Analysis of the coastal flooding exposure of embayed beaches at a regional scale using a video monitoring network: Basque Coast (Spain)

Abstract

Coastal zones are highly exposed to storm waves that can affect natural values and damage coastal facilities and engineering structures to varying degrees. The definition of a regional management strategy along the Basque Coast (150 km), composed mainly of embayed beaches, is challenging due to the high variability in the coastal flooding exposure, nearshore wave conditions and geomorphological characteristics. Here, 3 years of data (2019-2022) from a coastal video-monitoring network (KOSTASystem) deployed at 13 beaches were used to: i) assess the regional variability of storm impact associated to coastal flooding (hereinafter referred as storm impact), ii) analyse the relation between storm impact, nearshore hydrodynamics and pre-storm geomorphology and iii) find common beach behaviours, using an extensive video monitoring network operating along the Basque Coast on a series of embayed beaches. Different types of images were analysed during storms to characterise storm impact through the storm impact regime indicator (SIR, i.e. swash/collision/overtopping). The storm impact was compared against nearshore hydrodynamics (water level and energy flux) and geomorphological parameters, (1) the dry beach width (DBW) obtained from orthorectified Timex images in a predefined profile and (2) the dune/seawall toe (TH) and (3) crest (CH) height obtained from airborne LiDAR (Light Detection and Ranging) surveys. No clear relation was found between nearshore wave conditions and storm impacts. This might be related to the reduced range in the wave and tide values used to perform the analysis during high energy conditions. However, beaches with a wider dry beach and/or higher dune/seawall toe and crest lead to lower storm impact intensity (swash) and inversely (collision/overtopping). The beaches of the study site were described in function of the storm impact regime and the pre-storm geomorphological parameters (heights referred to the 0 of the Spanish topographic institute (NMMA IGN 2008)) as: Group 1) Beaches dominated by the collision regime that present low values of DBW and TH, specifically, DBW < 30 m and TH < 3.5 m. Group 2) Beaches dominated by the swash with few collision events that present moderate DBW and relatively high values of TH, specifically, 40 < DBW < 75 m and 4 < TH < 6.5 m. Group 3) Beaches fully dominated by the swash regime that present relatively high values of DBW and TH, specifically, DBW > 50 m and 4 < TH < 7.5 m. This coastal exposure assessment at regional scale benefits stakeholders and coastal managers by characterising the most exposed areas and identifying the drivers. The results presented here are expected to be valid in sites with similar wave climate and water level regimes. The application of the methodology to other coastal areas with a regional videometry network would help to generalize and give robustness of the results.