Inondabilité: a method for prevention of flood risk for a better integrated management of river catchments
In recent years, the incidence of flood damage in France and Europe has demonstrated that we still have some progress to achieve if we are to cope with inundations. To do so, prevention, forecasting and promotion of risk culture or awareness are possible tools. They should be used together because of their complementary nature and their common field of application. In risk prevention, it seems that conceptualizing risk by dividing it between a socio-economic dimension (vulnerability) and a hydrological dimension (hazard) is a productive approach. The first factor, "vulnerability", takes into account how a certain land use can deal with the phenomena, regardless of its occurrence. It is mostly relevant to the fields of sociology and economics. The second factor, "hazard", summarizes the natural phenomena, the occurrence of which could create human or economic damage. Both of these factors are relevant to hydrology and hydraulic sciences and have been studied for years. A given land area could be considered as "risky" when the probability of occurrence of the hazard is higher than what can be tolerated. Both hazard and vulnerability should be estimated as objectively as possible. The main difficulty in comparing these two factors is to define a similar measure for the two concepts, with a single unit. This is the purpose of the Inondabilité method: to produce a similar reference allowing an objective comparison. The aim of Inondabilité is to define a method to quantify both hazard and vulnerability in a compatible manner and then to build easy-to-understand, synthetic maps of risks as the basis for a real negotiation in the land-use planning process. Such maps allow the comparison of different evolution scenarios, taking into account the two components of risk, and their possible changes. Hydrology should give a synthetic description of the flow regime of the studied river. This can be based on flow-duration-frequency (QdF) models which describe the statistical functioning of the catchment and facilitate the calculation of the probability of events. Such synthetic models also yield synthetic hydrographs that are useful for hydraulic modelling and allow the quantification of vulnerability. Hydraulics is specially used to calculate hazard. Using hydrological results, a hydraulic model calculates flooded areas for flows of various return periods, from frequent to extreme. Each land parcel could be affected by the return period of the first flooding discharge corresponding to its hazard level. The same model will then be used to estimate the consequences of hydraulic works. Describing land use allows us to attribute a reasonable protection target to each land type. Varied areas such as natural (forests, grass lands, ...), rural (crops fields, orchards, ...) and urban (camping areas, leisure areas, cities ...) should be treated differently. Each land-use type is characterized by a target expressed in terms of frequency, duration and depth of an acceptable flood, resulting from local reviews and consultations. Then, an equivalent measure could be calculated, according to flow-duration-frequency models. Its unit is a return period expressed in years. After these two steps, that is to say hazard and vulnerability analysis, it is possible to build different maps. Hazard maps show the flooded areas for different return period flows. Vulnerability maps show the spatial distribution of this measure, equivalent to the need for protection allocated to each parcel according to its land use. The two variables being expressed in the same units, a return period in units of years, it is easy to compare them for each parcel and to convert this comparison into an easy-to-understand colour code: - either hazard is higher than vulnerability (the hazard's return period is lower than that of vulnerability): this shows an incompatibility between the desired protection target and the actual hydraulic constraint, and the parcel is colored in red ; - or hazard is lower than vulnerability (the hazard's return period is higher ...), which shows a reasonable safety of the parcel according to its land use, and the parcel is colored in green ; - or the parcel is away from the maximum flooded area: there is no risk at all, and the parcel is colored in yellow . The same formalism allows the representation of the results for any kind of solution, structural options such as hydraulic works, or non-structural options such as changes in land-use planning, and their comparison. Synthetic maps are a more efficient way to compare management scenarios and their effect on the distribution of risk all along the river than are classical design flow studies. They should facilitate real negotiation and lead to a more sustainable river management. The Inondabilité method offers advantages from other commonly used methods. First, it doesn't deal with only one referenced flood but with floods of various frequencies. It provides a good description of the overall regime of the catchment. Second, the definition of the risk as a difference ( Risk=∆ (Hazard, vulnerability)) implies a search of an acceptable solution instead of an optimized solution, usually impossible to reach. Third, the projection of vulnerability and hazard in a 1D space makes it very easy to draw a risk map, and this map is easy to understand for anybody, even non technical personnel. The originality of Inondabilité method lies in the quantification of vulnerability and hazard in the same unit, a flood return period. However, in the future more economic and social concepts have to be introduced to better fit the complex reality.