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Ocean & Coastal Management 102 (2014) 285e293

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Ocean & Coastal Management

journal homepage: www.elsevier .com/locate/ocecoaman

Coastal hazards from slope mass movements: Analysis andmanagement approach on the Barlavento Coast, Algarve, Portugal

Sebasti~ao Braz TeixeiraPortuguese Environment Agency, Rua do Alportel 10, 8000-503 Faro, Portugal

a r t i c l e i n f o

Article history:Received 19 May 2014Received in revised form12 October 2014Accepted 14 October 2014Available online 21 October 2014

Keywords:Rocky sea cliffsHazard areasBeachesRunout ratioAlgarvePortugal

E-mail address: sebastiao.teixeira@apambiente.pt.

http://dx.doi.org/10.1016/j.ocecoaman.2014.10.0080964-5691/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Barlavento Coast, Algarve, Portugal, is dominated by rocky sea-cliffs, cut on Miocene calcareniteswhich evolves through intermittent and discontinuous events of slope mass movements, along a 46 kmcliff front. Here, the main coastal geologic hazards result from the conflict between human occupationand sea-cliff recession. Most of the research on the dynamics of the cliffs has been directed to the riskwith the aim of defining long term set-back lines, for a preventive planning of the cliff top occupation.Little attention has been given to the hazard associated with mass movements on bathing beachesbacked by sea-cliffs. This article presents the results of a field inventory of 244 slope mass movementssingle events, collected in a rocky shore with tens of touristic pocket beaches, covering an nineteen yeartime span (1995e2014). Results show that landslides have seasonal pattern with higher incidence in theperiod between winter and early spring. More than 15% of movements occur during the Easter holidays(April) and 4% of landslides occur during the official bathing season (JuneeSeptember). The spatialdistribution of landslides shows that only 22% of the mass movements occur in capes and headlands,while 78% occur on the beaches, which demonstrates that the beaches are real hot spots of risk. Based onthe size distribution of slope mass movements runout ratio (the ratio between the radius of the base ofthe cone of and the height of the movement) a table of levels of security and hazard on beaches was built.Security levels enable the definition of cartographic hazard areas on beaches which can be provided tothe beach users on information boards at the beach entrance.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Algarve coastal region, with 451,000 inhabitants in 2011, is thepremier tourism destination of Portugal, with a mean annual of 15million overnight stays through the 2002e2011 period (INE, 2012).Almost 90% of the visitors' reasons of staying in the Algarve coastare leisure, recreational and holiday (INE, 2012). The BarlaventoCoast between Lagos and Albufeira (Fig. 1) is the core touristic areaand receives 60% of the Algarve visitors attracted with the “sun andbeach”, which is the Algarve' s top touristic product. About 50% ofthe Algarve visitors stay and bathe in beaches backed with rockysea cliffs of the Barlavento Coast.

Over the last three decades, there has been record of severalaccidents caused by the collapse of sea cliffs cut on Miocene rocks.On 22March 1998, a Portuguesemanwas killed while fishing at thecliff edge at the Mar�e das Porcas site, when a sudden planar land-slide dragged him down together with a volume of 2 � 104 m3

falling material; on 7 October 2000, three Swiss tourists were

injured by a block fall (volume 2 m3) on the Inatel beach; on 21August 2009 an instantaneous topple (volume 1 �103 m3) on a seastack killed five Portuguese tourists and injured another two,resting on the cliff base on the Maria Luísa beach; on 26May 2010 afour year Irish kidwas slightly wounded on the Vau beach hit by thedebris of a landslide (Fig. 2, volume 2� 102 m3); on 11 October 2010a German tourist was injured on the Beijinhos beach, hit by blocksof a small landslide (volume 1 m3). This record shows that theaverage number of accidents resulting directly from cliff collapse ofsea cliffs is 2 event/decade, causing two fatalities and two injurieseach decade.

