flushing flows in the lower ebro: monitoring and modelling · lower ebro: monitoring and modelling...

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Madrid Meeting, November 29th 2011

FLUSHING FLOWS IN THE LOWER EBRO:

MONITORING AND MODELLINGAlvaro Tena, Leszek Ksiazek, Damià Vericat and Ramon J. Batalla

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The lower Ebro River is experiencing a series of geomorphic and ecological alterations caused by human impacts.

Water and sediment transfer in the lower Ebro River are altered by dams. The Mequinenza-Ribarroja-Flix dam-complex retain, on average, up to 90% of the suspended load (Vericat and Batalla, 2006).

Downstream from this dam-complex, frequent floods (e.g. Q2 to Q25) have been reduced by around 25%(Batalla et al., 2004).

Persistent low flows, excess nutrients (nitrogen, phosphorus) and high light availability (due to the lack of suspended sediment) have been discussed as the likely causes for the recent uncontrolled growth of macrophytes.

This massive development of macrophytes generates a series of ecological and socio-economic problems (Palau et al., 2004).

INTRODUCTION

Logo grupoPotamogeton pectinatus

Ceratophyllum demersum Potamogeton friesii Rupr.

Myriophyllum verticillatum L.

Black Fly

Photo: Cristina Buendia www.heraldo.es

Photo: Cristina Buendia www.barranquismo.org

Clogging of water intakes of Flix Hydro-Electric Power Plant, AscóNuclear Power Plant, and irrigation stations.

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How to face this problem????Controlled water releases, have been implemented (i.e. designed, monitored, and modelled) in the lower Ebro River since 2002, with the objective of removing the excess of macrophytes and keeping sedimentary activity in the channel.Here we present an integrated monitoring design to assess the effectiveness of flushing flows.This design is based in the following scheme:

Monitoring and samplingFlushing flow designModelling adverse geomorphic effectsDesign validationFlushing flow monitoringFlushing flow evaluation

(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-

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Bed Entrainment

Topography

(A)MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentray Dynamics-

Particle Sizes

(((

((

(((

Embarcación con GPS y ecosonda

Sediment Transport

Bed Incision

Boat with GPS and ecosound

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Bathymetry

(B) FLUSHING FLOW DESIGN -Based on a specific objective and limitations-

Sediment Transport

(A) MONITORING AND SAMPLING-Sediment Transport, Hydraulics and Morphosedimentary Dynamics-Particle Sizes

Bed Entrainment

Bed Incision

Topography

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vτc = g · ρs’ · τc* · Di

(after Shields, 1936)

τ = d · g · ρ · s

Gravitational forces

Lift Forces

Drag Forces

vτ

τc

τ > τc‐Di → Di initial entrainment

The design of flushing flows was based on mobilizing an active layer, equal the maximum root depth of the macrophytes. This was based on the Shields entrainment function(1936).

Mac

roph

yte

Rem

oval

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Resulting peak flows were equivalent to the 1.5- to 2-year flood of the river’s post-dam flow series (i.e. ca 1350 m3s-1).

FLUSHING FLOW DESIGN


(C) MODELLING ADVERSE GEOMORPHIC EFFECTS-Based on Monitoring and Sampling-

vτ > τc-Di → Di will be entrained

τc = g · ρs’ · τc* · Di (after Shields, 1936)

τ = d · g · ρ · s

v

Submerged Weight

Lift Forces

Tractive Forces

Sediment Transport


Bed Entrainment

Bed Incision

Topography

Mac

roph

yte

Rem

oval

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Hyd

raul

ic a

nd S

edim

ent T

rans

port

Mod

ellin

g Bed Elevation Channel Roughness


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Hydraulic modellingVelocity Discharge

Shear stress

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Sediment transport modellingSuspended sediment concentration

Bed load transport rate

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Modelling geomorphic effectsBed changes



(D) DESIGN VALIDATION-Based on adverse effects, flooding limitations and water availability-



τ = d · g · ρ · s

v

Submerged Weight

Lift Forces

Tractive Forces

Sediment Transport


Bed Entrainment

Bed Incision

TopographyM

acro

phyt

e R

emov

al

Hyd

raul

ic a

nd

Sedi

men

t Tra

nspo

rt

Mod

ellin

g

Bed Elevation Channel Roughness Bed Changes


(E) FLUSHING FLOW MONITORING -Flushing Flow effects in Monitoring Sections-

a) Macrophyte density and topographyb) Flow hydraulicsc) Sediment transport

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a) Macrophyte density

Macrophyte densities were estimated in experimental sections before and after the FF by means of the backscatter of a sonar.

Depth

Distanceicc.cat

Macrófitos estables: 8,1 %Macrófitos incipientes: 3,3%Total macrófitos: 11,4 %

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A board-mounted ADCP was used to measure discharge and hydraulics in a monitoring section during the entire FF

b) Flow hydraulics

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c) sediment transport

Water samples were obtained at the same sections in order to calibrate the turbidity records, allowing us to obtain a continuous sedigraphs.

Punctual suspended sediment samples(Calibration of the turbidity record)

Continuous turbidity record

Turbidity has been measured in continuous at different sections by means of turbidity probes.





