Coastal Risk Management Plan in Saint-Malo

Coastal flooding is a temporary flood of a coastal area by the sea due to extreme weather conditions causing sea level rise. These conditions are generally caused by atmospheric depression, severe wind and high swell with specific detrimental tidal conditions. After the storm Xynthia and its tragic consequences on the French Western seaside (Vendée area) in February 2010, measures have been implemented by the French authorities in order to manage these events. One of these measures is to implement Coastal Risk Management Plans. These plans aim to delimit areas exposed to the risk of marine flooding and erosion and to define regulations about land use depending on the intensity of hazards and issues. In collaboration with the Ille-et-Vilaine Departmental Directorate of Territories and the Sea, DHI has been commissioned to take part in generating such a plan for the city of Saint-Malo (Brittany) and to implement a hydrodynamic model. Thus a coupled 1/2D model is set up with MIKE FLOOD and different scenarios of failure of sea defence structures are tested. Different types of failures are considered like breaches in the dikes, overflows when the sea level is higher than the structure level and wave overtopping due to wave run-up. Failure locations are defined according to topographical analyses, study of structure shapes, knowledge about historical events and feedback about accidentology and analyses of marine energy. In order to take into account the effect of climate change, current sea level and projected sea level for the year 2100 are considered. These levels are based on reference levels, wave set-up and uncertainty margin. The model is defined with a regular mesh with a 3-m-grid resolution. LIDAR surveys are used to represent the topography of the area. In order to be more accurate, wastewater networks are modelled with MIKE URBAN too and their effect to alleviate the flood propagation is highlighted. The dysfunction of a pumping station is considered too. After the modelling tasks, hazard maps are generated. These maps represent the intensity of the events based on the simulation outputs linked to water depth and flow velocity. Then, mapping flooded issues leads to divide the area in four main types of land use: urban spaces, natural spaces, historical urban centre and port area. Finally, a coastal risk regulatory zoning is generated by crossing hazard maps and issues maps. The area is then divided into different classes and each class is defined by a specific regulation about land use and building authorisation procedures depending on the hazardousness and the type of issues.