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Hazard assessment in rock cliffs at Central Algarve (Portugal): A tool for coastal management

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Coastal hazards are in the interface of human activities with natural coastal processes. The conflicts arising from this relationship require new approaches suitable for coastal management that consider the dynamic of coastal areas. A method to assess hazard in rock cliffs is presented, combining cliff evolution forcing mechanisms along with protection factors, according to a weighted factors system. This method provides a rapid evaluation of vulnerability for cliffed areas, supporting coastal management and hazard mitigation. The method was applied to the rocky cliffs of the densely populated coastal zone between Galé and Olhos de Água (Southern Portugal), where high and very high hazard values were found to be dominant. A method validation was made using the vulnerability areas and the recorded mass movements over a 45 year period in the same area.
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... In the last decades, several coastal hazard assessment approaches have been proposed all over the world to support decision making in coastal risk management and land-use planning (De Pippo et al., 2008;Del Río and Gracia, 2009;Gerivani, Stephenson and Afarin, 2020;Marques, 2018;Marques, Matildes and Redweik, 2011;Nunes et al., 2009). Most of these approaches, through the weighting and combination of selected geometrical, environmental, geological and structural factors (considered the main responsible for cliff instability), provide a hazard index that can be used to generate hazard maps. ...
Article
Cliff erosion is an unstoppable natural process increasingly occurring due to climate change and frequently causing crucial georisks on rocky coasts throughout the world. The resilience of a cliff depends on a variety of environmental, geometrical, geological and geotechnical conditions that have been included in several heuristic coastal hazard assessment approaches. In order to provide new quantitative insights into the relationship between some geometrical and structural characteristics of a sea cliff (height, basal erosion, discontinuities) and its stability conditions (failure mechanism, safety factor) this paper investigates numerically how the progressive undermining of a soft rock cliff affects its mechanical behaviour. It has been found that the undermining depth plays a significant role on cliff stability, whose mechanism of collapse changes according to the overhang slenderness. When a vertical joint is present, the higher is its persistence the lower is the global safety factor. Moreover, as the joint moves away from the cliff face the safety factor decreases, the worse condition being found when it is above the notch end. The results obtained can contribute to a deeper understanding of the failure mechanisms of seacliffs, helping in a reliable assessment of coastal risk and a proper design of the mitigation measures
... Protecting the rocky cliff toe with rigid structures has been preferred in the past, and although such structures reduce the potential cliff retreat and the probability of landslides, they do not eliminate it (Lee et al., 2001 ;Stavrou et al., 2011). The complexity of rocky cliffs' erosion dynamics, and the risk they represent requires adapted solutions based on the causes and mechanisms of mass movements behaviour (Nunes et al., 2009 Sediment or rock type is a key variable in the decision-making process as each class of sediment will react differently to the implementation of a CDM. For instance, on unconsolidated low shores, sand is more likely than pebbles to be subject to re-suspension due to hydrodynamic or aerodynamic processes. ...
