CRAF Phase 1, a framework to identify coastal hotspots to storm impacts

. Low-frequency high-impact storms can cause flood and erosion over large coastal areas, which in turn can lead to a significant risk to coastal occupation, producing devastation and immobilising cities and even countries. It is therefore paramount to evaluate risk along the coast at a regional scale through the identification of storm impact hotspots. The Coastal Risk Assessment Framework Phase 1 (CRAF1) is a screening process based on a coastal-index approach that assesses the potential exposure of every kilometre along the coast to previously identified hazards. CRAF1 integrates both hazard (e.g. overwash, erosion) and exposure indicators to create a final Coastal Index ( CI ). The application of CRAF1 at two contrasting case studies (Ria Formosa, Portugal and the Belgian coast), validated against existing information, demonstrates the utility and reliability of this framework on the identification of hotspots. CRAF1 represents a powerful and useful instrument for coastal managers and/or end-users to identify and rank potential hotspot areas in order to define priorities and support disaster reduction plans.


Introduction
Recent and historic lowǦfrequency, highǦimpact events such as Xynthia (France in 2010), Hercules (western coast of Europe in 2014), or the 1953 North Sea storm surge (Netherlands, Belgium and the UK), have demonstrated the exposure of coastal areas in Europe to flood and erosion induced risks. Typhoons in Asia (such as Typhoon Haiyan in the Philippines in 2013), hurricanes in the Caribbean and Gulf of Mexico, and Superstorm Sandy, impacting the north-eastern U.S.A. in 2012, have confirmed how coastal flooding events pose a significant risk worldwide and can impact large cities, regions and even countries. Whereas for certain coasts the regional risk is clearly recognized, assessed and managed, for other coasts less attention has been given resulting in a lack of tools and data to perform a coastal risk assessment. Risk assessment also often underestimates the induced losses [1]. Yet, the application of a suite of complex models at a full and detailed regional scale remains difficult to most coastal managers and may not be time and cost efficient or even feasible. Instead, a simplified approach based on simple models and on a screening process to identify and rank hotspots may be a useful and accessible instrument for most coastal managers. Risk assessment can, at a first instance, be performed by simple methods. Those should include simple hazard modelling/formulations and available datasets. Potential losses can also be estimated by using proxies towards the elements exposed to the hazards. To perform such risk assessment it is required to develop a framework that allows the identification of areas prone to coastal hazards and to rank the potential risk along a coastal region. This, in turn, will permit coastal managers to better allocate resources to prevent or minimise the risk.
This work presents the Coastal Risk Assessment Framework ± Phase 1 (CRAF1) that aims to identify hotspots caused by extreme events at coastal areas within a regional scale of about 100 km of coastal length, a W\SLFDO ³DGPLQLVWUDWLYH´ VFDOH &5$) LGHQWLILHV KRWVSRWV by calculating coastal indexes to different hazards and to the potential associated exposure for every alongshore sector (~1 km). The application of such framework aims to support coastal managers by providing a selection of ranked hotspots allowing them to take informed decisions on how to define priorities and improve their risk assessment. Here, we will present the application and testing of the CRAF1 focusing in two main hazard types: coastalǦflooding and coastalǦerosion. Simple parametric models have been selected to quickly assess their magnitude for a large number of events (to obtain reliable probabilistic distributions) and for a large number of positions along the coast (to properly characterize hazards at regional scale).
The hazard indicator (i h ) is ranked from 0 to 5 (none, very low, low, medium, high, and very high) with the null value referring to the absence of hazard. The exposure indicator (i exp ) embraces 5 types of exposure representative of the potential direct and indirect impacts: Land Use (i exp-LU ), Population (i exp-POP ), Transport (i exp-TS ), Critical Infrastructure (i exp-UT ), and Business (i exp-BS ). Each is ranked from 1 to 5 (non-existent or very low, low, medium, high, and very high). The overall exposure indicator (i exp ) is ranked similarly from 1 to 5.
The coastal index is calculated separately for each different hazard and associated return periods. As such if two hazards and three return periods need to be considered for a case study, six coastal indices are calculated.

Hazard Indicator
For each coastal sector identified hazards are assessed and reclassified into a specific-hazard indicator (i h ). The extent of the exposure has also to be determined.

Select and apply the appropriate method to assess hazard intensities
Coastal hazards and the corresponding stormǦinduced processes can be grouped in the following main hazard types: marine flooding and erosion.
Marine flooding groups all hazards related to variations in sea water level involving the temporary inundation of the coast at any degree. The lowest flooding level corresponds to overtopping and overwash, which essentially act on the most external coastal fringe. On the other hand, inundation specifically refers to the flood, for a longer time, of a relatively large portion of the coastal fringe due to increased water levels.
Erosion can include either storm induced retreat or long-term coastal erosion (e.g. by sediment scarcity or sea level rise). In order to assess the intensities and the extent of the hazard, several relatively simple methods can be used. As example, some of them are indicated in Table 1.