos, bem
documentados em jornais (locais e nacionais) e em revistas da época, sobre cheias e
catástrofes a elas associadas. São numerosas as notícias acerca das tumultuosas invasões do
rio Tejo na região de Abrantes, reportadas no jornal “Correio de Abrantes” (1941, 1978, 1979,
1989), desde 1884, de que se recordam algumas mais importantes.
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A cheia de 1876, conhecida como a maior cheia do Tejo de que há memória, registou na
escala hidrométrica do Rodão 25,40 m e atingiu quase 10000 m3/s de água em Tancos (fig. 1).
A repercussão da cheia em Ródão na região de Abrantes (cerca de 70 km a juzante) verifica-se
logo após 3 horas (Rodrigues et al., 2003).
Fig.1 – Cheia histórica de 1876, em Vila
Velha de Ródão e Tancos (4)
Já no séc. XX, na década de 40 (1940, 1941 e 1947) ocorreram várias inundações
dramáticas e que ocasionaram estragos consideráveis. A 2 de Fevereiro de 1941, a escala
hidrométrica do Rodão chegou aos 22,60 m e o Tejo subiu a pontos julgados inacessíveis,
como por exemplo, primeiros andares de casas do Rossio a Sul do Tejo. O Tejo teve então a
maior cheia que as gerações de então tinham visto, quase atingindo os níveis da de 1876.
Inundou, entre outros, o Rossio a Sul do Tejo (cota 32 m), a povoação de Rio de Moinhos
(35 m), a estação ferroviária de Abrantes (35 m), Arrifana (29 m), a estrada para o Tramagal
(30 m), a igreja e o cemitério de S. Miguel do Rio Torto (35 m), etc., para cotas de estiagem
próximas de 20 metros. A linha ferroviária ente Tramagal e Rossio a Sul do Tejo (30 m) esteve
também ela coberta de água, nas pontes sobre o Tejo os pilares ficaram cobertos quase até ao
tabuleiro, com mais de 1500 casas abandonadas precipitadamente numa área inundada
equivalente a 50 km2, “enfim, um horror.” A notícia faz, ainda, referência aos níveis de cheia
atingido em várias casas do Rossio.
Após alguns anos de perfeita acalmia, segundo os registos da época, em Março de 1978,
violentas tempestades originaram cheias de incalculáveis prejuízos em várias zonas da região.
No Rossio, estabelecimentos comerciais e habitações foram inundados, destruindo tudo o que
havia dentro, gado e aves foram arrastados pela corrente de cheia, conduzindo a prejuízos
elevados. Viveu-se uma completa desolação nas ruas do Rossio, onde a forte corrente das
águas arrancou calçadas, destruiu esgotos, derrubou muros, etc.
Apenas um ano depois, em Fevereiro de 1979, pluviosidade que ultrapassou o normal das
décadas anteriores, assolou de novo a região de Abrantes. O nível das águas foi subindo, até
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ultrapassar o máximo de 1941, que era o maior desde há longa data, provocando a maior cheia
do séc. XX. Desde Alferrarede até Constância, com passagem por Rio de Moinhos, “tudo foi
água” (fig. 2). Segundo as crónicas de então, estas cheias foram, em parte, também
provocadas pelas descargas da barragem de Alcântara em Espanha.
Fig. 2 - Cheia em Constância, 1979 (Soares,
M.M., 1995)
Fig. 3 – Cheia em Constância, 1989
(www2.snbpc.pt)
As chuvas intensas que se verificaram no mês de Dezembro de 1989, provocaram “como há
muito se temia”, enormes cheias no vale do Tejo. O Rossio ao Sul do Tejo e Rio de Moinhos
foram muito atingidos, alcançando o rio um nível próximo do de 1979. Também a vila de
Constância foi muito afectada pelas águas, tendo a sua parte mais baixa estado submersa
(fig. 3). Não houve vítimas a lamentar, mas os danos materiais foram colossais e muitas
famílias ficaram desalojadas.
Mais recentemente, a magnitude da cheia histórica de 1876 foi ultrapassada nos trágicos
episódios de Novembro de 1997, que provocaram a morte de 11 pessoas em Portugal
(Rodrigues et al., 2003).
Referências Bibliográficas Correio de Abrantes., 1941, 1978, 1979, 1989. Edição Minerva, LDA.
Madeira, M.C., 2001. Condições Geomorfológicas, Climáticas e Antrópicas das Inundações do Rio
Tejo em Abrantes. Diss. de Mestrado, Faculdade de Letras da Universidade de Coimbra, 104 p.
Ramos, C., 2000. Cheias e Impacte Ambiental. Seminário de Geologia do Ambiente, Departamento
de Ciências da Terra, Universidade do Minho, Braga. 2 p.
Rodrigues, R., Brandão C., Costa J.P., 2003. Breve Nota sobre as Cheias no Tejo e o seu Sistema de
Vigilância e Alerta. Direcção dos Serviços de Recursos Hídricos, Instituto da Água.
Soares, M.M., 1995. Navegando no Tejo. Comissão de Coordenação da Região de Lisboa e Vale do
Tejo, 161p.
http://www2.snbpc.pt/portal/page?_pageid=35,46761,35_46771:36_35022&_dad=portal&_schema=P
ORTAL
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
FLOODS IN THE CZECH REPUBLIC DURING THE PAST MILLENNIUM: DATA, FLUCTUATIONS, IMPACTS
RUDOLF BRÁZDIL Institute of Geography, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
e-mail: [email protected]
This work presents data on floods in the Czech Republic, based on historical documentary
evidence since the 12th century and measured hydrological data since the 19th century, in
order to detect long-term fluctuations of floods and their impacts. Floods in the Czech Republic
during the instrumental period were studied for the following rivers and hydrological stations: the
Elbe – Děčín (1851–2003), the Vltava – Prague (1825–2003), the Ohře – Louny (1884–2003),
the Odra – Bohumín (1896–2003) and the Morava – Kroměříž (1881–2003). Different data
sources were used for the study of floods in the pre-instrumental period: narrative written
sources, visual daily weather records, personal correspondence, special prints, official
economic records, newspapers, pictorial documentation, stall-keepers’ and market songs,
scientific papers and communications, and finally epigraphic sources (mainly flood water-
marks). The first credible report on a flood in Bohemia from sources of a narrative written
character relates to September 1118 and comes from the chronicle of Canon Kosmas of Prague
(Brázdil et al., 2005). In the period of systematic hydrological observations (i.e. since about the mid-19th century)
in the Czech Republic, the total number of floods, as well as their extremities expressed by the
N-year return period of maximum peak discharges, has been going down. This decline can be
ascribed above all to the reduction of frequencies of occurrence of floods of the winter synoptic
type (related to snow melting and ice damming, accompanied by rain), due to global warming,
through which, in consequence of the later onset of winters and lower accumulation of water in
snow cover, the number of floods has gone down, mainly in the months of February to April.
Documentary data on floods in the Czech Lands make it possible to extend the information
base about floods to far before the beginning of systematic hydrological observations, in fact
several centuries back. Work with this data requires, however, a critical analysis of the
employed sources, which is unfortunately lacking in most of the older papers in the Czech
Republic, as well as some of the newer, published overviews of historical floods. Critically
evaluated information can then be used to study the frequencies of occurrence for the floods,
their seasonality, synoptic typology and impacts. Through this, it becomes possible to document
disastrous floods in the Czech Lands for the last eight to nine centuries, with a relatively high
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
degree of reliability. Synthesis of floods based on instrumental data and documentary sources
shows long-term trends with a maximum of flood events during the 19th century and the second
part of the 16th century. From this analysis it is evident that, although several destructive floods
occurred in the 20th century, it can be considered as a very favourable hundred year period
(with the exception of the flood in July 1997).
The most disastrous well-documented floods affected the territory of the Czech Republic in
September 1118, January–February 1342, July 1432, August 1501, March and August 1598,
February 1655, June 1675, February 1784, February 1799, March 1845, February 1862, May
1872, September 1890, July 1897, July 1903, August–September 1938, July 1997 and August
2002. From the flood in May 1872 to the present, i.e. in the period subject to global warming,
they have been exclusively floods of the summer synoptic type (i.e. generated by continuous
heavy rains). The accumulation of floods of the winter synoptic type in February 1784, February
1799, March 1845 and February 1862 falls, from the climatological point of view, into the last
part of what is known as the Little Ice Age.
Information available until now, has not yet made it possible to give a definite answer to the
question of “to what extent the most recent floods – 1997, 1998, 2002 and 2006, which came
after a relatively very quiet period – are a possible consequence of the anthropogenically
conditioned process of global warming”, and “whether in the near coming years similar disasters
will occur with the same or greater intensity and frequency”.
Flood events today, in comparison with historical floods of the same extremity, have more
disastrous consequences than in the past, a fact closely linked to the increasingly more
complex infrastructure of human society and with a growing degree of anthropogenic
remodelling of the cultural landscape. Floods, as in the past, will also rise in the future whenever
meteorological, physico-geographical and anthropogenic conditions are favourable to them. The
problems of anti-flood protection may become still more pressing in the future than at present, if
assumptions of continuing anthropogenically-conditioned climatic change projected into more
marked fluctuation of the runoff process and flood activity are confirmed.
Acknowledgement This study was financially supported by the research project MŠM0021622412
(INCHEMBIOL).
References Brázdil, R., Dobrovolný, P., Elleder, L., Kakos, V., Kotyza, O., Květoň, V., Macková, J., Müller, M., Štekl,
J., Tolasz, R., Valášek, H. (2005): Historical and Recent Floods in the Czech Republic. Masaryk
University, Czech Hydrometeorological Institute, Brno, Praha, 370 pp.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
THE TAGUS RIVER AND ITS HISTORICAL FLOODS (SANTARÉM, PORTUGAL)
TERESA M. AZEVÊDO1, ELISABETE NUNES1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, CONCEIÇÃO FREITAS1,
CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3
1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016
Lisboa, Portugal
e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal
e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
e-mail: [email protected]
Since written information exists, river floods were recorded in local and regional archives in
the entire world. These are specially constituted by documents in churches about pilgrimages to
ask for rain, people perished in floods, chronicles of the kings, documents in libraries,
autarchies, old magazines, newspapers, etc. For thousands of years ago, countries like Egypt,
have been keeping records from the Nile floods. Other countries started the records too late or
lost the older ones due to natural catastrophes, e.g., fires or wars.
This is the case of Portugal. Being the Tagus one of the biggest Iberian rivers, the
information about historical floods was expected to be abundant. However, most of the old
archives were lost during invasions, fires, wars and the big earthquake of 1755. Therefore,
because of these reasons, there are so few records about Tagus floods.
