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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.
- 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).
- Use of a Bayesian Network for coastal hazards, impact and disaster risk reduction assessment at a coastal barrier (Ria Formosa, Portugal)Publication . Plomaritis, Theocharis; Costas, Susana; Ferreira, ÓscarCoastal communities are threatened by the impact of severe storms that may cause significant loss or damage of property and life. The main processes causing such impacts at sandy coastlines and nearby coastal communities are storm erosion, overwash and inundation. Coastal response under present conditions and under predicted climate change has been frequently assessed on the basis of numerical models, which in turn can be also used to evaluate the effectiveness of Disaster Risk Reduction (DRR) measures to mitigate the response of the coast to the imposed conditions. However, detailed morphodynamic models are computationally expensive and not commonly used by coastal managers. The present work proposes the construction of a probabilistic Bayesian Network (BN) as a surrogate for the numerical simulations. This BN is trained with a large number of morphodynamic simulations, under a variety of storm conditions and DRR measures, in order to serve as a front-end platform for visualising, analysing and evaluating combined results of the numerical modelling. The BN introduced in an early warning system will be able to serve both, as a predictive and as a working tool to determine impacts and evaluate risk reduction after measures implementation. Here, an example of the implementation and results of such a BN system is presented. The BN system was built for a coastal sector of the Ria Formosa barrier island system (South Portugal) to inform the degree of impact derived from overwash and erosion over the study area. The BN boundary conditions include variable wave height, water level, and wave period. The impact on receptors, including houses and infrastructure, was assessed. In addition, this tool can inform about the effectiveness of a particular DRR measure. The evaluated DRR measures were two primary measures (partial house removal and beach replenishment) and a non-primary measure (improve channels of communication), all measures proposed by local stakeholders. Results show that for a storm with wave characteristics of the 1 in 50 year return period and spring tide conditions, the house removal DRR reduces the overwash impact by 15% and erosion impact by 58%. The implementation of beach replenishment could reduce the erosion impact of the same event by 96% while it would have a smaller effect on the overwash impact (16%). The implementation of non-primary measures would have a much smaller effect on risk reduction. The combined effect of the above DRR measures (mainly house removal and beach nourishment) reduces storm impacts at the study area to a value near zero. The BN surrogates the model simulated onshore hazards and translates them into impacts for the current conditions, which give a high degree of confidence in the potential application of the BN as a management tool.