Searching for the accidents of the cliff top walkers, mostlysightseers and fishing anglers, in the decade 2003e2012, Teixeiraand Dores (2013) identified a record of 50 accidents on the Barla-vento rocky cliffs with 11 fatalities and 41 injured. 45% of the vic-tims were foreigner tourists. Although statistically not verysignificant when compared to accidents resulting from the use ofthe top of the cliffs, accidents caused by landslides on beaches havegreat impact on public opinion. While accidents of suicide or deathof fishermen by falling from the cliffs typically occupy small news inlocal newspapers, after the collapse recorded in Maria Luisa beach

Fig. 1. Barlavento coast. Study area.

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the accident was the subject of daily news in all national newspa-pers for more than a month. Five years on, the accident is stillremembered every year at the start of the bathing season(JuneeSeptember).

Slope mass movements on steep rocky sea cliffs are instanta-neous phenomena with virtually no warning. The velocity of themovement is typically over 5 m/s, within the landslide velocityclass 7, the extremely rapid events of the Cruden and Varnes (1996)classification. On the Barlavento rocky coast, where sea cliffs aresubvertical with height varying from 5 to 40 m, the duration of theslope mass movements lies in a narrow time window of 1e2 s. Theinstantaneous nature of the phenomenon precludes any action tominimize the damage after the onset of the collapse process. Onsteep subvertical rocky sea cliffs prone to slope mass movements,the actions to minimize risk and damages are therefore exclusivelybased on prevention.

The first step in the implementation of prevention is theknowledge and definition of the areas potentially affected by amassmovement, i.e. the spatially definition of hazard areas. Hazardareas, limited by hazard lines, correspond to areas parallel to theshoreline where, in a pre-defined period, it is likely that effects ofslope mass movements will be felt (Fig. 3).

Most studies and bibliography, oriented strand planning, fo-cuses on the land hazard areas at the top of the cliffs worldwide

Fig. 2. Rock fall on the Vau beach occurred in the 26 May 2010 (locat

(see, for example, Hall et al., 2002; Lee and Clark, 2002; Moore andGriggs, 2002; Del Rio and Gracia, 2009; Stravou et al., 2011; Epifanioet al., 2013) and on Algarve (Marques, 1994, 1997, 2003; Teixeira,2003, 2006; Bezerra et al., 2011; Marques et al., 2011; Nuneset al., 2009). Very few papers deal with sea hazard areas on seacliffs (Marques, 2009), although studies on the identification of riskareas associatedwith damage caused by slopemassmovements arevery common in land areas (e.g, Copons et al., 2009; Michoud et al.,2012).

In the case of touristic areas centered on beaches backed bycliffs, the sea hazard area has particular interest in that it is in thisarea where accidents occur with people affected by the debris of amass movement. In Portugal, beach tourism is an important eco-nomic activity, all the coastal plans regulations include a seawardhazard area on beaches backed by sea-cliff extending seawardsfrom the cliff toe, where beach support structures are interdicted(Marques, 2009). On the study area coastal plan regulations, inforce since 1999, a sea hazard area is defined on beaches backed bysea cliffs with a width of 1.5 times the cliff height and correspondsto the maximum extent of cliff failures debris displacement nearthe toe.

In this paper we present the results of slope mass movementsinventories gathered on Barlavento Coast sea cliffs, for the lastnineteen years (1995e2014), we assess the adequacy of the legal

ion on Fig. 1); mean width (Wm) ¼ 2.5 m; runout ratio (R) ¼ 0.8.

Fig. 3. Hazard areas on rocky sea cliffs.

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provisions of the hazard areas and we propose a solution tospatially define the seaward hazard lines on beaches backed byrocky cliffs prone to slope mass movements.