The Tagus floods were however referred in many old historical texts mainly due to the very
good nature of the lands left back by the floods. Several Arabic references and even the
description presented by the English crusader Osberno, engaged for the Lisbon request to the
Moors in 1147, tell us the magnificent properties of the Tagus lands at those times.
The research about historical floods is based on cartographic documents, old written
documents, newspapers, magazines and photos. The field survey and the data gauge are the
main documents used for the study of present floods.
One of the goals of this historical research was to establish a flood magnitude scale
(Azevêdo et al., 2004) and a flood risk map using data from the documents analysed (Nunes,
2001). Another important component was the comparative cartographic analysis of all the maps
since 1866 to the present day, at four different scales.
An example of the research is the artificial change of the Tagus channel, described in the
documents archived in “Torre do Tombo”, which gave place to several papers (Dias, 1984;
Azevêdo & Nunes, 2000, Azevêdo, 2001, Azevêdo et al., 2003, Azevêdo et al., 2004). This
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
amazing case is the one in which the Prince D. Luis, brother to the king of Portugal D. João III,
requests permission to change the Tagus’ course between Tancos and Chamusca, due to the
river sand deposition that every flood ruined its cultivations causing big damages.
The Tagus was then pushed 1 km to the north in a 10 km long
section and a new, perfectly straight channel was opened (fig. 1).
However, the river was not stable in this new course. It began to
meander and eroded an area of 30 km2 in only 15 years, causing great
damages, now to the Order of Christ monks, located in Quinta da
Cardiga. Nowadays, it is visible that the Tagus’ main channel runs right
next to the walls surrounding the farm.
Fig. 1 – Old scroll
showing the 3 different
courses of the Tagus
in the XVIIIth Century
(in A.N.T.T, undated).
Another major change to the Tagus’ main channel was ordered by
king D. João V two centuries later, in the downstream segment between
Valada and Vila Franca de Xira. His intention was to improve the river’s
navigability and so the channel was once again shifted, widened,
deepened and the existing five channels were joined together forming
the present-day one.
Other important sources of information are inquiries to the population, inscriptions and flood
marks. Floss can also be used to estimate water levels from recent floods when there aren’t any
other elements available. Inscriptions and marks can be preserved for centuries on gates,
bridges, and houses or can be expressly built to record a particular flood. The old and
centennial flood marks drawn on the outside walls of the houses, where the inhabitants wrote
the water levels and dates, have recently been substituted by glazed tiles only with levels,
having all the dates disappeared. This is an excellent example of the degree of knowledge of
the people directing water governmental institutions.
However, the old flood marks face a great problem, that is, they have to be renovated
every now and then (the case of the renovated marks in Reguengo do Alviela) and by doing
so, we can never be 100% certain if they are still representing the original position.
One of the issues with flood marks is its meaning.
(i) Taking into account that the big Tagus floods take place when the river waters surpass 7
m high in Santarém bridge, 50 large floods were registered (Almeida D’Eça, in Sobrinho, 1980)
between 1852 and 1996. In the same period, the flood marks surveyed in 20 places in the
floodplain, registered 19 floods. Hence, the flood marks’ method gives a trial of 38% of the total
big floods that occurred in the Tagus floodplain.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
(ii) The flood marks evaluation shows great flood
events irregularity, with decades of no big floods and
others with 2, 3 or 4 large floods (table 1).
(iii) Flood marks also show that the months with a
higher flood risk are December, January, February
and March (table 1).
(iv) Along the studied period, the highest
magnitude flood event occurred in February 1979. If
the two places (of the 20) with more flood marks
events registered (Quinta do Monteiro e Reguengo do
Alviela) are considered, it can be seen that the flood
marks’ method gives that information precisely.
(v) Finally, the flood marks of the February 1979
event are the best represented along the floodplain (in
18 of the 20 places).
Another aspect related to floods frequency is its
influence on regional architecture: the riverside
villages of Caneiras and Aldeia da Palhota are very
curious examples of how the local inhabitants are
familiarised with floods. Their houses are pile dwelling buildings, built circa 5 m above the
normal water level.
Table 1 - Tagus floods referenced by flood marks
Year Month Day Number of Places
1816 12 6 1
1895 2 17 17
1896 12 7 1
1909 12 23 2
1912 1
1919 1
1936 3 16 2
1940 1 3 3
1941 1 25-27 5
1946 1 25 2
1947 3 5 1
1963 1
1964 1
1969 1
1970 1 13 1
1978 3 4 5
1979 2 1-11 18
1986 2 19 1
1989 12 23-27 3
1996 1 09 1
1999 2 1
2001 1
Bibliography A.N.T.T., undated. Conventos de Tomar, Ordem de Cristo, Maço 30 (vermelho), nos 1, 2 e 4, fl. 21.
Azevêdo, M. Teresa M., 2001. A utilização dos dados históricos no estudo das cheias do Tejo. Estudos
do Quaternário, 4, APEQ; 69-77. Porto.
Azevêdo, T.M. & Nunes, E.V., 2000. A acção antrópica na mudança da tipologia do Tejo ao longo do
tempo. Actas. In: II Jornadas do Quaternário da APEQ. Porto.
Azevêdo, T.M.; Nunes, E.; Ramos, C. & Ramos Pereira, A., 2003. Historical floods of the Tagus River in
the Santarém region, Portugal. Abstracts. Ciências da Terra, Vol. Especial, Nº V. VI Congresso
Nacional de Geologia, Univ. Nova Lisboa, Costa da Caparica, p. 92 (CD-Rom Ed., p. H10-H13).
Azevêdo, T.M.; Nunes, E. & Ramos, C., 2004. Some morphological aspects and hydrological
characterization of the Tagus floods in the Santarém region, Portugal. Natural Hazards, 31. Kluwer
Academic Publishers, Netherlands. p. 587-601.
Nunes, E., 2001. Estudo Multidisciplinar da Variabilidade Temporal e Espacial do Tejo na Região de
Santarém. Tese de mestrado (não publ.). Univ. de Lisboa, 209 pp.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Dias, J.J. Alves, 1984. Uma grande obra de engenharia em meados do século XVI. A mudança do rio
Tejo. Rev. Nova História, Ed. Estampa, Lisboa.
Pereira, A. Ramos; Ramos, C.; Azevêdo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C. & Pereira, D.I.
2006. Geomorphological assessment of the Middle Tagus alluvial plain. Abstracts. Tagus Floods ’06
Workshop, Lisbon, Portugal.
Sobrinho, A.S., 1980. Os temporais de Fevereiro de 1979 no Ribatejo e região de Lisboa. Finisterra, vol.
XV, 29, 85-93.
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PALAEOENVIRONMENTS OF THE TAGUS VALLEY DURING THE LAST 15 KA: SEDIMENTOLOGICAL, PALYNOLOGICAL AND MICROMORPHOLOGICAL EVIDENCE OF
THE ENTRE VALAS SEV CORING (SANTARÉM, PORTUGAL)
M. TERESA AZEVÊDO1, S. FAVARETTO2, A. MIOLA2, PAOLO MOZZI3, C. NICOSIA4, ELISABETE NUNES1 &
I. SOSTIZZO2
1 Centro de Geologia, Universidade de Lisboa, Edifício C6, 3º Piso; Campo Grande, 1749-016 Lisboa, Portugal
e-mail: [email protected], [email protected] Dipartimento di Biologia, Università degli Studi di Padova, via U. Bassi 58/B, 35121 Padova, Italy
e-mail: [email protected], [email protected], [email protected] Dipartimento di Geografia, Università degli Studi di Padova, via del Santo 26, 35123 Padova, Italy
e-mail: [email protected] Department of Geology and Soil Science, University of Gent, Krijgslaan 281 S8 9000 Gent, Belgium
The 20 m deep coring object of this study was drilled in the Tagus River alluvial plain in
Entre Valas, few kilometres upstream of Santarem on the northern side of the valley, less than 1
km from the valley margin (SEV is the acronym for the site Santarém-Entre Valas). The
lowermost portion between 20 and ca. 15 m is mainly sandy, locally with pebbles. The
sediments above are mostly silty-clay with an increase in sand in the top 3 m. The radiocarbon
dating of carbonaceous organic material (peat, wood) at different depths gave the following
ages: 18,109 cal BP at 16.7 m; 11,219 cal BP at 14.7 m; 8097 cal BP at 12.3 m; 6960 cal BP at
10.7 m; 3550 cal BP at 6.5 m; 3086 cal BP at 4.5 m; 816 cal BP at 1.0 m. Grain size analyses
have been performed in the whole coring, with sample spacing of some decimetres. Pollen and
non-pollen-palinomorph (NPP) analyses have been carried out on 22 samples from different
depths between 15.5 and 3.9 m. Pollen and spore identification was based on Moore et al.
(1991), Reille (1992, 1998) and the modern reference collection at the Department of Biology,
University of Padova. NPPs identification and nomenclature follow van Geel (1978), van Geel et
al. (1980-81), van Geel et al. (1983), van Geel et al., (1989). Micromorphology has been carried
on 11 samples in the 13.5–3.0 m interval. Thin section description follows Bullock et al. (1985)
and Stoops (2003).
Relatively coarse, fluvial channel sediments were deposited during the Late Glacial, followed
by finer overbank deposits of Holocene age. Some macroscopic evidence of pedogenesis have
been described in the coring at depths of ca. 3-4 m, but no specific indicators of pedogenesis
have been observed in thin section. The abundant calcite nodules are generally ascribed to the
gradual infilling of microvoids under vadose circumstances and are not necessarily related to
soil forming processes. At the bottom of the coring (15.5-13.7 m) palynomorphs are absent, in
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the middle part (13.7-10.0 m) there is the highest palynomorph concentration and at the top
(10.0-3.9 m) there is a poor concentration and the worst preservation of the pollen grains.
Crossing the palynological and micromorphological data, the Holocene sedimentary series
corresponding to the top 15 m of the coring can be split in two main sequences, with the
boundary at a depth of ca. 8 m. The presence of Quercus ilex, Juniperus type and Arbutus
pollen types, together with some pollen grains of Vitis vinifera and Olea europea, shows that the
lower sequence formed under mediterranean climatic conditions. The aquatic plant community
was apparently dominated by Isoetes, indicative of freshwater condition and of shallow ponds
which seasonally dried out. The presence of foraminifera, observed during pollen analysis,
indicates possible inputs of marine water. The abundant pyrite, often oxidized to form sulphuric
minerals such as jarosite in association with gypsum, observed in thin section, may also be
explained in terms of an occasionally brackish underground water table. Even though the
vegetation was not that of a coastal salt marsh, the backswamp ponds in the floodplain could
probably receive marine water. Such events were presumably related to the interplay between
the floodings of the Tagus and the remounting tides. Sea surges and consequent spillout of sea
water in the plain may have happened during storms with strong western winds from the Atlantic
Ocean and/or exceptionally high tides. These processes could take place in the floodplain of a
deltaic/estuarine system.