Thesis
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Les caractéristiques des environnements côtiers varient à l’échelle du Québec maritime. Plusieurs types d’ouvrage de protection côtière (OPC) existent pour résoudre une problématique d’érosion ou de submersion dans ces environnements. Les effets des OPC sur le système socioécologique côtier (SSEC) sont complexes en raison des nombreuses rétroactions entre les éléments hydrodynamiques et géomorphologiques qui ont aussi des répercussions sur les aspects écologiques et socio-économiques des communautés côtières. Le choix d’un OPC dépend des caractéristiques socioécologiques propres à un secteur de côte, ainsi que des effets souhaités. Cependant, entre 1980 et 2000, au Québec, les OPC ont été aménagés en situation d’urgence et en réaction aux événements de tempête, sans considération des effets indésirables qu’ils pourraient produire. L’objectif principal de cette thèse doctorale est de développer un outil d’aide à la prise de décision élaboré sur une structuration cohérente de l’information permettant de prendre en considération les données tant géomorphologiques et hydrodynamiques, qu’écologiques et socio-économiques nécessaires à l’identification des meilleures alternatives en termes d’OPC au regard des conditions spécifiques d’un SSEC et des besoins exprimés par les acteurs du territoire (professionnels et gestionnaires). Pour ce faire, plusieurs méthodes ont été utilisées : (1) des consultations des acteurs de la zone côtière; (2) un traçage par système d’information géographique des composantes du SSEC; (3) une revue et méta-analyse de la littérature sur les effets des OPC; (4) le développement d’un algorithme; (5) une application d’une analyse multicritère. La thèse est composée de deux volets : (1) caractérisation des interventions passées et établissement d’une base afin d’orienter les décisions futures; (2) développement d’une approche d’évaluation des OPC. Premièrement, en 2017, 97,6 % des OPC présents dans le Québec maritime étaient des enrochements et des murs de protection. Par le passé, en plus de l’urgence des interventions, les principaux facteurs évoqués par les acteurs consultés pour justifier leur choix d’aménagement de ces OPC étaient un manque de connaissance, de financement et de processus collaboratif. Or, les acteurs consultés en 2017-2018 ont démontré une ouverture pour l’utilisation d’une plus grande diversité d’OPC. Ils ont également soulevé un besoin d’acquisition de connaissances scientifiques sur les effets des différents OPC pour pouvoir prendre de meilleures décisions. Les résultats de la méta-analyse de la littérature internationale sur les effets des OPC sur le milieu côtier démontrent que 52,7 % des 355 sites étudiés sont des côtes basses sablonneuses, que les études portent en majorité sur les recharges de plage (40,9 %), les murs de protection (16,7 %) et les brise-lames (12,5 %) et qu’il y a une absence d’études dans un contexte de climat nordique avec la présence de glaces côtières. Ce qui suggère un déséquilibre dans les connaissances scientifiques se rapportant aux effets produits par les OPC sur des environnements côtiers variés, déséquilibre qui doit être redressé afin d’améliorer le processus décisionnel. Deuxièmement, une approche d’évaluation a été développée pour répondre au besoin d’outils d’aide à la décision soulevé par les acteurs du territoire. Cette approche est basée sur la combinaison d’un algorithme d’identification et d’une analyse multicritère. L’algorithme permet d’évaluer et de hiérarchiser des OPC en fonction de leurs effets sur les différents environnements côtiers, et ce en trois étapes. (i) La caractérisation du SSEC est effectuée au moyen d’indicateurs de suivis géomorphologiques (type de côte, substrat) et hydrodynamiques (marnage, vagues, courants). Des données cartographiques (caractérisation côtière, écosystèmes, activités et usages) et hydrodynamiques (vagues et marnage) servent à définir l’état initial des sites à l’étude. (ii) Tirés d’une revue de littérature, les énoncés d’effets observés associés aux caractéristiques environnementales qui y correspondent (type de côte et de substrat, marnage et vagues) ont été compilés dans une base de données, catégorisés et pondérés selon une échelle qualitative de pondération (-5 à 5). Cette échelle est basée sur la pertinence et sur le caractère positif ou négatif des énoncés. (iii) L’information est traitée par l’algorithme sur la base d’une correspondance entre les caractéristiques environnementales du site d’étude et celles enregistrées dans la base de données. L’évaluation et la hiérarchisation des OPC sont réalisées en colligeant et en classant les effets observés connus, produits par ces OPC dans des contextes environnementaux similaires. (iv) Les résultats de l’algorithme présentent la hiérarchisation des OPC selon une structure d’agrégation à plusieurs niveaux qui peut être utilisée par les gestionnaires, les décideurs et les ingénieurs côtiers pour la planification et la conception de projets d’intervention pour protéger des infrastructures ou des milieux sensibles. L’analyse multicritère est ensuite utilisée pour hiérarchiser les OPC présélectionnés selon les résultats de l’algorithme en trois étapes. (i) Les critères d’évaluation ont été identifiés et pondérés par les acteurs du territoire selon leurs priorités dans le cadre d’une série de cinq ateliers. (ii) Les OPC ont été évalués en regard de chacun des critères et des caractéristiques socioécologiques de quatre secteurs d’études. (iii) Les OPC sont hiérarchisés avec la méthode PROMETHEE. Les résultats de la hiérarchisation montrent que le premier rang est occupé par la végétalisation dans trois des quatre sites et par l’enrochement dans le quatrième site. De manière plus générale, les résultats montrent la pertinence de l’utilisation d’une méthode d’analyse multicritère et de l’implication des acteurs du territoire dans le processus de sélection d’un OPC qui tienne compte des priorités locales et soit adapté aux conditions environnementales. Globalement, cette thèse offre des connaissances permettant d’améliorer le processus décisionnel menant à la sélection d’un OPC. Elle est appuyée sur une approche intégrée et holistique d’identification des OPC adaptés aux conditions spécifiques d’un SSEC, en tenant compte d’une part des effets des OPC sur l’évolution du SSEC et, d’autre part, des besoins exprimés par les acteurs du territoire.
