Evaluation techniques of storm induced coastal risks

As with many natural phenomena such as earthquakes, droughts or floods, storms are relatively unpredictable. Indeed, these phenomena vary considerably in duration and intensity, and generally occur very irregularly in space and time. However, thanks to statistical and modelling methods, or the use of measuring instruments and observation tools, it is possible to predict relatively precisely when and where a given phenomenon will occur, and to determine some of its characteristics. By taking into account the occurrence and characteristics of coastal storms, estimating the degree of exposure of coastal sites, and the vulnerability of the economic, natural and human assets that characterize them, it is then possible to assess the risks involved.

The Xynthia storm in Europe in 2010, which caused numerous levee failures in coastal areas, reminded us of the importance of protecting ourselves against extreme events, and the need to develop a risk culture to avoid serious disasters. Wind has a major influence on the risk of erosion and, in the most extreme cases, flooding in coastal areas, particularly as its characteristics directly influence sea state and, more specifically, wave height, which is one of the main factors in coastal erosion. Offshore wave heights depend on wind speed, duration and fetch. However, close to the coast, other factors – such as the width and slope of the low-water platform – also have significant effects on wave height. It is therefore necessary to take into account the characteristics of waves coming from the open sea and the local geomorphology in order to determine the behavior of waves propagating towards the coast and the impact of waves breaking on the shore. The geomorphological study of the beach and its evolution requires consideration of both hydro- and morphodynamic aspects. In addition to wave formation, which is partly responsible for coastal erosion, storms can also cause a rise in sea level: this is the phenomenon of marine surge. This is defined as the positive difference in water level between the observed tide and the astronomically predicted tide, and consists of a temporary, local rise in sea level generally linked to low atmospheric pressure and wind action. Combined with high tidal coefficients, this effect represents a major risk of erosion and even marine submersion of coastal areas.

Moreover, these coastal risks are likely to be exacerbated by climate change. Indeed, according to the IPCC (Intergovernmental Panel on Climate Change) report on "Climate Change 2001", climate change, particularly sea-level rise, will result in increased coastal flooding due to storms and coastal erosion. Although there is no real consensus as to the extent of the phenomenon and the regions affected, a number of studies have identified regions...