Pollen analysis suggests that mediterranean climatic conditions persisted in the upper
sequence. It also gives some indication of deforestation and heathland expansion which, in the
region, are known to start significantly from the Bronze Age onwards. Similarly,
micromorphology indicates the presence of soil relicts of former topsoil horizons, which have
been eroded and re-deposited, at a depth of 3.5 m. The groundmass also contains finely
comminuted charcoal and charred organic matter fragments. The existence of a phase of
topsoil erosion, which could be man- or natural fire-induced, should be carefully taken into
account. Sedimentation took place by overbank deposition of the Tagus River, with no evidence
of coastal influx. Further hypotheses on vegetation development and local environments for this
part of the sequence cannot be formulated because the pollen grains were generally
deteriorated.
The transition from a deltaic/estuarine situation to an alluvial plain indicates a more inland
position of the coastline in the Middle Holocene, followed by a regression. The maximum
transgression was reached at the culmination of the post-glacial eustatic sea-level rise, while
the regression has been due to the basinward progradation of the alluvial wedge of the Tagus
fluvial system during the Upper Holocene sea highstand. The palaeoenvironmental evolution of
the Tagus valley during the last 15 ka may serve as an analogue of the sedimentary response
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of the Tagus system to past glacio-eustatic sea level rise. This may help to better understand
the Pleistocene sequences of the Tagus alluvial terraces which are present on the southern
side of the valley (Zbyszewski, 1946, 1958; Mozzi et al., 2000).
References Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., 1985. Handbook for soil thin section
description. Waine Research, Wolverhampton, United Kingdom.
Faegri, K., Iversen, J., 1989. Textbook of pollen analysis, IV Ed., John Wiley and Sons, London.
Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis. II Ed., Blackwell. Oxford, Great Britain.
Mozzi, P., Azevedo, M.T., Nunes, E., Raposo, L., 2000. Middle terrace deposits of the Tagus river in
Alpiarça, Portugal, in relation to early human occupation. Quaternary Research, 54 (3).
Reille, M., 1992. Pollen et Spores d’Europe et d’Afrique du Nord. Laboratoire de Botanique Historique et
Palynologie. Marseille, France.
Reille, M., 1998. Pollen et Spores d’Europe et d’Afrique du Nord. Supplement 2. Laboratoire de
Botanique Historique et Palynologie. Marseille, France.
Stoops, G., 2003. Guidelines for analysis and description of soil and regolith thin sections. Soil Science
Society of America, Inc. Madison WI, U.S.A.
van Geel, B., 1978. A palaeoecological study of Holocene peat bog sections in Germany and the
Netherlands, based on the analysis of pollen, spores and macro- and microscopic remains of fungi,
algae, cormophytes and animals. Rev. Palaeobot. Palynol., 25, 1-120.
van Geel, B., Bohncke, S.J.P., Dee, H., 1980-1981. A palaeoecological study od Upper Late Glacial and
Holocene sequence from “De Borchert”, The Netherlands. Rev. Palaeobot. Palynol. 31, 367-448.
van Geel, B., Hallewas, D.P., Pals, J.P., 1983. A Late Holocene deposit under the Westfriese Zeedijk
near Enkhuizen (Prov. of N-Holland, The Netherlands): palaeoecological and archaeological aspects.
Rev. Palaeobot. Palynol. 38, 269-335.
van Geel, B., Coope, G.R., van der Hammen, T., 1989. Palaeoecology and stratigraphy of the Lateglacial
type section at Usselo (The Netherlands). Rev. Palaeobot. Palynol. 60, 25-129.
Zbyszewski, G., 1946. Etude géologique de la region d'Alpiarça. Comun. Serv. Geol. Portugal, XXVII, 145-268.
Zbyszewski, G., 1958. Le Quaternaire de Portugal. Boletim da Sociedade Geològica de Portugal, 12, 1-
227.
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SISTEMAS METEOROLÓGICOS DE PRECIPITAÇÃO E AS CHEIAS NAS BACIAS HIDROGRÁFICAS: RIO TEJO
ILDA Mª SANFINS NOVO VILLA SIMÕES Instituto de Meteorologia, Rua C do Aeroporto, 1700 Lisboa
e-mail: [email protected]
Em Portugal Continental, as cheias nas bacias hidrográficas estão associadas às situações
meteorológicas de fluxo perturbado de oeste persistente que origina períodos prolongados de
precipitação e que geralmente afectam quase todas as bacias hidrográficas do território.
Sistemas depressionários e passagem de sistemas frontais de forte actividade, podem originar,
também, situações de cheias em uma ou mais bacias hidrográficas.
Apresentam-se algumas situações meteorológicas que exemplificam os principais sistemas
meteorológicos de precipitação e que foram casos de cheias na bacia hidrográfica do rio Tejo.
Refere-se a importância da Vigilância Meteorológica e da Previsão do Tempo a curto e
médio prazo na previsão de cheias e no apoio à tomada de decisão em situação de cheias
assim como a importância das previsões a longo prazo (10 dias, mensal e trimestral) na gestão
dos recursos hídrícos.
Referências Bibliográficas Godinho, S. Contribuição para o Estudo da Cheia da região de Lisboa de 25-26 de Novembro de 1967.
Instituo de Meteorologia, Lisboa.
Godinho, S. Aspectos Meteorológicos das Inundações de Novembro de 1983. Instituto de Meteorologia,
Lisboa.
Malheiro, A.C., Ferreira, I.B., Freire, J.A., 1984. Análise à Escala Sinóptica da Situação Meteorológica
que condicionou o estado do tempo em Portugal Continental nos dias 18 a 21 de Novembro de 1983.
Instituto de Meteorologia, Lisboa.
Meteorological Office College, 1997. Source Book of the Forecaster's Reference Book. The
Meteorological Office, Bracknell.
Miranda, P.M.A., 2001. Meteorologia e Ambiente: Fundamentos de Meteorologia, Clima e Ambiente
Atmosaférico. Universidade Aberta, Portugal.
Condições Meteorológicas nos meses: Dezembro de 2000, Janeiro, Fevereiro, Março, Novembro e
Dezembro de 2001, Janeiro, Feveiro, Março, Novembro e Dezembro de 2002. Publicação mensal do
Instituto de Meteorologia, Departamento de Vigilância Meteorológica, Divisão da Análise e Previsão
do Tempo.
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POSSIBILIDADES DE APLICAÇÃO DA LUMINESCÊNCIA OPTICAMENTE ESTIMULADA (OSL) NA DATAÇÃO DE SEDIMENTOS DO RIO TEJO
ANTÓNIO ANTUNES MARTINS1 & PEDRO PROENÇA E CUNHA2
1Depart. de Geociências da Univ. Évora, Rua Romão Ramalho, 59, 7002-554 Évora
e-mail: [email protected] de Geociências, Depart. de Ciências da Terra da Univ. Coimbra, 3000-272 Coimbra
e-mail: [email protected]
Nas técnicas de datação de sedimentos por luminescência, a idade de uma amostra obtém-
se, no essencial, dividindo a paleodose pela taxa de radiação ambiental (dose-rate). A
paleodose é a energia (electrões) acumulada em defeitos (traps) do edifício cristalino de
minerais, desde a última vez em que estes foram expostos à luz solar. Os minerais
normalmente utilizados na OSL são o quartzo (Qz) e o feldspato potássico (Fk). A exposição à
luz descarrega a energia latente anterior, processo designado por bleaching. Nessa altura é
como se o “relógio” da contagem do tempo fosse colocado a zero. Logo que os grãos são
enterrados começam a receber radiação ionizante proveniente de elementos radioactivos (ex.
T, U e 40K) existentes nos sedimentos e também a dos raios cósmicos. A energia acumulada
pode ser medida em laboratório (obtendo-se a dose equivalente - De). É portanto crucial que a
energia latente da amostra de sedimento seja colocada próximo de zero antes deste ser
enterrado; caso contrário, a idade da amostra ficará sobrestimada. A sobrestimação da idade
resultante do incompleto bleaching pode ser significativa em sedimentos do Holocénico, mas
tem pouca relevância em sedimentos do Plistocénico, pois nestes a energia presente na altura
do enterramento é muito pequena relativamente à acumulada após a última exposição à luz.
O sinal OSL resultante da estimulação pela luz é mais facilmente “limpo” (bleached) do que
o sinal luminescente resultante da estimulação pela temperatura (termoluminescência - TL),
sendo essa uma das principais vantagens da OSL relativamente à TL. Na medição de grãos de
Qz usando o protocolo SAR (Murray & Wintle, 2000), a OSL permite datar sedimentos até
cerca de 150.000 anos, caso a taxa de radiação ambiental não seja elevada; idades da ordem
da dezena de anos podem ser obtidas usando técnicas apropriadas (Huntley & Lian, 1999;
Wallinga, 2002). A principal limitação do quartzo nas datações por OSL é a saturação dos
grãos que ocorre à volta de 300 Gray (Gy)1. Arenitos e lutitos do Holocénico podem ser
também datados com precisão por esta técnica e, consequentemente, servir para determinar a
crononogia de eventos geológicos recentes, tais como as cheias fluviais. O cálculo de taxas de 1 Gray é uma unidade no Sistema Internacional de medidas, que representa a quantidade de energia de radiação ionizante absorvida (ou dose) por unidade de massa. 1Gy = 1 J/kg
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
sedimentação pode também ser efectuado, desde que se disponha de uma coluna
estratigráfica com suficiente constrangimento cronológico. Também se podem usar sedimentos
de idade já conhecida (ex: depósitos do tsunami de 1755 ou da cheia de 1909 do rio Tejo) para
aferir o processo de medição laboratorial ou determinar a importância do incompleto bleaching
nos ambientes com transporte aquoso de elevada turbulência. No processo de averiguação do
incompleto bleaching, normalmente reduz-se o número de grãos em cada aliquot para evitar
valores médios, chegando-se a fazer medições grão a grão da De. Uma distribuição muito
heterogénea da De ou a existência de alguns aliqutos com valores muito superiores aos
valores mais frequentes da população são indicadores de incompleto bleaching (Murray et al.,
2002). A presença de sinal latente anterior à deposição tem de ser tomada em conta no cálculo
das idades dos depósitos modernos.
Como exemplo, apresentam-se duas medições da De em grãos de Qz, feitas em
sedimentos holocénicos da planície aluvial do Tejo, na depressão de Montalvo. O enchimento
holocénico tem aqui 9 m de espessura visível. A amostra 052239 foi colhida a 1m do topo do
enchimento aluvial e a amostra 052238 4 m abaixo daquela. Relativamente à amostra 052238
(Fig. 1), parece óbvio que os aliquots com dose equivalente superior a 1 Gy correspondem a
situações de incompleto bleaching. Na amostra 052239 existem muito menos aliquots com
incompleto bleaching (apenas dois com 1,3 Gy). Nas duas amostras, aliquots com mais de 1
Gy muito provavelmente devem ser portadores de energia latente, anterior ao último
enterramento do sedimento; por isso, não foram considerados no cálculo das idades.