... Following this division of the study area in TUs, a set of cliff failure predisposing factors was defined (f 1 , …, f 9 ), which could be obtained with the desired level of detail adequate for a regional scale of analysis, which were correlated with the cliff failures identified and mapped (Nunes et al., 2009;Marques et al., 2011Marques et al., , 2013Marques, 2018) (Fig. 5). ...
Article
The considerable increase in the occupation of coastal areas requires strategies to conceive and apply hazard prevention and mitigation measures, especially in coastlines with sea cliffs where mass movements are more likely to occur. To predict the potential of sea cliff failures and identify the more unstable cliff sections, analytical, physical and/or statistical tools can enable the assessment of cliff failure susceptibility, which is a progress towards a better understanding of sea cliff failure hazard assessment and mapping. In this study, statistical approaches for the assessment of sea cliff failure susceptibility and definition of safety set-back lines located landwards of the cliff top crest were adopted, in the coast located to northeast of Peniche, between Baleal and the Óbidos lagoon, in the western coast of Portugal. For the susceptibility assessment, the logistic regression was used to correlate a set of predictive factors, obtained from thematic maps of geology (lithology, structure, faults), geomorphology (cliff height, slope angle, exposure, curvature, toe protection) and wave action, with an inventory of cliff failures that caused cliff top retreat, compiled from aerial photographs from 1947 to 2010. This method enabled the production of cliff failure susceptibility models using terrain units with different sizes, encompassing cliff crest length segments of 50 m, 20 m, and 10 m. The ordinal logistic regression was applied for the setback lines definition. The modeling results were analyzed using diagnostic measures and external validation, with each model being validated against the cliff instability inventory which indicated very good statistical performance and allowed the production of reliable susceptibility maps. The application of such statistical techniques enables estimation of the degree of susceptibility for cliff failures at regional scales of analysis, and assists the definition of cliff setback lines, which provide answers to planning and hazard prevention problems of coastal areas with sea cliffs.
... In Portugal, Marques et al. [36] undertook a statistically-based study to assess the capacity of a set of conditioning factors to express the occurrence of sea cliff failures at a regional scale. In the same country, Nunes et al. [37] presented an expeditive method to assess hazard in rock cliffs by combining cliff evolution forcing mechanisms along with protection factors according to a weighted factors system. ...
Article
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High coastal sectors constitute the most widespread coastal environment and, under the present accelerated sea-level rise scenario, are suffering huge impacts in terms of erosion. The aim of this paper is the proposal of a new methodological approach for the assessment of their susceptibility to erosive processes. The method is based on the combination of two matrices, i.e., a matrix considering the main physical elements (essentially morphological and geotechnical characteristics) that determine the proneness to erosion of a specific high coastal sector, and a forcing matrix, which describes the forcing agents affecting the considered sector. Firstly, several variables were selected to construct each one of the two matrices according to existing studies and, in a second step, they were interpolated to obtain the susceptibility matrix (CSIx). The approach was applied to Procida Island and Cilento promontory, both located in southern Italy. Results obtained were validated by comparing them with cliff retreat data obtained by means of aerial photographs and satellite images. The analysis shows that the greater part of the analyzed high coastal sectors belongs to the high-susceptibility class due to the combination of adverse morphological, geotechnical and forcing characteristics. Such sectors can be considered “hotspots” that require an increase in monitoring programs and, at places, urgent protective actions.