2. Study area

The south coast of the Algarve, Portugal, located at SouthWestern end of Europe, has a typical Mediterranean climate withhot dry summers and mild rainy winters. Average annual precipi-tation is 500e600 mm, 80% of which is concentrated in a wetseason (OctobereMarch). Wave regime is moderate with meanannual significant height of 1 m (Costa et al., 2001) and 85% of seastorms (Hs � 2.5 m) occur in the wet season Tide is semi-diurnal;mean tidal range is 2 m and spring-tide range is 3 m.

The Barlavento Coast is a rocky coast with a continuous front ofsubvertical sea-cliffs cut in carbonate Mesozoic and Miocene rocks.The study area is the cliffed coast, between Porto de M�os beach andOlhos de �Agua beach (Fig. 1), cut in Miocene rocks, included in theLagos-Portim~ao Formation (Cach~ao et al., 1998). The total length ofsea cliffs cut in Miocene is 46 km as measured using detailedtopographic maps (scale 1:2000). Sea cliffs are composed of alter-nate decimetric layers of fine grained calcarenites and calcareniteswith high content of macrofossils (Manupella, 1992). Carbonatecontent is 60e75% in fine-grained calcarenites and greater than80% in fossiliferous calcarenites (Marques, 1997). Cliff height variesfrom 5 to 40 m and displays a sequence of strata, horizontal orgently sloping to the South or Southeast except in the vicinity ofmajor tectonic features of the area (the Portim~ao fault and Albufeiradiapir; Terrinha, 1998) where the slope is greater (10e20�). CoastalMiocene formations have a deep and well developed karst, with adense network of caves, sinkholes and galleries covered by Plio-Quaternary (Manupella, 1992) reddish silty-clayed sands. Thecombination of the network of karsic cavities with the action ofcoastal erosion provides a modeling of the coastline very diverseand irregular, very attractive for tourists that seek the region.

Fig. 4. Head collapse and generation of a new stack in Marinha beach, occurred on 6 Octo(R) ¼ 0.5.

The quantitative assessment of cliff retreat rates in Miocenecalcarenites was first performed by Marques (1994, 1997) based onidentification and measurements of slope mass movements bycomparative analysis of aerial photographs. The recession ratebased on values of lost area of mass movements in the 1947e1991period is 1e2 cm/year (Marques, 1997). Based on a continuous fieldinventory, of mass movements on Miocene cliffs, started in 1995,Teixeira (2003, 2006) calculated amean recession rate of 1 cm/year.Themaximum local retreat measured byMarques (1997) was 45 m,in an arch collapse in plunging cliffs that occurred between 1974and 1980; in the 1995e2014 period the maximum local retreatmeasured was 25 m in a head collapse occurred in 6 October 1998in Marinha beach (Fig. 4).

The irregular shape of the coast promotes the formation ofseventy small pocket beaches, with lengths of tens to hundredmeters, and thin sand cover about 2 to 4 m thick, accumulated overcut shore platforms. Most beaches (80%) are accumulated in theirregularities generated by erosion of the cliffs. Only sixteen of thebeaches are located in small river mouths, four of them in hangingvalleys. One third of the cliff front (15 km) has a beach on their base.The total length of the fifty official bathing beaches extends for9 km.

3. Methods

The regional coastal management authority of Algarve(Portuguese Environment Agency e APA) has been conducting acontinuous observation of the coast since 1995, which includes thesystematic recording of slope mass movements occurring on thefront of rocky sea cliffs cut on Miocene calcarenites. The very firstsource of information of the slope movements is the reporting ofoccurrences by public regional or local field authority staff, as wellas by the tourism operators, private owners and fishermen.Complementarily, a periodic systematic observation in situ of thecoast, by land, sea and air, is conducted several times a year,

ber 1998. Postcard from Ediç~ao Vistal 1992; mean width (Wm) ¼ 20 m; Runout ratio

Fig. 5. Topple occurred on 2 Out 2009, in Santa Eul�alia beach; (location on Fig. 1) Wm ¼ 1.5 m; R ¼ 1.9.

