Estimation of mortality in case of coastal flood: proposal for a new methodology and application to Storm Xynthia

International audienceStorm Xynthia (2010) was responsible for an important coastal flood on French Atlanticcoast. This flood has impacted low-lying urbanized areas, which conducts to the death bydrowning of 41 people who were trapped inside single-story houses without chances toescape. Review of the storm pointed out the recent growth of urbanization in potentiallyflooded area, non adaptation of constructions to the risk, and predominance of elderly as acombination of reasons to explain the human toll. Based on these facts and existing methodsof mortality estimation (dutch example of mortality function), we propose an adaptedmethod to predict potential number of death in case of coastal flood. To evaluate thepotential number of death, this methodology uses information about: the location andconfiguration of residential houses (based on previous work, the VIE index) in order to assesconstructions as a threat for people, the characteristics of population (age) on coastalmunicipalities in order to asses human vulnerability, and the occupation of housesdepending if there are residential or secondary homes. Following the dutch mortalityfunction, it allows to estimate a probability of death amongst a number of people exposed.The novelty is that we can propose a probability of death for each town depending of theabove mentioned criteria. This work has been validated using information about stormXynthia. It gives good correlation with the toll of this event (for three impacted towns of Baiede l’Aiguillon: La Faute-sur-Mer, l’Aiguillon-sur-Mer and Charron). It has been applied to anon impacted but potentially vulnerable area, Noirmoutier Island, to assess its vulnerabilityto futures coastal flood. The usefulness of the method is based on the possibility to use it ina Cost-Efficiency Analysis (CEA) in a way of supporting decision process at a local scale. Itallows to compare different scenarios of vulnerability reduction for houses and to evaluatethe number of death with and without measures. It gives a number of life potentially savedwith the measure. The authors conducted this type of CEA comparing four vulnerabilityreduction measures. Presently applied to the two study areas, the CEA could be applied toother territories in order to identify the best solutions in order to reduce vulnerability forpeople, depending on local characteristics of population and danger of buildings. It will beuseful for land use planning, for example for European flood directive