... Being located at the physical contact of land and water basin (Fu et al., 2017;Alicandro et al, 2019;Pugliano et al., 2019;Liu, 2017;Appeaning et al., 2008), coastal zones are natural territories with a complex geological-geomorphological structure, unique ecological environment, biological diversity and climatic conditions (Ashis, 2002;Zollini et al., 2020). In recent times, coastal areas have become vulnerable to the effects of natural disasters (typhoons, tornadoes, tsunamis, abrasions, floods, etc.) and anthropogenic impacts (deforestation of coastal zones, exploitation of sediments at the mouth of rivers, construction of coastal dams and breakwaters, etc.) and are home to more than 45 % of the world's population with increasing density (Mubarak, 2018;Dominici, 2020;Jackson et al., 2013;Nunes, 2009;Komar, 2010). ...
Article
Based on the analysis performed in the Kura geomorphological sub-region of the Caspian coastal zone, it was determined that the sea level rise of 2.43 m in 1976–1996 caused an average annual rate of erosion processes about 75 times higher than accumulation processes. In 1996–2019, the stabilization and the following drop of the sea level of 1.39 m led to an average annual rate of accumulation processes up to 14 times higher than the average annual rate of erosion processes. This pattern is observed along the entire coastline of those periods. Erosion processes in the initial stage (1976–1996) increased the indentation and length of the coastline by almost 2 times, and in the next stage (1996–2019) the intensification of accumulation processes resulted in the smoothing and reduction of the coastline by 35 %. Statistical analysis of coastlines was carried out by processing Landsat satellite images for 1976– 2019; each coastline was divided into 14 segments according to geomorphological zones and their morphostructures in the study area. The development forecast for the next 10 and 20 years is provided.
... A ocorrência de morfologia cársica, principalmente de algares, foi identificada como o principal mecanismo responsável pelos elevados valores de recuo das arribas rochosas no Algarve meridional (Marques, 2008). Nestas, as taxas de erosão costeira aumentaram substancialmente, duplicando em vários sectores ao longo do toda a extensão nas últimas 4 décadas (Teixeira, 2004(Teixeira, , 2006Marques, 2008;Nunes et al., 2009Nunes et al., , 2011Bezerra et al.,2011). A perda de território, que é potenciada pela densidade de algares perto da linha de costa, tem repercussões socioeconómicas. ...
Book
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Recent research at University of Algarve. Divulgation to general public (in Portuguese)
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The intricate convergence of tectonic plates and the interplay between landmasses and oceans in subduction zones give rise to marine geo-hazards, encompassing catastrophic events in marine environments, posing significant risks to ecosystems, coastal communities and infrastructure. The Makran Subduction Zone (MSZ), with its remarkable history of devastating earthquakes and tsunamis, is a subject of significant attention from both academic and industrial sectors in recent decades. In this comprehensive review, we investigated various marine geo-hazards in the north Arabian Sea (NAS), particularly those associated with the MSZ, providing valuable insights for risk mitigation in the coastal regions with a population of over 45 million. The review employed bibliometric methods to comprehensively analyze relevant publications from databases such as Web of Science, Scopus, and China National Knowledge Infrastructure. By conducting a systematic review of 133 publications, this study deepens our understanding associated with MSZ, uncovering 07 distinct categories of geo-hazards. The earthquakes and tsunamis hazards have received extensive attention, with a tentative recurrence interval of around 500 years, while the remaining categories, including seabed fluid flows, mud-volcanism, sub-marine mass movements, subsidence, and erosion, were similarly explored in their respective order. The eastern side of the MSZ demonstrated greater instability compared to the western side, attributed to the ongoing subduction process. The ‘Gang of Four’, consisting of faults, has been identified as a primary causative factor for seismic activity in the NAS, largely influenced by transpressional tectonics. The identified geo-hazards exhibit complex interdependencies, where the initiation of one hazard can amplify the severity of another. An integrated approach is essential for assessment of the complex and interrelated risks and hazards. The research emphasizes the significance of long-term seafloor observatories in the MSZ for real-time monitoring, enabling proactive management and mitigation strategies to address these geo-hazards effectively.