Obtiveram-se valores de 117 e 114 anos, que concordam com a posição estratigráfica das
amostras, mas com um intervalo de tempo demasiado curto para explicar os 4 m de espessura
de aluviões que separam as duas amostras. Os valores da radiação ambiental revelaram-se
altos (3,44 e 4,03 Gy/ano), mas não são impeditivos na determinação das idades destes
sedimentos modernos.
Casal Montalvo 052238 QZ
0
5
10
15
20
25
0,0 0,3 0,5 0,8 1,0 1,3 1,5 1,8 2,0 2,3 2,5 2,8 3,0 3,3 3,5 3,8 4,0 4,3 4,5 4,8 5,0 5,3 5,5
Gy
Média= 0,79 ± 0,13 Gy σ = 0,98n = 61
Casal de Montalvo 052239 QZ
0
5
10
15
20
25
0,0 0,3 0,5 0,8 1,0 1,3 1,5 1,8 2,0 2,3 2,5
Gy
M édia = 0,51± 0,03 Gyσ = 0,19n = 32
Fig. 1 – Histogramas relativos aos valores de De medidas em aliquots com grãos de Qz de duas
amostras da planície aluvial do médio Tejo português, do alvéolo tectónico de Montalvo. Na amostra
052238 foram medidos 61 aliquots e na 052239 medidos 32.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Sedimentos do Holocénico podem ser datados utilizando o Qz, mas o incompleto bleaching
poderá ter um peso relativo importante no cálculo da De, sobrestimando as idades. As
datações OSL em Qz, apesar de precisas, revelaram-se praticamente inviáveis para datar
terraços do rio Tejo, cujos sedimentos possuem altas doses de radiação ambiental (geralmente
3 a 7 Gy/ka). Julga-se que as elevadas taxas de radiação ambiental possam estar relacionadas
com substratos graníticos existentes na bacia hidrográfica do rio Tejo, quer do lado espanhol,
quer do lado português. Aponta-se como exemplo o batólito de Nisa, com concentrações de
Urânio. Para os terraços, a datação OSL em Fk surge como alternativa; com este mineral pode
datar-se até 1 milhão de anos (Wallinga, 2002). Contudo, esta metodologia é mais complexa,
morosa e as idades obtidas precisam de ser corrigidas relativamente à perda de energia
(anomalous fading) que ocorre nos grãos de Fk ao longo do tempo (Wintle, 1973) e mudanças
de sensibilidade nos defeitos do cristal (Wallinga, 2002). Os dois fenómenos são ainda mal
compreendidos e as correcções são problemáticas. Apesar destas dificuldades, foi possível
obter datações OSL em Kf nos três níveis inferiores terraços do rio Tejo (sectores de Ródão e
Abrantes). Aqui existem cinco níveis geomorfológicos de terraços tendo-se alcançando idades
mínimas da ordem dos 200 mil anos para o terceiro nível de terraço (Martins & Cunha, 2006;
Cunha et al., submetido para publ.). Devido às características dos sedimentos não foi possível
obter qualquer idade, para os dois níveis superiores de terraço.
Projecto POCTI/CTE-GEX/58120/2004, da Fundação para a Ciência e a Tecnologia e co-financiado pelo FEDER
Referências bibliográficas: Cunha, P. P., Martins A. A., Huot, S., Murray A., Raposo, L., submetido para publ. Dating the Tejo River
lower terraces (Ródão, Portugal) to assess the role of tectonics and uplift. In: Impact of Active
Tectonics and Uplift on Fluvial Landscapes and River Valley Development (Eds P.G. Silva, F.A.
Audemard & A.E. Mather). Geomorphology, Special Issue).
Huntley, D.J., Lian, O.B., 1999. Using optical dating to determine when a sediment was last exposed to
sunlight. in: D.S. Lemmen and R.E. Vance (eds.), Holocene Climate and Environmental Change in the
Palliser Triangle: A Geoscientific Context for Evaluating the Impacts of Climate Change on the
Southern Canadian Prairies: Geological Survey of Canada Bulletin 534, 211-222.
Murray, A. S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of
sedimentary quartz: a status review. Geochronometria 21, 1-16.
Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot
regenerative-dose protocol. Radiaton Measurements 32: 57-73.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Martins, A. A. & Cunha, P.P., 2006. Vantagens e limitações da luminescência opticamente estimulada na
datação de terraços do rio Tejo, sua importância na determinação da taxa de incisão fluvial. VII
Congresso Nacional de Geologia (Resumos), Univ. Évora.
Wallinga, J. 2002. Optically stimulated luminescence dating of fluvial deposits: a review. Boreas, 303-322.
Wintle, A.G. 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143-144.
75
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
GEO-ARCHAEOLOGICAL EVIDENCE OF DESTRUCTIVE FLOODS DURING THE SETTLEMENT OF FAIAL (AZORES)
JOSÉ MADEIRA & ANTÓNIO BRUM DA SILVEIRA Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, LATTEX – Laboratório de
Tectonofísica e Tectónica Experimental e IDL – Instituto D. Luís
Edifício C6, Campo Grande, 1749-016 Lisboa, PORTUGAL
e-mail: [email protected], [email protected]
Four different regions characterize the morphology of the island of Faial, in the Azores
archipelago: the “Caldeira Volcano”, the “Graben of Pedro Miguel”, the “Horta-Flamengos
Platform”, and the “Capelo Peninsula” (Fig. 1A; Madeira, 1998).
The Horta-Flamengos Platform is located on the SE of Faial, around the town of Horta, and
includes the localities of Feteira, Granja, Flamengos, Farrobo and Conceição. It is a low-lying
region crossed by the Flamengos creek that drains part of the upper southeast slope of the
central volcano. The altitudes at which this flat area stands range from sea level to
approximately 140 m, with scattered scoria cones rising to altitudes up to 257 m.
Around Flamengos the region is almost horizontal (< 1º). This was a former depression
limited by the slopes of the Caldeira Volcano, to the west, the reverse of the Espalamaca fault
scarp (Graben of Pedro Miguel), to the north, and closed to the south and east by the
construction of a set of late Pleistocene scoria cones (Lameiro Grande, Quinta de S. Lourenço
and Conceição); this depression was gradually filled by lava flows, Holocene plinian and
phreato-plinian pyroclastic deposits from the Caldeira Volcano, and alluvium. Thus, the flat area
of Flamengos is an aggradation surface, ranging in altitude from 140 m to around 85 m, in
which 2 (or 3) river terraces can be recognised.
The alluvial deposits did not outcrop until 1992, when two roads were enlarged and
straightened.
In Cruz do Bravo, the new road cuts revealed a thickness of at least 2 m of alluvial deposits
below the present soil. Near by, the terrace surface, at an altitude of 140 m, drops to a lower
terrace level at altitudes of 125-120 m.
Entering Flamengos from Quinta de S. Lourenço, a road cut exposed a sequence of deposits
from the lower terrace of Flamengos creek. The succession is composed of two alluvial deposits
separated by a pyroclastic intercalation, and covered by a flood deposit (Fig. 1B):
1 – alluvial deposit (> 2 m), composed of lenses of well-rounded pebbles, millimetre to
decimetre size, without matrix, showing imbrication, intercalated in sand containing small yellow
pumice clasts, and presenting horizontal or cross-stratification. A soil (40 cm) developed on this
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
unit; 2 – Phreato-magmatic pyroclastic deposit (85 cm), resting on a erosive surface; it is
composed of stratified grey trachyte pumice and ash, forming centimetre thick layers; 3 –alluvial
deposit (80 cm), similar to level 1, but presenting a fresher aspect; 4 – flood deposit, erosive on
the underlying units; it is a clay matrix massive deposit containing scattered pebbles and
cobbles, bits of coal, bones and teeth of mammals, fragments of tiles and pavement bricks,
earthenware (bowls and pots) and glazed pottery; 5 – present soil (40 cm).
Fig. 1: (A) – Digital terrain model for Faial, with the indication of phisiographic regions and localities
referred in the text; CV – Caldeira Volcano; GPM – Graben of Pedro Miguel; CP – Capelo Peninsula;
HFP – Horta-Flamengos Platform; 1 – Horta; 2 – Conceição; 3 – Farrobo; 4 – Quinta de S. Lourenço; 5 –
Flamengos; 6 – Cruz do Bravo; 7 – Granja; 8 – Feteira; 9 – Porto Pim; 10 – Praia do Almoxarife. (B) –
Log of the Flamengos section: 1 and 3 – river terraces; 2 – phreato-plinian pyroclastic layer; 4 – flood
deposit. Rectangles indicate samples dated by radiocarbon.
The presence of remains of mammals alone would indicate an event post dating 1432, when
cattle was introduced in the islands in order to reproduce and help the survival of the first
settlers; the association with pottery indicates a post settlement (circa 1466) occurrence.
However, even the oldest of the Flamengos inhabitants had no memory, not even from oral
tradition, of floods reaching the village altitude; effectively the Flamengos creek incised a narrow
gorge, some 5 metres deep, within which there are some old houses.
To find references to such destroying events one must search in the ancient chronicles,
where descriptions of destructive floods exist. Monte Alverne (circa 1700) refers that after the
initial settlement in Praia do Almoxarife, where they were subject to the attack of pirates and
privateers, the settlers moved to another place inland. This place was later called Flamengos
because there were many Flemish among the first settlers. The description mentions, however,
that they were frustrated because all they built was destroyed by the floods of the creek, fact
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
that made them move once more to the place of Porto Pim, which would became the nucleus of
the present city of Horta.
Radiocarbon dating of the paleosol developed on level 1 and coal fragments from the
inundation deposit yielded conventional radiocarbon ages of 1860 ± 70 BP (sample FA1A;
Madeira et al., 1995) and 380 ± 80 BP (sample FA1B), respectively. Calibration by
dendrochronology of the latter indicates a central date of 1480 cal AD (with the following
intervals: 1440-1641 cal AD within a 68% probability and 1409-1666 cal AD within a 95%
probability). Considering that the settlement of Faial begun in 1466 the radiocarbon dates are
convincing and agree with flood descriptions during the initial stages of settlement.
Another deposit of this type, which also contained ancient glass and pottery, was observed
on the south slope of Monte das Moças, in Horta.
These facts have implications on the assessment of natural hazards in the area of Horta and
Flamengos, and show that although large floods did not occur in recent times, the potential for
destructive events exists and must be considered.