(F) FLUSHING FLOW EVALUATION -Based on flushing flow monitoring and post-sampling-



τ = d · g · ρ · s

v

Submerged Weight

Lift Forces

Tractive Forces

Sediment Transport


Bed Entrainment

Bed Incision

TopographyM

acro

phyt

e R

emov

al

Hyd

raul

ic a

nd

Sedi

men

t Tra

nspo

rt

Mod

ellin

g

Channel Hydraulics, Bed Topography and Bed Mobility

Sediment Transport


Macrophyte density control

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0 10 20 30 40 50 60 70 80 90-4,0

-3,5

-3,0

-2,5

-2,0

-1,5

-1,0

Distance (m)

Wat

er d

epth

(m

)

Bed Changes

ArmouringMacrophyte Removal

RB LB

RB LB

0 10 m

0 10 m

a)

b)

Incision

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0

5

10

15

20

25

30

35

40

08/07/2009 24/04/2010 03/06/2010 18/10/2010 05 y 22 /11/2010

Macrófitos %

t1

t2

t3

t4

t5

FFFF

MACROPHYTES COVER EVOLUTION IN THE LOWER EBRO (2009-2010)M

acro

phyt

es c

over

(%)








τ = d · g · ρ · s

v

Submerged Weight

Lift Forces

Tractive Forces

Sediment Transport


Bed Entrainment

Bed Incision

Topography

Mac

roph

yte

Rem

oval

Hyd

raul

ic a

nd

Sedi

men

t Tra

nspo

rt

Mod

ellin

g

Channel Hydraulics, Bed Topography and Bed Mobility

Sediment Transport


Bed Changes, Entrainment, Armouring

Macrophyte Removal

Macrophyte density control

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FLUSHING FLOWS IN THE LOWER EBRO

Flushing flows do not exhibit severe geomorphic impacts after evaluation.

Effectiveness of flushing flows (i.e. rate of macrophyte removal) attains 95%, but decreases substantially downstream.

Flushing flows are an important instrument of river management in rivers subject to regulation specially in large Mediterranean rivers such as the Ebro.

Flushing flows exhibited high transport capacity for suspended sediment.

Fine to medium gravels are mobilized but bedload rates are typically low given the short duration of the events

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o BATALLA, R.J., GOMEZ, C.M & KONDOLF, G.M. (2004): Reservoir-induced hydrological changes in the Ebro River basin (Northeastern Spain). Journal of Hydrology, 290, 1-2, 117-136.

o BATALLA, R.J., VERICAT, D. (2009): Hydrological and Sediment Transport Dynamics of Flushing Flows: Implications for Management in LargeMediterranean Rivers. River Research and Applications, 25, 297-314.

o PALAU A, BATALLA R, ROSICO E, MESEGUER A, VERICATO D. (2004). Management of water level and design of flushing floods for environmental river maintenance downstream of the Riba–roja reservoir (lower Ebro River, NE Spain). HYDRO 2004: A New Era for Hydropower. Porto, Portugal, 18–20 October 2004.

o TENA, A., KSIAZEK, L., VERICAT, D., BATALLA, R.J. (2011):Assessing the geomorphic effects of a flushing flow in a large regulated river . River Research and Applications (under review).

o VERICAT, D. & BATALLA, R.J. (2006): Sediment transport in a large impounded river: The lower Ebro, NE Iberian Peninsula. Geomorphology, 79, 72-92.

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- Elaboración de una metodología de base física para la preparación de crecidas generadoras aguasabajo de embalses: Aplicación al tramo inferior del río Ebro, CICYT (REN2001-0840-C02-01/HID)- Diseño y análisis de crecidas generadoras como estrategia de reequilibrio hidrológico y sedimentario del curso inferior del río Ebro, CICYT (CGL2005-06989-C02-02/HID)- Diseño y aplicacion de crecidas generadoras como estrategia de reequilibrio hidro-sedimentario en rios regulados, CICYT (CGL2006-11679-C02-01/HID)- Encomienda de gestión relacionada con el medio ambiente en el marco del Proyecto ‘ModeloConceptual para comprender el riesgo de los fangos tóxicos contenidos en el embalse de Flix. Propuestas para su estudio científico’, CSIC, 2006-2008- Desarrollo y experimentacion de un sistema de crecidas de mantenimiento en cascada con base en criterios fisicos y economicos para la mejora hidrosedimentaria del bajo Ebro y sus principales afluentes, CICYT (CGL2009-09770 (subprograma BTE)- Assessing and predicting effects on water quantity and quality in Iberian rivers caused by global change SCARCE, MICINN, Consolider Ingenio 2010 CSD2009-00065- Diseño y monitorización de una crecida generadora en el río Ebro aguas abajo del sistema de embalses Mequinenza-Ribarroja, Endesa Generacion SA, 2002-2011- Analisis de la dinámica de las poblaciones de macrófitos en el tramo bajo del río Ebro. URS-España & Confederacion Hidrográfica del Ebro, 2008-2010

ACKNOWLEDGEMENTS

flushing flows in the lower ebro: monitoring and modelling · lower ebro: monitoring and modelling...

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