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Decision-making in a coastal socio-ecological system involves managing a site-specific complexity arising not only from the interaction between hydrodynamic and morphological conditions, but also from the interactions between human structures and ecological systems, as well as the conflicting needs and interests of local actors. In addition, the projections of climate change impacts on coastal systems have a degree of uncertainty, which increases the general unpredictability of coastal dynamic behaviour, and adds a layer of complexity to the decision-making process. Decision-making in a coastal socio-ecological system involves managing a site-specific complexity arising from the interaction between hydrodynamic and morphological conditions, and resulting in uncertain patterns. In addition, projections show that sea level will rise in the near future, thus increasing the uncertainty of coastal dynamic behaviour, and adding a layer of complexity to the decision-making process. Scientific knowledge can help reduce some of the inherent uncertainties, and is essential when it comes to making sound decisions on the choice of appropriate coastal defence measures (CDMs) that are adapted to specific coastal environments and improve the resilience of coastal communities. This paper is based on a meta-analysis of 355 CDMs case studies drawn from 301 publications. From these published case studies, the objectives were to analyze the geographical and physical contexts in which CDMs monitoring was carried out and, based on the findings, to recommend areas of improvement necessary to help make sound decisions in any type of coastal environment. The meta-analysis showed that study sites are not evenly distributed around the world. Most originate from Europe (n=106), the USA (n=151) and Australia (n=30), while few studies have been carried out in Africa and Asia where dense population resides in high-risk zones. Also noticeable is the absence of sites in high latitude climates where ice plays a major role in the erosion process. Five basic variables (coastal type, sediment type, wave characteristics, tidal range, and currents or sediment transport characteristics) are used in publications to characterize study sites according to their physical components. However, only 13 of the 355 sites included a complete characterization using the 5 variables, of which coastal and sediment types are the most frequently identified (77.2% and 72.7% respectively). In general, CDMs are studied in the context of unconsolidated low shore (59.4%) and in sandy environments (74.6%). Information on tidal range, wave climate, and currents, or sediment transport characteristics, is much scarcer. Since 1990, 3 of the 10 CDMs identified in the studies have received more attention than the others; these are beach nourishments, seawalls and breakwaters, with respective cumulative study sites of 164, 67 and 50. The geomorphological effects of CDMs are the most studied (55.1%), followed by ecological (31.2%), hydrodynamic (9.1%), and social (4.6%). Overall, this meta-analysis helped identify knowledge gaps regarding geographical and physical contexts in which CDMs monitoring was held. It also gave an indication of the kind of improvement necessary for global-scale adaptation planning, and for a better decision-making process to reduce coastal risks in the most vulnerable coastal communities. Finally, the analysis shows that 4.4% of the studies on defence measures include monitoring of their effects on the coastal zone. A conceptual scheme is proposed for the evaluation of adaptation solutions based on the global monitoring of coastal zones to measure coastal change trajectories in the context of climate change.
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O litoral do Barlavento do Algarve é dominado por morfologias de erosão, essencialmente representadas por arribas talhadas em rochas carbonatadas do Miocénico, cuja geodinâmica natural determina a existência de risco quer para os utentes das praias por elas suportadas, quer para as estruturas implantadas no topo das arribas. Aqui apresentam-se os resultados do registo contínuo e sistemático dos movimentos de massa nas arribas do litoral compreendido entre as praias de Porto de Mós (Lagos) e Olhos de Água (Albufeira), no período entre Outubro de 1995 e Setembro de 2002, e analisam-se as relações entre a ocorrência de rupturas e a actividade dos agentes mesológicos. Os resultados mostram relação directa anual e mensal entre a magnitude da precipitação ou a frequência de tempestades e a ocorrência de movimentos de massa. A uma escala temporal mais fina, verifica-se uma resposta imediata das arribas perante os picos de actividade dos factores externos, num período que não excede cinco dias e que, mais frequentemente, não ultrapassa três dias. A precipitação e a agitação marítima de tempestade são os factores externos mais importantes no desencadeamento do processo de ruptura das arribas, sendo possível associar a sua intervenção a cerca de 75% das ocorrências. A influência da intervenção antrópica nas rupturas é marginal, havendo sido identificada em cerca de 5% dos movimentos registados.