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especially after wave or rainy storms. At least, once a year acampaign of capture of oblique photographs either on a low alti-tude flight (since 2006) or on a boat tour (since 2002) is done inorder to get a complete view of the area and identify recent slopemass movements. Fresh scars on the sea cliff face, detritus coneswith fine sediment content, and a yellow plume in the water,resulting fromwashing out of the debris, provide the most obvioussigns of a recent mass movement (Teixeira, 2006). The main sourceof the inventory comes from periodical observation of the sea clifffront that enabled the identification of a considerable amount ofunreported slope mass movements (66% of the inventory). Thegathered inventory covering a nineteen year time span (July1995eJune 2014) has a total of 244 slope mass movements, with amean length�1 m, which is the threshold of completeness of in situinventories on Barlavento Miocene calcarenites (Teixeira, 2006).

After each report of an occurrence or during the periodical fieldobservation, a trip to the site is carried out and a characterization ofmass movement type and geometry is performed in situ asdescribed in Teixeira (2006). In the field inventory (244 events),mass movements were classified onto four types, according to the

Fig. 6. Karst collapse in Armaç~ao de Pera beach, in Febru

Fig. 7. Block fall on Albandeira beach, occurred on 12 Sept

generic mass movements proposed by Sunamura (1992) andMarques (1997): rock fall (Fig. 2), topple (Fig. 5), karst collapse(Fig. 6) and block fall (Fig. 7).

For the purpose of the present study, the radius of the debriscone (the distance between the cliff base and the seaward limit ofthe debris), geometrically the same as the runout or travel distanceused on landslide motions, (Hungr et al., 2005; Michoud et al.,2012) is an important parameter, crucial for computing the run-out ratio (R) parameter. The runout ratio of a slope mass movementis defined as the ratio between the radius of the base of the cone ofdebris and the height of movement, a dimensionless scalingparameter. When the mass movement reaches the total cliff face, Ris equal to the ratio between the radius of the base of the cone ofdebris and the cliff height (Fig. 8). A cliff collapse in which the coneof debris projecting from the base of the cliff at a distance equal tothe height of the cliff has a runout ratio 1; a collapse in which thecone of debris occupies the sandy beach up to a distance equal tothe height of 1.5 times the cliff height has a runout ratio 1.5.

The measure of the runout is sometimes impossible in massmovements identified weeks after the occurrence as waves tend to

ary 2011 (location on Fig. 1); Wm ¼ 1.5 m; R ¼ 1.5.

ember 2007 (location on Fig. 1); Wm ¼ 1.5 m; R ¼ 0.9.

Fig. 8. Definition of the runout ratio of a slope mass movement on steep rocky sea cliff.

Fig. 9. Stack collapse occurred on 1 Dec 2010, in Olhos de �Agua beach.

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rapidly wash out and remove the debris. In movements recorded inplunging cliffs and promontories (22% of the field inventory)sometimes is not possible to calculate the runout as the cone ofdebris extends to the immersed area below the low tide line,making it difficult to measure the radius of the debris cone. For atotal of 244 registered mass movements in the 1995e2014 period,the R was computed in only 170 (70% of the field inventory).

4. Results

After each report of an occurrence or during the periodical fieldobservation, a characterization of mass movement type and ge-ometry was performed in situ, measuring mass movement meanand maximum linear parameters (length, height, width), directlyon the cliff scar, as described by Teixeira (2006). The horizontal areaof loss was computed as the product of mean length and meanwidth, and the volume of themass movement was calculated as theproduct of horizontal area and mean mass movement height. Thevolume of the fresh debris produced by the mass movement pro-vided a supplementary control of the evaluation of dimensionalparameters.

Between July 1995 to June 2014, covering a nineteen years span,244 slope mass movements on sea cliffs, have been recorded and

Fig. 10. Cumulative absolute frequency size of mean width of mass movementsoccurred between July 1995 and June 2014 (n ¼ 244).

measured, spatially dispersed along the sea cliff front. Rock fall isthe dominant movement (61%). Topples (21%) and karst collapse(16%) have approximately the same importance. Block fall is the lessfrequent mass movement type (3%) During the study period fivestacks collapsed (Fig. 9) and two new ones were formed (Fig. 4).