ACKNOWLEDGEMENTS This study is a contribution from research project GEODYN (LATTEX, POCTI-ISFL-5-32).
REFERENCES Madeira, J., 1998. Estudos de Neotectónica nas ilhas do Faial, Pico e S. Jorge: uma contribuição para o
conhecimento geodinâmico da junção tripla dos Açores. PhD Thesis, Lisbon University: 481 p.
Madeira, J.; Monge Soares, A. M.; Brum da Silveira, A. & Serralheiro, A., 1995. Radiocarbon dating
recent volcanic activity on Faial island, Azores. Radiocarbon 37(2), 139-147.
Monte Alverne, A., ca. 1700. Crónicas da Província de S. João Evangelista das ilhas dos Açores ...
Edição do Instituto Cultural de Ponta Delgada, 2ª edição, 1988, 3º vol.: 336 p.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
GEOMORPHOLOGICAL ASSESSMENT OF THE MIDDLE TAGUS ALLUVIAL PLAIN
ANA RAMOS PEREIRA1, CATARINA RAMOS1, TERESA M. AZEVEDO2, ELISABETE NUNES2, CONCEIÇÃO FREITAS2,
CÉSAR ANDRADE2 & DIAMANTINO I. PEREIRA3
1Centro de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal
e-mail: [email protected], [email protected] de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016
Lisboa, Portugal
e-mail: [email protected], [email protected], [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
e-mail: [email protected]
The goal of geomorphological research within the Geotarif Project has been the recognition
of the alluvial landforms of the Middle Tagus alluvial plain.
The research focus on the Middle Tagus, between Tancos and Vila Franca de Xira / Benfica
do Ribatejo, where the River incises the sediments of its Tertiary Basin, the channel changes
direction and the valley widens rapidly leading to sediment deposition on a 2 to 13 km wide
alluvial plain, bounded by Pleistocene terraces. Despite being the largest Portuguese alluvial
plain, few geomorphological studies have been done.
Based on georeferenced topographic maps, contour lines each one-meter have been drawn.
The automatic interpolation was blind to the fluvial morphology, so hand-drawing interpretation
had to be done, using 5 m contour lines and numerous elevation points. This arduous work
allowed the definition of the alluvial landforms (several generations of natural levees and bars,
inactive channels, floodbasins and yazoo rivers) and the construction of a digital elevation
model (DEM) of the floodplain in the studied area with ArcView and ArcGis.
As one of the aims of the Geotarif project was the identification of different fluvial sediments
in the cores, each landform (present day and inactive) was sampled. The results from the study
were used to compare their textural parameters with those of the core samples (fig. 1). This
methodology leads to the identification, in subsurface sequences, of episodes of channel
avulsion and channel shifting.
In the alluvial plain, the middle reach of the present Tagus River is defined as a single
bedload channel with alternate bars, according to the classification of Miall (1996).
The channel has a low sinuosity (ρ = 1.05), but in this reach it shows a large variability: just
downstream Tancos it describes a large meander (ρ = 1.5) evolving to an almost straight reach
before the next bend.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
The channel width (W) varies between 270 and 590 m, its depth (D) ranges between 3.58
and 6.3 and the mean W/D ratio is 97. Therefore, since this reach of the Tagus shows: (i) highly
variable sinuosity, (ii) high variable W/D ratio, (iii) a very low channel slope (0.0006) and (iv) a
sand dominant bed load, according to Rosgen’s classification of natural rivers (Rosgen 1994
and 1996), it is an anastomosing river (DA5 in Rosgen classification). The present situation (a
single channel, however, is the result of multiple human interventions, known to have happened
since the Romans.
The main in-channel
bars are longitudinal
alternate bars close to the
banks (marginal bars) or
located in the middle of the
channel, reaching 1 km in
length and 200 in width.
The Tagus floodplain,
between Tancos and
Benfica do Ribatejo, a
down-valley distance of
47 km, varies in height from 22 m upstream to 7 m downstream, with a 0.31m/km-1
(approximately half of the channel slope). The floodplain shows a large lateral variation in width
of between 2 and 13 km. The former and present natural levees and crevasse splays reach 1 to
4 m above the average elevation of the floodplain and the floodbasins and abandoned channels
are 1 to 5 m below. The spatial distribution of these elements can be used to define the former
positions of the Tagus channels.
Fig. 1 – Tagus alluvial plain superficial deposits
One of the most impressive examples of the anthropogenic intervention in the foodplain can
be seen near Tancos (Azevêdo et al., 2006a). Initially, the river flowed near the southeastern
limit of the floodplain, close to Arrepiado and Carregueira terraces, adapting its course to the
NNE-SSW lower Tagus fault (fig. 2). However, in 1550 the course of the Tagus River was
changed to a straight channel in this area. The river, however, was not stable in this location
and began migrating to the northwest, adopting two new courses before stabilizing in the
present one – the previous floodbasin, in a lower position. Historical documents report that in
about a sixty-year period the channel shifted from the artificial position to the present-day one.
All three successive avulsions (to WNW), still recognizable in the field, built a major point bar
where abandoned channels and natural levees can be seen.
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Fig. 2 – Cross section of the alluvial plain between
the present-day channel and Chamusca (Arrepiado)
and the digital elevation model (DEM) in the same
area. Key: T1 – the original channel, T2 – artificial
channel, T3 and T4 – former channel positions
before the present-day channel.
References Pereira, A. Ramos; Ramos, C.; Reis, E.; Azevedo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C., 2002. A
dinâmica da planície aluvial do Baixo Tejo no Holocénico recente: aplicação de métodos de análise
geomorfológica e sedimentológica. Abstracts. 1º Seminário de Geomorfologia. Dinâmica
Geomorfológica. Progressos recentes na investigação e aplicações ao ordenamento do território.
Associação Portuguesa de Geomorfólogos, vol. I, Lisboa, p. 66-76.
Ramos, C.; Reis, E.; Pereira, A. Ramos; Azevedo, T.M.; Nunes, E.; Freitas, M.C.; Andrade, C., 2002.
Late Holocene evolution of the Lower Tagus alluvial plain and heavy metals content: Preliminary
results. In J.M. García-Ruiz, J.A.A.Jones & J.Arnáez, eds. Environmental change and water
sustainability. Zaragoza, p.167-182.
Azevêdo, T.M., Pereira, A. Ramos, Ramos, C., Nunes, E., Freitas, M. C., Andrade, C. & Pereira, D.I.,
(accepted). Floodplain sediments of the Tagus River, Portugal: avulsions and channel shifting
assessment and human impact over time, Sedimentology, Special Number dedicated to Peter Friend.
Ramos, C., Pereira, A. Ramos, Azevêdo, M.T. & Nunes, E., 2006. Dinâmica fluvial: o caso das mudanças
do traçado do Tejo na sua planície aluvial, II Congresso da Associação Portuguesa de
Geomorfólogos. Ciência e Sociedade – Associação Portuguesa de Geomorfólogos, vol. II, p.41-48.
Pereira, A. Ramos, Ramos, C., Azevêdo, M.T., & Nunes, E., 2004. Geomorphological and textural
analysis as a tool to evaluate the migration of Tagus river channel during the late Holocene (Portugal).
Geophysical Research Abstracts, Vol. 6, 04749 (EGU 04-A), European Geosciences Union, Nice.
Azevêdo, T.M., Nunes, E., Ramos, C., Pereira, A. Ramos, Freitas, M. C., Andrade, C. & Pereira, D.I.,
2006a). The Tagus River and its historical floods (Santarém, Portugal). Abstracts. Tagus Floods ’06
Workshop, Lisbon, Portugal.
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CHARACTERIZATION OF THE TAGUS FLOODPLAIN SEDIMENTS: THEIR IMPORTANCE IN THE SEARCH OF CHANNEL VARIATIONS AND HUMAN IMPACTS OVER TIME
TERESA M. AZEVEDO1, ELISABETE NUNES1, CATARINA RAMOS2, ANA RAMOS PEREIRA2, CONCEIÇÃO FREITAS1,
CÉSAR ANDRADE1 & DIAMANTINO I. PEREIRA3
1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016
Lisboa, Portugal
e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal
e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
e-mail: [email protected]
The Tagus floods’ research implied a multidisciplinary
work using a diversity of methods such as
sedimentological, geochemical and geochronological
analyses, as well as geomorphological and hydrological
studies, performed in order to characterize the flood
sediments and the dynamics of the river during the
Holocene.
Fig. 1 – Simplified geology of the Portuguese
Tagus basin (modified after Daveau, 1970) and
digital elevation model (DEM). Key: 1 – dune
and coastal sands; 2 – recent alluvium; 3 –
Pleistocene terraces; 4 – Pleistocene Cano
limestone; 5 – Plio-Pleistocene; 6 – Tagus right
margin Miocene lacustrine limestones; 7 –
Miocene detrital series including the Palaeogene
in its eastern part; 8 – Palaeogene; 9 –
Mesozoic Lusitanian basin; 10 – basement; 11 –
location of the DEM of the studied floodplain and
the morphological position of cores and surface
samples (Pereira et al., 2004).
Following a preliminary regional sedimentological and
stratigraphic survey, in order to study the river’s
behaviour during the Holocene, four cores (FB – Fonte
Bela, QB – Quinta da Boavista, SEV – Santarém Entre
Valas and G – Goucharia), ranging in depth from 3.70 to
8.04 m were obtained from the floodplain (fig. 1) and 232
samples were analysed.
The SEV core site, 6 km north of Santarém and 5 km
from the main channel, was selected due to its distal
geomorphological position in the floodplain and its
frequency of flooding, i.e., it is inundated mainly during
major floods. Although this core reached 19.45 m, only
the first 8.04 m were selected in order to compare with
the other three cores.
Core QB was located in the lowest part of the
embankment in the main channel. Core site FB was
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
selected due to its position in the medial zone of the floodplain; the core, totalling 7.4 m, is
composed of two different parts: an upper 2.5 m thick section corresponding to an existing ditch
in agricultural land and a lower 4.9 m section below the base of the ditch. Core G was selected
due to its position on the left bank, opposite to the SEV core site, is 5 km from the main channel
and frequently flooded.
A detailed textural analysis of the core samples and present analogues (bars, natural levees,
crevasse splay deposits, flood basin and abandoned channels) (Pereira et al., 2006b) was
emphasized due to the lack of preservation of sedimentary structures in the cores (fig. 1). Both
the present analogues and core sediments were well discriminated using mean diameter versus
standard deviation and average mean diameter versus average mud percentage graphical
correlations. The textural parameters defined (sand:mud ratio, mean, standard deviation,
skewness) and particularly the interparameter correlations together with twelve 14C numerical
ages of organic matter obtained, allowed the evaluation of:
(i) four different domains to be defined: the present day channel, with a low content of mud (<
5%) and the coarsest material (< 1 φ, medium sand); channel bars and natural levees from
previous channel-belts, with mud contents from 15 to 30% and a mean grain size range from
2.5 to 3.5 φ (fine-very fine sand); the floodplain itself, with 40 to 60% mud (3.5 to 5.5 φ, very fine
sand to coarse silt) and the more distal environment (flood basin and floodplain channel fill) with
the highest content of mud (> 85%) and a mean grain size ranging from 6.5 to 7.5 φ (fine to very
fine silt) and,
(ii) the chronological evolution of the different energetic environments of the floodplain for the
last 4 kyr.