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Dias, J. Alveirinho (1984)-Evolução geomorfológica das Arribas do Algarve. 3º Congresso sobre o Algarve. Textos das Comunicações, 2:705-712, Racal Clube, Silves, Portugal
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The United States Federal Emergency Management Agency (FEMA) is assessing technical methodologies and procedures for the collection, analysis, and computation of coastal erosion rates. It is likely that the methodology selected will involve the use of historical shoreline data compared with current shoreline information. This paper largely addresses the sources of errors inherent in the raw data. Source data include historical and recent National Ocean Service (NOS) T-sheets (produced ca. 1840s to present) and air photos (taken ca. late 1930s to present) as well as any other types of accurate map and photographic data. -from Authors
Book
The text begins with an introduction to concepts and terminology, and the factors that have affected coastal evolution and coastline changes (Chapter 1). This is followed by a discussion of waves, tides, currents and other nearshore processes (Chapter 2), and a study of the effects of land and sea level changes, notably the Holocene marine transgression, which has played a major part in shaping modern coastlines and can be regarded as a unifying theme in coastal geomorphology (Chapter 3). Cliffs are discussed in Chapter 4 and the shore platforms that border them in Chapter 5. Chapter 6 deals with the origin of beaches and the changes taking place on them, and Chapter 7 with the beach erosion problem. Spits, barriers and bars are discussed in Chapter 8 and the formation of coastal dunes in Chapter 9. Intertidal wetlands, including mudflats, salt marshes and mangroves are dealt with in Chapter 10, followed by estuaries and lagoons, including other inlets (rias, fiords, fiards, calanques, sharms and sebkhas) in Chapter 11. Chapter 12 considers deltas produced by deposition at river mouths, and Chapter 13 reviews coral and algal reefs. Chapter 14 deals with future coasts. There are 187 illustrations and 15 tables. Details of articles cited are given in the References section, which includes many pre-2000 publications that remain relevant.
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Digital, aerial orthophotomosaics with 0.5-3.0 m horizontal accuracy, used with NOAA topographic maps (T-sheets), document past shoreline positions on Maui Island, Hawaii. Outliers in the shoreline position database are determined using a least median of squares regression. Least squares linear regression of the reweighted data (outliers excluded) is used to determine a shoreline trend termed the reweighted linear squares (RLS). To determine the annual erosion hazard rate (AEHR) for use by shoreline managers the RLS data is smoothed in the longshore direction using a weighted moving average five transects wide with the smoothed rate applied to the center transect. Weightings within each five transect group are 1,3,5,3,1. AEHR's (smoothed RLS values) are plotted on a 1:3000 map series for use by shoreline managers and planners. These maps are displayed on the web for public reference at http://www.co.maui.hi.us/ departments/Planning/erosion.htm. An end-point rate of change is also calculated using the earliest T-sheet and the latest collected shoreline (1997 or 2002). The resulting database consists of 3565 separate erosion rates spaced every 20 m along 90 km of sandy shoreline. Three regions are analyzed: Kihei, West Maui, and North Shore coasts. The Kihei Coast has an average AEHR of about 0.3 m/yr, an end point rate (EPR) of 0.2 m/yr, 2.8 km of beach loss and 19 percent beach narrowing in the period 1949-1997. Over the same period the West Maui coast has an average AEHR of about 0.2 m/yr, an average EPR of about 0.2 m/yr, about 4.5 km of beach loss and 25 percent beach narrowing. The North Shore has an average AEHR of about 0.4 m/yr, an average EPR of about 0.3 m/yr, 0.8 km of beach loss and 15 percent beach narrowing. The mean, island-wide EPR of eroding shorelines is 0.24 m/yr and the average AEHR of eroding shorelines is about 0.3 m/yr. The overall shoreline change rate, erosion and accretion included, as measured using the unsmoothed RLS technique is 0.21 m/yr. Island wide changes in beach width show a 19 percent decrease over the period 1949/ 1950 to 1997/2002. Island-wide, about 8 km of dry beach has been lost since 1949 (i.e., high water against hard engineering structures and natural rock substrate).