A total of 2100 m of the sea cliff front (4.5% of the total cliff frontlength) was altered by slope mass movements, during the nineteenyear period of observation. The average value of the renewal of thecliff front of 0.24%/year gives an estimate for the cliffs mean lifeperiod of 425 years. The average loss of horizontal areawas 400m2/year, the mean annual volume was 7400 m3, and the averagerecession rate was 9 mm/year, which are values similar to previousestimates obtained by Marques (1997) and Teixeira (2003, 2006).

The field inventory showed an average frequency of 13 move-ments/year with a range of 2e42 annual movements. The series ofmass movement mean width (the width of the instantaneousretreat) show a magnitude decay pattern, i. e. the wider the massmovements are, the less frequent they are (Figs. 10 and 11) aspreviously showed by Teixeira (2006). Increasingly, recent studiesdemonstrate that the dimensions (width, area or volume) of clifffailures fit to inverse power-law relationships (e. g. Marques, 2008;Lim et al., 2010; Young et al., 2011; Barlow et al., 2011, Katz andMushkin, 2013).

Fig. 11. Return period distribution of mean width of mass movements occurred be-tween July 1995 and June 2014 (n ¼ 244).

Fig. 12. Annual distribution of maximum date uncertainty of slope mass movements.Year starts in July and ends in June.

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4.1. Date uncertainty

The effort for permanent observation of the 46 km long coastand the level of dating of the slope mass movements is such that77% of the events are monthly dated and less than a half of theevents are daily dated (46%). In 94% of the occurrences the date ofthe event is within a tree month period. Maximum uncertainty ofthe date of the movements is less than a half year (Fig. 12).

4.2. Monthly distribution

The dataset dated monthly (monthly inventory, 188 events,Fig. 13) enable more detailed analysis of the annual distribution ofmass movements and their relationship to the beach use. Theseasonal distribution of monthly dated movements shows clearseasonal pattern with predominance of cliff failures during the wetseason, where 83% of cases occurred between November and April.In half of the movements in which the exact day is known thecollapse occurred under sea storms (Hs � 2.5 m) or daily rainfallexceeding 10 mm. 78% of failures occurred within five days afterstorms or heavy rain.

A closer and detailed look at the monthly distribution of thecollapses suggests a two stage dynamics. The forerunner stagehappens at the beginning of autumn when a first peak of collapsesoccurs in October triggered by the first heavy rains and sea storms.

Fig. 13. Monthly distribution of slope mass movements (n ¼ 188), rain and sea storms. PrecipInstituto Portugues do Mar e Atmosfera (IPMA).

At this stage, the destabilizing effect of the water seems to have animmediate effect. The second stage of failures that extendsthroughout the winter has the highest expression in early spring.Here the progressive increase in the frequency of collapses suggeststhe preponderance of the cumulative undermining effect of water.Storms and heavy rains in early spring seem more effective intriggering cliff failure than in autumn.

In fourmovements itwaspossible to relate thecollapsewithdirecthuman intervention. The most dramatic episode was the break in aswimming pool in June 1997 previously reported by Teixeira (2006).The other three are associated with the concentration of runoffresulting from poor sealing and channeling rainwater in May 1997,October 2008 and March 2010. In 18% of the monthly inventory thetrigger is unknown, possibly associatedwith fatigue failure of the cliffmaterial. Only 4% of mass movements occurred during the officialbathing season (June to September) but three of the eight recordedcliff failure resulted in casualties (5 fatalities and 6 wounded).