These approaches permitted the determination of sedimentation rates for the different alluvial
plain environments. The highest sedimentation rates occurred in the flood basin and channel fill
domains, with values ranging from 2.23 mm yr-1 to 4.72 mm yr-1 and the lowest in the channel
(0.27 mm yr-1). Values from 0.81 to 1.60 mm yr-1 were recorded in sedimentary environments
proximal to the channel, where several crevasse splay episodes have been recognized. In the
period common to the four cores, i.e., the last 4000 years, the sedimentation rates decreased to
the present.
Bibliography Azevêdo, T.M., 2001. A utilização dos dados históricos no estudo das cheias do Tejo. Estudos do
Quaternário, 4, 69-77.
Azevêdo, T.M., 2004. As mudanças de percurso do Tejo nos tempos modernos. Causas naturais e
antrópicas. In: Evolução Geohistórica do Litoral Português e Fenómenos Correlativos. Geologia,
83
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
História, Arqueologia e Climatologia. Eds António A. Tavares, Maria José. F. Tavares and João Luís
Cardoso, Universidade Aberta, Lisbon. 517-567.
Azevêdo, T. M., Nunes, E. and Ramos, C., 2004. Some morphological aspects and hydrological
characterization of the Tagus floods in the Santarém region, Portugal. Natural Hazards, 31, 587-601.
Pereira, A. R., Ramos, C., Reis, E., Azevêdo, T. M., Nunes, E., Freitas, M. C. And Andrade, C., 2002. A
dinâmica da planície aluvial do Baixo Tejo no Holocénico recente: aplicação de métodos de análise
geomorfológica e sedimentológica. Pub. Assoc. Port. Geom., I, 67-76.
Pereira, A. R., Ramos, C., Azevêdo, M. T. and Nunes, E., 2004. Geomorphological and textural analysis
as a tool to evaluate the migration of Tagus river channel during the late Holocene (Portugal).
Geophys. Res. Abstr., 6, 04749, European Geosciences Union, Nice.
Pereira, A. Ramos; Ramos, C.; Azevedo, T. M.; Nunes, E.; Freitas, M. C.; Andrade, C. & Pereira, D.I.
2006. Geomorphological assessment of the Middle Tagus alluvial plain. Abstracts. Tagus Floods ’06
Workshop, Lisbon, Portugal.
Ramos, C. and Reis, E., 2002. Floods in southern Portugal: their physical and human causes, impacts
and human response. Mitigation and Adaptation Strategies for Global Change, 7, 267-284.
Ramos, C., Reis, E., Pereira, A. R., Azevêdo, T. M., Nunes, E., Freitas, M. C. and Andrade, C., 2002.
Late Holocene evolution of the Lower Tagus alluvial plain and heavy metals content: preliminary
results. Environmental Change and Water Sustainability (Eds. J. M. García-Ruiz, J. A. A. Jones and J.
Arnáez), 167-182, Ins. Pirenaico de Ecología, Zaragoza.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
UNDERSTANDING LATEGLACIAL AND HOLOCENE ENVIRONMENTAL CHANGES THROUGH THE SEDIMENTOLOGICAL STUDY OF A CORE (SANTARÉM REGION)
MARIA DA CONCEIÇÃO FREITAS1, CÉSAR ANDRADE1 , TERESA M. AZEVEDO1, ANA RAMOS PEREIRA2, CATARINA
RAMOS2, ELISABETE NUNES1 & DIAMANTINO I. PEREIRA3
1Centro de Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C2, 5º Piso, Campo Grande, 1749-016
Lisboa, Portugal
e-mail: [email protected], [email protected], [email protected], [email protected] de Estudos Geográficos, Alameda da Universidade, Faculdade de Letras, 1600-214 Lisboa, Portugal
e-mail: [email protected], [email protected] de Ciências da Terra, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
e-mail: [email protected]
Under the Geotarif Project, the paleoenvironmental evolution of the Middle Tagus during the
Lateglacial and Holocene has been addressed. For this purpose, a deep core was drilled in
Santarém Entre Valas (SEV). This paper aims to report preliminary results obtained from the
sedimentological study of the more inclusive SEV sedimentary column.
The SEV core was retrieved from the Tagus alluvial plain (8.60 m above MSL) using Shelby
samplers driven by hydraulic pressure down to 19.4 m below surface, where a coarse gravel
unit, interpreted as Pleistocene in age, was found. The core was sub-sampled for each 10 cm
interval and studied for grain-size, organic matter and CaCO3 contents and geochemistry; seven
radiocarbon dates were obtained from selected levels to provide time boundaries. This
radiocarbon dating of carbonaceous organic material (peat, wood), at different depths, gave the
following ages: 18,109 cal BP at 16.7 m; 11,219 cal BP at 14.7 m; 8097 cal BP at 12.3 m; 6960
cal BP at 10.7 m; 3550 cal BP at 6.5 m; 3086 cal BP at 4.5 m; 816 cal BP at 1.0 m.
The sediments cored are essentially detrital and of terrestrial provenance. The results of the
sedimentological analysis of the first 8.04 m of the SEV core, of three other cores, the
comparison with present day analogues and the definition of avulsions events and shifting
episodes were presented in Ramos et al. (2006) and Azevêdo et al. (accepted). On the basis of
textural and compositional attributes the core has been divided in three main units (figs. 1 and
2).
Unit I – 19.4 to 13.5 m (> 18100 to circa 9900 cal BP) – consists of sandy mud to muddy
sand, with mean diameter in the range of fine sand to medium silt, usually poorly sorted and
positively skewed. This unit is free of organic matter and shows in its lower section the highest
values of CaCO3 and total Ca found throughout the core, although no shell remains were
observed. The concentration in Br is bellow detection limit, and U is present in the range
2-5 ppm. The La/Sm decreases up-unit but remaining in the highest values of the whole core.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Bearing in mind existing sea level data, this unit is contemporaneous of a rapid rise in mean
sea-level (-100 to -20 m), which, however, must have remained at least 20 m below the
topographical surface at that time. This suggests a higher gradient of the paleo-river system
than today and sediments found in this unit must have been deposited marginally to a main
channel; in general sedimentation must have been more energetic than in later stages and
confined to a valley that probably was narrower, single-channel and more incised. This
interpretation is consistent with absence of organic material and dominant terrestrial
provenance, as suggested by CaCO3 and Br, the former eventually associating with
remobilization of particles consisting in carbonate-cemented aggregates remobilized from
nearby Miocene outcrops; the consistent change in La/Sm found up-unit suggests congruent
change in relative influence of distinct sources. We hypothesise that the coarser sedimentation
episode found circa 15 m, may represent a reactivation associated with Younger Dryas low-
stand.
18109 Cal BP
11219 Cal BP
8097 Cal BP
6960 Cal BP
3550 Cal BP
3086 Cal BP
816 Cal BP
0 20 40 60 80 100
20
18
16
14
12
10
8
6
4
2
0
0 10 20 30 0 1 2 30 2 4 6 8 10 12 0 1 2 3 4 5 -4 -2 0 2 4
mud (%) Mean grain size (φ) Sd (φ) Sk (φ)
Dep
th (m
)
CaCo3 (%) Organic matter (%)
Uni
t IU
nit I
I B
Uni
t III
AA
B
Fig. 1 – Mud, CaCO3 and organic matter content, textural parameters, unit division and dated horizons
of Santarém Entre Valas (SEV) core.
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Tagus Floods '06 Workshop, Lisbon, Portugal 19-21 July, 2006
Unit II – 13.5 to 2.9 m (circa 9900 to circa 2000 cal BP) – sediments of this unit contrast
markedly in texture with underlying materials and the contact with unit I is sharp. Two sub-units
have been considered:
Sub-unit IIA – 13.5 to 8.0 m (circa 9900 to circa 5100 cal BP) – is essentially made of mud
and slightly sandy mud, mean grain size falling in the range of clay and the size distribution
being negatively skewed. Sorting is better in the lower half of the sub-unit, where the sediment
is mud and decreases upward with the addition of secondary coarser fractions. Small amounts
of organic matter were found, the higher values in the basal section and decreasing further
upward. Br and U are higher than in Unit I suggesting marine influence and low oxygenation
and the vertical profiles show relative enrichment in the lower half (below 10 m) and smaller
concentrations above; the La/Sm is lower than in unit I and relatively constant, indicating
unvarying sources. Favaretto et al. (2006) and Azevêdo et al. (2006b) refer the presence of
foraminifera in this unit up to 10 m indicating limited marine-brackish water influence that may
have persisted until 6900 cal BP.
0 20 40 60 80 100
20
18
16
14
12
10
8
6
4
2
0
0 4 8 12 4.8 5.2 5.6 6 6.4 0 2 4 6 8 10Br (ppm) Ca (%) La/Sm U (ppm)
Uni
t IU
nit I
I B
Uni
t III
AA
B
Dep
th (m
)
Fig. 2 – Vertical profiles of some geochemical parameters and ratios of Santarém Entre Valas (SEV)
core.
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This date is in agreement with the age range accepted for the Holocene transgressive
maximum in the western Portuguese coast. We interpret sub-unit II A as deposited in a paralic
shallow and low energy environment, essentially dominated by fresh-water with occasional
marine incursions that allowed accumulation of some organic matter and eventually favoured
development of anoxia. The study of pollen, spore and non-pollen palynomorphs (Azevêdo et
al., 2006) indicates the occurrence, under Mediterranean climate, of such an acidic, oligotrophic
freshwater environment which could dry out in the summer. This interpretation is in agreement
with data indicating the marine base level having reached an elevation slightly below present-
day level circa 5500 BP; this would account for the abrupt decrease in energy of this location
that must have shifted into a distal estuarine environment with a week signal of marine
influence.
Sub-unit IIB – 8.0 to 2.9 m (circa 5100 to circa 2000 cal BP) – this sub-unit is quite uniform
and homogeneous, and just slightly coarser and more sandy than sub-unit II A. Organic matter
is vestigial, carbonate (again not associated with bioclasts and probably of terrestrial and local
source) and Ca increase upwards, while Br decreases markedly. U remains fairly constant and
La/Sm shows a similar pattern, yet with higher values than in the sub-unit II A. These data,
together with the absence of foraminifera tests, point to low energy, freshwater fluvial
sedimentation, related with an aggrading floodplain. Comparison with present-day analogues
suggests strong resemblance with a floodbasin environment (Azevêdo et al., accepted).
