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- Regional assessment of storm related overwash and breaching hazards on coastal barriersPublication . Plomaritis, Theocharis A.; Ferreira, Óscar; Costas, SusanaCoastal communities are threatened by the impact of severe storms that may cause significant loss of life and damage to properties. Among the main processes behind such impacts on coastal barriers are the occurrence of overwash and breaching during storm events. In order to estimate potential losses associated with a particular event, the above processes must be properly parameterized. Here, we propose a novel methodology to estimate overwash and breaching hazards suitable for a regional scale analysis (Ø 100 km). For the overwash hazard assessment, the method is based on the application of the approach developed by Donnelly (2008) that allows the parametrisation of the overwash hazard considering both flow velocity and flow depth. Moreover, the inland extension of the associated hazard, which is critical to assess subsequent vulnerability, can also be estimated following this methodology. The proposed method requires the selection of a runup formula validated for the study area, a storm beach profile, a runup lens angle, and a percolation constant for infiltration. To assess the breaching, hazard, a new multivariable evaluation is proposed that allows ranking the potential of breaching. The multivariable evaluation combines overwash and erosion hazards as well as their extensions with the main morphological characteristics of the barrier, resulting in the breaching hazard index, that ranks from 0 to 5 (no breaching to inlet formation). Inland breaching extension is also relevant for the vulnerability assessment. The breaching extension can be estimated using historical or contemporary analogues of the nearest flood deltas. The developed approaches were applied to Ancão Peninsula (Algarve, Portugal) as a demonstration example. The advantages of the present approach are: adaptability to various environments where overwash and/or breaching processes are important, time efficiency on evaluating overwash and breaching hazards, and the assessment of hotspot areas at a regional scale.
- Barrier island resilience assessment: Applying the ecological principles to geomorphological dataPublication . Kombiadou, Katerina; Matias, Ana; Costas, Susana; Carrasco, A. Rita; Plomaritis, Theocharis A.; Ferreira, ÓscarApplying the ecological resilience principles to barrier island geomorphological evolution requires approaches that perceive and interpret resilience far from predefined barrier characteristics and static views. Accepting that barrier islands, like all natural systems, are dynamic and adaptively changing in response to external disturbances is fundamental to the formulated approach. To this aim, geomorphological units and dimensions were used to describe barrier island stability landscape as an actively shifting ‘topography’, reshaping in response to exogenous events and in relation to intrinsic properties. The structure of the subaerial barrier was characterised using the environmental units of Beach, Dune and Marsh (or BDM), where different combinations of BDM structure define distinct barrier stable states, under a simplified framework that is applicable over a wide range of barrier structures. The methodology is based on reconstructing resilience trajectories of barrier islands through identifying the distinct BDM states and related shifts (thresholds crossed) and assessing resilience dimensions (latitude, resistance and precariousness defined as barrier width and height and proximity to mainland, respectively) that, jointly, define the shape of the stability domain. The approach was applied to the Ria Formosa barrier island system (S. Portugal), using multi-decadal geomorphological data and gradually decreasing spatial discretisation, passing from individual barrier transects to sectors and to entire barriers. The joint evolution of two inland-migrating barriers (Cabanas Island and Cacela Peninsula) was used as an exemplar of adaptive capacity in barrier geomorphic change and, therefore, of resilient response to external pressures. Resilience pathways showed that the Ria Formosa barriers have been resilient over the studied timeframe, with a tendency for maintaining or increasing BDM structural complexity. In general, the stability domain tends to shift from low latitude and high resistance forms (narrow-deep basins of attraction) in the west part of the barrier chain, to higher latitudes and lower resistance ones (wide-shallow basins) towards the east. Precariousness peaks near the edges of the system (low lagoon width) and minimises towards the central part (most detached barriers). Scaling issues regarding smoothing of longshore variability and potential consequences on masking thresholds and critical dimensions are highlighted and discussed, along with the key role of the meaning of specified resilience (of what?) in the assessment. The methodology is a novel approach, easily transferable to different systems and spatiotemporal scales of analysis, representing a step forward in interpreting and assessing barrier island resilience.