4.3. Runout ratio size distribution

The amplitude of the runout ratio covers the 0.1e1.9 range andvaries with the type of mass movement (Fig. 14). The most frequenttype of movement (rock fall, 98 records) presents a normal type dis-tributionwithameanvalueof0.90. In the caseof topples (34 records),with more pronounced horizontal component, the distribution alsoresembles a normal distribution with mean value 1.11, but withgreater variance. The karst collapse failures produce the lowest R(mean 0.49, 30 records). Values show great dispersion, with a modecentered in the lower values associated with the prevalence ofmovements with almost exclusive vertical component. In the case ofblock fall (8 records) values have very tiny dispersion, with all valuesin between 0.75 and 1.0. Aggregating the result set (Fig. 15) it isapparent that in all the results fit a normal distribution (mean 0.88),with a secondarymodeonvalues of the lower runout ratio, associatedwith movements with predominance of the vertical component.

5. Discussion

5.1. Beaches as hot spots

Of the 244 mass movements recorded in field inventory, 190(78%) occurred in the cliffs of the beaches. The remaining 54 (22%)

itation data from Algoz station (Agencia Portuguesa do Ambiente); sea storm data from

Fig. 14. Runout ratio distribution according to slope mass movement type (n ¼ 170).

Fig. 16. Distribution of the probability of exceedance of the runout ratio (n ¼ 170).

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were recorded in plunging cliffs and promontories. Over theobservation period the field inventory gives a ratio of 12 move-ments/km on beaches whereas in the remaining area that ratio is1.7 movement/km. Rocky coast sheltered areas (where beachesaccumulate) are sevenfold more prone to slope mass movementsthan exposed sites (headlands and promontories). From the pointof view of coastal erosion Barlavento beaches are real hotspots.

Results show that the most vulnerable areas are not located inthe areas most exposed towave attack despite wave action is one ofthe most effective triggers on the evolution of sea cliffs. Plungingcliffs are permanently exposed to direct wave action, promontoriesare daily exposed (at flood tide), while in cliffs backed by beacheswave action acts episodically under storm conditions and springtides. This dynamic pattern of the Barlavento cliffs coincides withthe evolutionary pattern of rocky cliffs defined by Sunamura (1992)in which plunging cliffs occur on resistant rocks and shore plat-forms develop on weaker rocks. Bezerra et al. (2011) used a wavepropagation model to quantify breaking wave conditions and waveenergy to estimate the relative role of wave action and lithology incontrolling cliff erosion and evolution along a 12 km of Miocenecalcarenites of Central Algarve. They found that lithology repre-sents the dominant control on mass movement occurrence. Whenthe distribution of mass movements along the coast is analyzedwithout considering the lithological variation, there is no rela-tionship between the number and displaced volumes of massmovements and wave energy for each sector, with the majority ofthe movements and the greater volumes occurring in the leastenergetic sector. Although subject to higher wave energy, capes andpromontories are more resistant to erosion than the adjacent areas.On the other hand, the most sheltered areas (where beachesaccumulate) are more prone to slope mass movements.

Fig. 15. Runout ratio distribution of all mass movement types (170 events).

5.2. Hazard and security zones

Data collected in situ at 170 movements between 1995 and 2014have resulted in the size distribution of runout ratio illustrated inFig. 15. The distribution of the probability of exceedance shown inFig. 16 allows setting security levels (inverse of hazard levels) forusers depending on the runout ratio value (Table 1). The securitylevel of 95% (risk level of 5%) is reached for R¼ 1.5, that is to say thatif a mass movement occurs on a beach backed by cliffs, a userresting on the sand at a distance of 1.5� height of the has a 95%probability of not being struck by debris that collapse (and 5%probability of being hit). This probability drops to 63% if the user isat a distance from the base equal to the height of the cliff and falls to20% if he stands at a distance from the base equal to half the heightof the cliff. The hazard level equals the level of security for a runoutratio value of 0.86.