Unit III – 2.9 cm to surface (circa 2000 cal BP until present) – this unit comprehend two sub-
units.
Sub-unit III A – 2.9 to 0.9 cm (circa 2000 to circa 700 cal BP) – in this sub-unit the mud
content drops drastically and sediment coarsens, being essentially slightly muddy sand to
muddy sand. Organic matter is vestigial, carbonate content decreases and Br values are under
detection limits. This reflects a period of higher energy sedimentation, sharing similarities with
the present day natural levees that may correlate with a channel approach to the coring site
(avulsion episodes).
Sub-unit III B – 0.9 cm to surface (circa 700 cal BP until present) – the uppermost unit is
essentially muddy, showing strong oscillations in the sand content. The relevant differences in
composition associate with Br and Ca, which significantly increase. These materials correspond
to the present-day floodplain environment, located far from any marine influence. We
hypothesise an anthropic origin for Br, eventually related with the wide spreading of pesticides
and atmospheric fallout after combustion of fuel; both sources reduced after the 1980’ and this
may explain the reduction noticed in the topmost sediment layers.
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In summary, the sediments cored record a three-staged paleoenvironmental evolution of a
section of the Tagus floodplain since the Late Pleistocene, which are marked by distinct
compositional and textural breaks. The whole sequence represents a sedimentary cycle, with a
transgressive phase (until 6900 cal BP) followed by a forced regressive one.
References Azevêdo, M.T., Favaretto, S., Miola, A., Mozzi, P., Nicosia, C., Nunes, E. & Sostizzo, I., 2006b).
Palaeoenvironments of the Tagus valley during the last 15 ka: sedimentological, palynological and
micromorphological evidence of the Entrevalas SEV coring (Santarem, Portugal). Abstracts. Tagus
Floods ’06 Workshop, Lisbon, Portugal.
Azevêdo, T.M., Pereira, A. Ramos, Ramos, C., Nunes, E., Freitas, M. C., Andrade, C. & Pereira, D.I.,
(accepted). Floodplain sediments of the Tagus River, Portugal: avulsions and channel shifting
assessment and human impact over time, Sedimentology, Special Number dedicated to Peter Friend.
Favaretto, S., Sostizzo, I. & Miola, A. (2006). Pollen and NPP analyses of the Entre Valas sequence
(Santarém, Portugal), Unplubished Technical Report, Geotarif Project, 4 p.
Ramos, C., Pereira, A. Ramos, Azevêdo, M.T. & Nunes, E., 2006. Dinâmica fluvial: as mudanças de
traçado do Tejo na sua planície aluvial. Pub. Assoc. Port. Geom., 3, 41-48.
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POSTERS
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ESTIMATION OF RECENT ALLUVIAL SEDIMENTATION RATES, UPPER ODRA, POLAND
AGNIESZKA CZAJKA University of Silesia, Faculty of Earth Sciences, Bedzinska 60, 41-200 Sosnowiec, Poland
e-mail: [email protected]
The Upper Odra River is a sub-mountainous river of ground-snow-rain regime. It flows in the
temperate climatic zone. Before the regulation the Upper Odra (drainage area of 4666 km2,
mean flow of 41,5 m3/s) used to be a meandering, sand-bed river. As an effect of the training
works, agricultural, urban and industry development, the natural processes of erosion and
sedimentation were interrupted in the 19th century (Klimek, 1999). The embankment
constructions narrowed the width of natural floodplain to about 200 m and changed the natural
way of alluvial sedimentation. The construction of groins in 19th century resulted in a decrease
of the channel width from over 150 m before the rectification to approximately 50 – 70 m after
the training works. Nowadays, the Upper Odra channel sinuosity reaches from 1.3 to 2.3, while
the valley floor slope reaches 0,3 m/km. Construction of regular arrays of groins along the
Upper Odra channel created a new environment for sediment deposition. They keep the main
flow away from the river banks but also result in deposition of material adjacent to the groin
structures. This paper describes the rate of sediment accumulation within the channelised reach
of the Upper Odra River, in the lee-side of the groins.
The summer floods occur in the Upper Odra almost every year, promoting an opportunity to
overbank deposition (Ciszewski, 2002). The inter-groin infill is a significant sink for material
transported in the Upper Odra channel. Sediment accretion rates were derived using artefacts
as an alluvia age indicator. In the last 40 years, the average rate of sediment accretion has
been 5 cm per year, while shortly after channel stabilization(the rates ranged from 1.2 to 2.8 cm
per year. During large floods, up to 30 cm per event have been deposited (Czajka, 2005). Three different age indicators were used to calculate rates of vertical accretion since
approximately 1750: coal dust, plastic artefacts and 137Cs. The coal dust and the plastic objects
are very common in the youngest sediments. The coal dust can be used to estimate rates of
sedimentation on the Odra River since about 1750 when the first coal mines within Upper Odra
catchment started to work. The artificial radioactive element, 137Cs was used as an age indicator
for the sediments deposited after the year of 1964 (detectable caesium-137 began in 1954 and
the first peak appeared in 1958/1959. The second peak occurred at 1962-1964).
By determining the depth of the strata containing artefacts, more detailed age estimates were
obtained Rates of sediment accumulation per year were defined in thickness (cm a-1) and
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can be expressed as
calculated by dividing the thickness of sediments deposited above the stratum containing the
dated artefact by the number of years since the artefact deposition. The rate of sedimentation
)/(
ASPAs YYSR −=
where RS = average rate of sedimentation (cm a -1), SA = depth of stratum containing artefact of
known age (cm), YP = year of deposition of top layer of the unit and YSA = year of artefact
deposition. The average rate of the accretion of overbank sediments in the zone adjacent to the
river channel reaches1.7–6 cm/year.
Synthetic artefacts in the alluvia provide evidence of increasing rates of vertical accretion
during the last 50 years. These results correlate with data concerning the sediment budget
within the investigated sectors of the Upper Odra. On average, 195.000 tons of suspended
material transported by the Upper Odra has been yearly trapped as overbank deposits within
the investigated sections.
References Ciszewski D., 2002. Heavy metals in vertical profiles of the middle Odra River overbank sediments:
evidence for pollution changes, Water, Air, and Soil Pollution 143, 81-98
Czajka A., 2005. Accumulation of sediments within the channelized reach of the Upper Odra River,
Poland. In: Ramon J. Batalla, Celso Garcia (ed.), Geomorphological Processes and Human Impacts in
River Basins IAHS Publ. 299, 2005. str. 191-196
Klimek K., 1999. A 1000 year alluvial sequence as an indicator of catchment/floodplain interaction: the
Ruda valley, sub-Carpathians, Poland. In: Fluvial Processes and Environmental Change (ed. by
Brown A. G., Quine T. A.) Wiley & Sons Ltd.
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PELIGROS E IMPACTOS PRODUCIDOS POR LAS MODIFICACIONES DE ORIGEN ANTRÓPICO DURANTE LA AVENIDA DEL ARROYO RIVILLAS EN 1997 (SW DE ESPAÑA)
MARIA EUGÉNIA MOYA-PALOMARES 1 & JUAN D. CENTENO 2
1Departamento de Geología. Facultad de Ciencias. Universidad de Alcalá. Carretera Madrid-Barcelona, Km. 33,600.
Alcalá de Henares, Madrid, (España).
e-mail: [email protected] de Geodinámica. Facultad de Geología. Universidad Complutense de Madrid (España)
Introducción
Las avenidas relámpago o flash floods constituyen en la actualidad el peligro natural más
importante que se produce en redes secundarias de drenaje de la Península Ibérica. Las
avenidas relámpago causan una media de 20 muertos al año y perdidas materiales superiores
a los 210 millones de euros anuales. Muchas de estas consecuencias podrían ser evitadas con
una correcta ordenación territorial en consonancia con el conocimiento de los peligros de las
áreas implicadas. Un claro ejemplo de magnificación de un evento natural por modificaciones
antropicas lo constituyeron las inundaciones ocurridas en Noviembre de 1997 en el arroyo
Rivillas.
El arroyo Rivillas y la tormenta del mes de noviembre de 1997 El arroyo Rivillas es un afluente temporal del río Guadiana cuyo cauce permanece seco gran
parte del año. Presenta una cuenca de 321 km2 rellena de materiales cenozoicos (arenas y
arcillas fundamentalmente) que otorgan a la cuenca una permeabilidad baja. En la primera
semana del mes de noviembre de 1997 una borrasca extratropical atravesó la mencionada
cuenca con dirección SW originando una ciclogénesis explosiva (Elizaga y Martín 2001). Las
precipitaciones acumuladas en 24 horas superaron los 120 l/m2 en la mayoría de las estaciones
pluviométricas de la región (Núñez y del Pino, 2001) acompañadas de rachas de viento
superiores a los 100 Km/h. Las intensas precipitaciones en un corto espacio temporal (días 5-6
de noviembre de 1997) provocaron una importante crecida en el río Guadiana y el
desbordamiento de algunos tributarios de mencionado río como el Rivillas. El estudio de las
formas erosivas y sedimentarias permitió reconstruir el proceso. En este sentido, pueden
consultarse los trabajos de Moya-Palomares et al. (1998, 2006) y Ortega et al. (1998).
La avenida catastrófica y el papel jugado por las modificaciones antrópicas sobre los efectos del flujo
Debido a las actuaciones humanas en el valle y a parcial canalización del arroyo, el Rivillas
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presenta una baja sinuosidad (casi se podría decir que es un canal rectilíneo y constreñido por
los leeves artificiales de sus márgenes) con un flujo discontinuo y efímero durante gran parte
del año. Con un incremento de flujo las aguas tienden a migrar sobre la llanura aluvial,
rompiendo los leeves y trazando meandros como ocurrió en los primeros estadios de la
avenida de 1997. Cuando se produce el pico de la crecida, la llanura de inundación se
comporta como si de un fondo de canal aluvial se tratase. En condiciones naturales este
proceso no tendría mayor importancia pero la intervención humana hizo que dos barrios de la
ciudad de Badajoz se vieran convertidos en una trampa mortal para sus habitantes.