- Exploring foredune growth capacity in a coarse sandy beachPublication . Costas, Susana; Bon de Sousa, Luísa; Kombiadou, Katerina; Ferreira, Oscar; Plomaritis, Theocharis A.Dunes are key elements of coastal landscapes in almost every latitude. They host high levels of biodiversity and provide important benefits to society; e.g. protection against floods and erosion, or recreation. Coastal dune growth is constrained by intrinsic factors, which are critical when managing dune systems or choosing coastal dune restoration as an alternative green solution for coastal protection. Here, the evolution of a beach-dune system, characterized by a reflective coarse sandy beach and low dunes, is explored to identify the favourable and optimal conditions for dune growth in these settings. Dune growth capacity is evaluated by analysing the topographical changes observed along a coastal dune over two different temporal scales (interannual and event scale) and comparing the observations with theoretical approximations of sediment transport potentials. Observations and predictions over interannual scale document that (1) temporal variability in external conditions (wind regime) and spatial variability of estimated wind fetch length, alone, fail to explain alongshore dune growth patterns and (2) optimal conditions for dune growth occur when storms (strong winds) impact the study area, jointly with low runup levels, at zones of shoreline progradation and absence of direct human influence. Conversely, lowest values of dune accumulation are associated with areas where shoreline retreat was documented. Observations from event timescales suggest that sediment transport potential can be reached over zones with no significant signs of beach erosion, if runup levels remain low and the event duration is shorter than the time scale of sand surface depletion within the upper beach.
- Process-based indicators to assess storm induced coastal hazardsPublication . Ferreira, Óscar; Plomaritis, Theocharis A.; Costas, SusanaStorms are responsible for several hazards (e.g. overwash, erosion, inundation) in coastal areas, leading to the destruction of property and loss of life in populated areas. Various indicators are used to express potential storm impact and describe the associated hazards. The most commonly used indicators include either forcing parameters (e.g. wave height, sea level) or coastal morphologies (e.g. dune height or berm width). Whereas they do not represent the processes associated with storm induced hazards in coastal areas. Alternatively, a hazard could be better characterised if process-based indicators are used instead. Process-based indicators express the result of the forcing mechanisms acting over the coastal morphology and reflect both hydrodynamic and morphological characteristics. This work discusses and synthesizes the most relevant process-based indicators for sandy shores subject to overwash, erosion and inundation promoted by storms. Those include: overwash depth, potential and extent; shoreline, berm or dune retreat; vertical erosion; and inundation depth and extent. The selection of a reduced set of process-based indicators to identify coastal hazards induced by storms in sandy coasts will facilitate comparison of different coastal behaviours for distinct storm return periods, and help to optimise coastal management plans, thereby contributing to the reduction of coastal risks.
- Overwash hazards assessment using a simplified process based approachPublication . Plomaritis, Theocharis A.; Costas, Susana; Ferreira, ÓscarCoastal communities are threatened by the impact of severe storms that may cause significant loss or damage of property and life. One of the main processes behind such impacts is the overwash of coastal barriers. In order to estimate the losses associated with a particular event, overwash must be properly parameterized. Here, we propose a novel approach to estimate potential overwash hazards, which includes the associated major processes and crucial parameters. For that purpose it was used the parametrisation of the physical processes developed by Donnelly (2008), and the overwash hazard was related to both flow velocity and flow depth, which are in turn a function of lateral spreading of the flow and percolation. The proposed method requires the selection of a validated run-up formula for the study area; a percolation constant for infiltration; a typical value for the run-up lens; and a storm beach profile. Combining these parameters, the overwash depth and velocity for different return periods can be estimated together with the associated hazards. The advantages of the present approach are: adaptability to any environment where overwash processes are important, time efficiency on evaluating overwash hazards, and the assessment of hotspot areas at a regional scale (tens to hundred kilometres).
- Identification of risk hotspots to storm events in a coastal region with high morphodynamic alongshore variabilityPublication . Celedón, Victoria; Del Río, Laura; Ferreira, Óscar; Costas, Susana; Plomaritis, Theocharis A.High-energy storm events induce hazards that promote damage and destruction of property and infrastructure. Defining high-risk areas is therefore fundamental to prioritise management actions. This work presents the application of an approach to identify hotspots of storm impact at a regional scale (tens to hundreds of kilometres). The Coastal Risk Assessment Framework Phase 1 (CRAF1) is a hotspot selection method based on a coastal index that combines the potential hazard (i.e. overwash and erosion), the exposure (based on land use) and the vulnerability (based on socio-economic data) along each kilometre of the coast to assess the risk level. The suitability of the approach was tested on the southeastern coast of the Gulf of Cadiz (South Spain). CRAF1 was applied considering a morphological worst-case scenario and events of 10/50/100-year return period. The region shows a high overwash and erosion hazard level. Nevertheless, a relatively low number of risk hotspots were identified due to the low level of occupation in the study area. Comparison against available information of previous overwash and erosion events proved the reliability of the method to identify hotspots at a regional scale, even in a coastal area with high alongshore variability (geomorphology, wave exposure and tidal range). The results support the utility of the tool for coastal managers to prioritise and support risk reduction plans. Furthermore, the method presents two aspects that enlarge its potential applicability: (1) it is relatively easy to apply at a regional scale, and (2) it can be updated with new data to test different scenarios (e.g. sea-level rise).