Morphological response and coastal dynamics associated with major storm events along the Gulf of Lions Coastline, France

Abstract

Along the coast, anticipating the different morphological responses induced by storm events is crucial for managers to evaluate coastal risks and to develop the best measures to mitigate them. In this paper, a methodology is developed to determine the best storm intensity parameter to derive storm thresholds for different morphological responses. The methodology is applied to the northern part of the Gulf of Lions coastline where storm events can induce important morphological changes. These include shoreline retreat, beach and dune erosion, significant migration of nearshore bars, overwashes and even breaches of coastal barriers, as well as damage to coastal defences and coastal infrastructure. In order to evaluate historical storm characteristics and impact, an extensive review was undertaken to obtain quantitative datasets (beach profiles, wave records), aerial photographs and more qualitative information on morphological evolution and coastal damage. Re-analysis of hydrodynamic and morphology data was undertaken, and hindcast wave modelling results were used to characterised storm intensities. The methodology developed to evaluate storm thresholds consists of obtaining morphological evolution indicators (evidence of breaching, overwash processes, volume variations and migration of morphological patterns) that can be directly linked to a storm event and its characteristics. Results demonstrate that in such a quasi-non-tidal wave-dominated environment, with intermediate double-barred beach shoreface morphology, major coastal changes occur following the maximum significant wave height reached during the storm, or else according to maximum wave run-up elevation regarding upper beach impact, a wave height dependant parameter. Inversely storm surge (and water level) alone, as well as total storm energy, do not explain any storm impact scale. Storm-specific datasets indicate that important morphological evolution is observed during moderate storm events (significant wave heights over 2.7m). Above this threshold, the morphological behaviour changes radically. The main characteristics are a rapid offshore migration of the nearshore bars and large deposition of sand on the upper beach. However, the major morphological changes are associated with even more energetic events. When the significant wave height reaches 5m, important impacts are observed: breaching and overtopping of natural coastal barriers, and severe dune erosion and impacts to coastal infrastructure on urbanised beaches. Qualitative observations show an important increase in damage when the storm waves reach 5m. The methods employed can be applied easily to any coastal segment and provide coastal managers with tools to evaluate different coastal evolution and coastal damage induced by storm events.