A lo largo del Rivillas se han identificado numerosas prácticas agrícolas y canalizaciones
sistemáticas del cauce, edificaciones, caminos sobre la llanura de inundación que
incrementaron los efectos del flujo. Una de las causas de la magnificación del evento fue sin
duda, la gran cantidad de material trasportado convirtiendo el flujo en un mudflow. La
sustitución de las viejas dehesas por pastizales y viñedos redujeron la tasa de infiltración e
incrementaron la cantidad de material sedimentario transportado. Esta cantidad de material se
acrecentó por el elevado número de construcciones que actuaron a modo de represas sobre la
llanura aluvial. Cuando estas presas fueron destruidas y sobrepasadas por las aguas, el flujo
adquirió “picos” de concentración energética.En Badajoz, la ocupación por edificaciones hasta
el borde del canal produjo un aumento de la altura de las aguas. Además, la estructura de
canalización en la confluencia de los arroyos Rivillas y Calamón que cubre parte de los ojos del
puente de la N-5 se convirtió en un obstáculo de salida de las aguas hacia el río Guadiana.
Agradecimientos Este trabajo ha sido financiado por el proyecto de Investigación PI2005/055 de la
Universidad de Alcalá.
Referencias Elizaga, F. y Martín, F., 2001. Estudio diagnóstico de la ciclogénesis explosiva del 5-6 de noviembre de
1997. V simposio Nacional de predicción. INM. Madrid 20-23 Noviembre de 2001. En CD.
Moya, M.E., Garzón, G., Ortega, J.A. & Centeno J.D., 1998. Estructuras sedimentarias resultado del
desbordamiento del arrollo Rivillas, Badajoz (España). Noviembre de 1997. Actas do V Congresso
Nacional de Geología. Tomo 84, Fascículo 1. Lisboa. pp. 209-212
Moya-Palomares, M.E. & Centeno, J.D., 2006. Morphologycal analyses of the flash floods in ouadis and
torrents with lower declive, the case of the Rivillas River (SW of Spain). 3rd Conference of the
Association of African Women Geoscientists. Women, Geosciences and Development, El Jadida.
Marroc. pp. 118-120.
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Nuñez, M. y Del Pino, J.D., 2001. Ciclogénesis Atlántica del 5 y 6 de noviembre de 1997, análisis
fenomenológico y mesoescala. V simposio Nacional de predicción. INM. Madrid 20-23 Noviembre de
2001. En CD.
Ortega, J.A., Garzón, G., Moya, M.E., 1998. Parámetros climáticos y ambientales de la avenida del
arroyo Rivillas en Noviembre de 1997. V Reunión nacional de Geomorfología. Granada. pp. 237-246.
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DESARROLLO ESPACIAL Y EVOLUCIÓN DE UNA FLASHFLOOD A TRAVÉS DE LA INFORMACIÓN APORTADA POR DEPÓSITOS EFÍMEROS
JOSÉ A. ORTEGA1, GUILHERMINA GARZÓN2
1 Depto. Medio Ambiente, E. S. Politécnica, Universidad Europea de Madrid, Madrid
e-mail: [email protected] Depto. Geodinámica, Facultad de Geología, UCM, 28040 Madrid
e-mail: [email protected]
Los estudios de delimitación de áreas inundables se centran cada vez mas en cálculos de
modelos numéricos, aún sabiendo las limitaciones de estos métodos debido no solo a la poca
calidad general de los datos hidrológicos de partida si no también a la propia naturaleza
cambiante del sistema aluvial. Frente a ello la cartografía de áreas inundables debería basarse
lo más posible en datos directos de campo, con los que puedan contrastarse los modelos
numéricos teóricos. Los métodos geológicos presentan (Costa y Baker, 1985) el principal
problema de la correlación no univoca entre formas y magnitud de la avenida, pero constituyen
elementos tangibles reales de la dinámica y del flujo de desbordamiento. La dificultad de
correlación se basa en la propia variabilidad de los elementos naturales, pero especialmente
por la interferencia con obras y obstáculos antrópicos, por lo que la información de los efectos
de una avenida resulta una información básica para el estudio de la dinámica e insustituible a
nivel de referencia para cualquier análisis hidrológico posterior. Por ello el esfuerzo de los
investigadores debe centrarse en recoger la máxima información inmediata posible después de
un evento catastrófico (Williams y Costa, 1987).
En el caso de la avenida del Arroyo Rivillas de noviembre de 1997 en Badajoz, el
levantamiento cartográfico inmediato a la inundación, ha servido para la elaboración del modelo
hidráulico, basado fundamentalmente en las marcas de altura del agua y sus depósitos (Ortega
y Garzón, 2005). Éstos se han clasificado de acuerdo con su ambiente genético y según su
localización respecto a la crecida, aspectos hidrológicos e hidráulicos, tamaño de grano y
energía de formación, naturaleza sedimentaria o erosiva y tamaño de la forma. El resultado es
un conjunto de 18 formas, la mayoría de ellas previamente descritas en la literatura, pero no
asociadas en su conjunto tanto con el resto de las morfologías, como a una cartografía a
escala 1:4.500.
A su vez, la relación de estos depósitos con el modelo hidráulico ha permitido establecer
una clasificación de rangos de estabilidad para las formas resultantes. Se ha obtenido el caudal
máximo de la crecida en tres de los tramos analizados (elegidos por su gran variedad de
formas y que son representativos puesto que se distribuyen a lo largo de todo el perfil
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longitudinal) a partir de la modelización mediante HEC-RAS y así se han obtenido resultados
sobre variables importantes en la estabilidad de las formas como son: profundidad de la lámina
de agua, velocidad, esfuerzo de cizalla y potencia fluvial. Empleando la cartografía
morfosedimentaria obtenemos unas relaciones entre elementos y los rangos de estabilidad
bajo los cuales las formas no quedan destruidas. Estos resultados son orientativos, puesto que
el HEC-RAS es un método hidráulico de cálculo unidimensional, que solo permite establecer
tres valores medios para cada sección (margen derecha e izquierda, y canal) por ello los
campos y condiciones obtenidas no pueden ser consideradas como las originales para su
formación. Aún así, estimamos que el trabajo base puede ser muy útil en investigaciones
futuras mejorando el modelo hidráulico, que debería ser 3D en vez de unidimensional.
La interferencia antrópica a través de cambios de uso del suelo ha modificado el aporte de
sedimentos originando una avenida relámpago más catastrófica. Aún así, las formas erosivas
dominan sobre las sedimentarias por lo que el balance sedimentario es negativo. Los
resultados preliminares obtenidos muestran que la profundidad no parece ser un factor
limitante en la permanencia de las formas posteriormente a la crecida. Algunas formas tienen
rangos muy grandes que llegan hasta los 140cm en las barras longitudinales arenosas y 150
en las barras de derrame. En ambas el umbral superior de calado es bastante elevado lo que
muestra la gran resistencia de la forma a ser erosionada por los caudales de descenso. Los
abanicos aluviales que llegan a la llanura de inundación permanecen estables al sumergirse en
profundidades bajas no superior al metro. Las formas erosivas muestran los valores más altos
de profundidad, entre 100 y 150cm para surcos, mientras que los canales antiguos encajados
tienen valores de calado máximo de 170cm.
La velocidad sí se revela como un factor más determinante en la aparición de formas
sedimentarias o formas erosivas. El umbral entre ambas lo hemos situado alrededor de 1m/s,
aunque se trata de un límite difuso. Las formas de erosión son las que requieren los mayores
valores de velocidad (no indicativa del momento de formación). Los surcos o acanaladuras se
dan en zonas de alta velocidad (superan 1m/s hasta 1,67 m/s), valor también superado por los
canales antiguos encajados. En relación con el esfuerzo de cizalla existe una clara delimitación
sobre la llanura entre las formas de depósito y las formas de erosión. En las morfologías
erosivas el valor del esfuerzo de cizalla medio se incrementa con cifras que alcanzan los 93
N/m2 de valor máximo para los surcos, siendo normales valores superiores a 30 N/m2. La
potencia del canal, tiene una distribución similar entre formas de erosión y sedimentación.
Como conclusiones más relevantes obtenemos que las efímeras formas de erosión y
depósito generadas tras una flashflood constituyen una valiosísima fuente de información para
conocer el desarrollo de la misma. Los eventos energéticos, mostrados en erosiones y
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depósitos de material grueso pueden llegar a intercalarse con otros de menor energía, buena
muestra de los pulsos seguidos por la crecida de los cuales tenemos información directa de
testigos presenciales. La relación abanico-depósito y erosión en la llanura de inundación es
también una buena fuente de información sobre la llegada de caudales por parte de afluentes,
y también sobre el momento temporal en que esta se produce. Encontramos dos tipologías
muy diferentes en cuanto a esta ocurrencia temporal. Los derrames guardan una especial
relación con las actuaciones antrópicas (Moya et al. 1998 a y b; Ortega et al. 2004) y la
evolución de las modificaciones en el canal. Con una sencilla modelización, en el mapa
aparecen las zonas más sensibles a presentar este tipo de formas, como se ha comprobado
posteriormente en campo. Además son el origen del sedimento en la mayoría de las
macroformas sobre la llanura.
Sin duda los cambios humanos determinan el riesgo posterior, esto se ha podido comprobar
mediante estudio de las fotografías aéreas, localizando los puntos más conflictivos, que suelen
coincidir con zonas de erosión, y por tanto de alta energía en casi todas las fases de la crecida.
Un adecuado registro de formas, acompañado de un levantamiento topográfico y modelización
hidrológica-hidráulica permite establecer por tanto un gran número de características, y ayuda
a entender mejor el funcionamiento de las flashfloods.
BIBLIOGRAFIA Costa, J.E. y Baker, V.R., 1995. Surficial Geology. Techbooks, 498 p.
Ortega, J.A., Garzón, G. y Muñoz Barco, P., 2004. Incidencia de los acondicionamientos en pequeños
cauces y de los cambios de uso del suelo en las inundaciones de Badajoz de 1997. En: Cachón de
Mesa, J. y López-Piñeiro Pérez T. (eds). Congreso de Restauración de Ríos y Humedales. CEDEX.
Madrid, pp. 207-218.
Moya, M.E., Garzón, G., Ortega, J.A., 1998a. Depósitos de la avenida del Arroyo Rivillas, Badajoz.
Noviembre de 1997. V Reunión Nacional de Geomorfología. Granada. pp 229-236
Moya, M.E., Garzón, G., Ortega, J.A. y Centeno, J.D., 1998b. Estructuras sedimentarias resultado del
desbordamiento del Arroyo Rivillas, Badajoz (España), Noviembre de 1997. Congreso Nacional de
Geología. Lisboa. Sociedade Geológica de Portugal, 84-1. pp. A-209-212.
Ortega, J.A. y Garzón, G., 2005. Ephemeral deposit analysis from a riverine flashflood:
morphosedimentary evolution and significance in flood interpretation. VI Internacional Conference on
Geomorphology. Zaragoza, p. 93.
Williams, G.P. y Costa, J.E., 1987. Geomorphic Measurements after a Flood. In: Baker, V.R., Kochel,
R.C., Patton, P.C., (eds). Flood Geomorphology. Wiley, N. York. pp. 65-77.
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