5.3. Reducing risk by improving information

Given the described concentration of mass movements on thebeaches backed by sea cliffs cut on Miocene calcarenites and takinginto account the intense occupation of beaches by tourists is theimmediate conclusion that in the Algarve coastal managementmust focus on risk. On beaches, risk can be reduced in two ways:reducing exposure (removing users from areas prone to slope massmovements) or decreasing the vulnerability of the cliffs to slopemass movements acting with measures of active intervention.Provide information on the risk to the users of the cliffs and beachesis basic low-cost preventive action.

After the accident occurred at Maria Luisa beach the Algarvecoastal authority proceeded to significant enhancement of rock fallhazard signaling. Information boards with the definition of riskareas were placed on every access to the beaches. This initiativeaims to provide all tourists with information and knowledge ofhazard areas at each beach.

The distribution of the runout set enabled to draw a boardmodel that discriminates two bands with different hazard levels,

Table 1Hazard and security level as a function of runout ratio value.

Exceedance probability (hazard level) Security level Runout ratio

50% 50% 0.8625% 75% 1.1110% 90% 1.385% 95% 1.500% 100% 2.0

Fig. 17. Model of sea cliff hazard sign.

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limited by the line of R ¼ 0.86 (Fig. 17). The area of high hazardcontains the fringe between the cliff base and a distance of 0.86�the cliff height, which corresponds to the zone in the event of amass movement occurs, the probability of being occupied by debrisis greater than the probability of not being occupied. The range ofmoderate risk is contained in the range of 0.86e1.5 runout ratio, thearea in which, in the event of a mass movement occurs, the prob-ability of being hit by mass movement debris is less than 50%.

6. Conclusions

The pattern of occupation of the Algarve tourism and the naturalgeodynamics of the rocky sea cliffs, reflected in the discontinuousand intermittentoccurrence of slopemassmovements, determiningthe existence of risk to users of beaches backed by rocky cliffs. Thecontinuous inventory ofmassmovements collectedduringnineteenyears in the Barlavento Coast (46 km of cliffs that support 15 km ofpocket beaches) shows that on average 13 mass movements occurper year, mainly rock fall type, concentrated during the winter andearly spring. However, during periods of increased beach use occur aconsiderable number of movements: during Easter holidays (April)about 16% of annual slope mass movements occur and during theofficial bathing season (JuneeSeptember) occur 4% of annualmovements. The spatial distributionof slopemassmovement showsthat 78% of movements occur in cliffs that support beaches, incontrast in natural resistant headlands and promontories occur only22% of mass movements.

The analysis of the size distribution of the runout ratio discussedin this article confirms that the coastal plan regulations are welladjusted to the field reality. The width of sea hazard area as definedin those regulations corresponds to a security level of 95% for usersof the beaches. The size distribution of the runout ratio permits thedesign of continuous distribution of levels of security and hazard

along the sandy beaches, which may be reproduced cartographi-cally. As a preventive measure of risk, the information collectedmay be provided to users, on boards at the beach entrance, in orderto contribute to a more aware and safe use of the beach areaavailable. Mapping of hazard areas should be updated whenever amass movement that will significantly change the configuration ofthe cliff occurs.

Signaling is the cheapest action to prevent risk on beachesbacked by rocky cliffs. On beaches with increased use or where thecliffs show higher activity other measures progressively moreexpensive can be used, like beach nourishment. Beach nourishmentof pocket beaches, besides the increased protection of the cliffsunder the action of the sea, provides extra area available for sun-bathers out of the cliffs hazard zones. The hazard mapping pre-sented in this article is very relevant information in the design ofany sandy replenishment project.

Acknowledgments

The author thanks to Marcos Rosa, Celso Pinto, Ricardo Gomesand Fernando Engr�acia for their assistance in fieldwork. The authorgratefully thanks the anonymous reviewers for their helpful com-ments and suggestions that significantly improved the manuscript.This is a contribution of SHORE Project (PTDC/MAR-EST/3485/2012)funded by the Portuguese Foundation for Science and Technology(FCT).

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