Development of Methodologies for the Analysis of the Efficiency of Flood Reduction Measures in the Rhine Basin on the Basis of Reference Floods

After some years of extreme flooding in the 1990s extended efforts were made to improve flood protection by means of an integrated river basin management. Part of this strategy is the implementation of decentralised flood reduction measures (FRM). With this in mind, the CHR/IRMA-SPONGE Project DEFLOOD was initiated. By establishing a set of methodological tools this project aims at making a step further towards a quantitative hydrological evaluation of the effects of local FRM on flood generation in large river basins. The basin of the River Mosel and in particular, the basin of its tributary Saar served as case study area for testing the methodological approach. A framework for an integrated r iver basin modelling approach (FIRM – Flood Reduction) based on generation of hydrometeorological reference conditions, precipitation-runoff modelling and flood routing procedures was set up. In this approach interfaces to incorporate the results of scenario calculations by meso-scale hydrological modelling are defined in order to study the downstream propagation of the effect of decentralised flood reduction measures including the potential retention along minor rivers in large rivers. Examples for scenario calculations are given. Based on the experience gained the strategy for the use of the methodological framework within the context of river basin management practice are identified. The application of the methodology requires a set of actions which has to be installed in the Rhine/Meuse basins. The recommendations suggest that - beside progress in hydrological modelling - a base of knowledge needs to be built up and administered which encompasses hydrologically relevant information on the actual state and prospected developments in the River Rhine basin. Furthermore, problem-oriented hydrological process studies in selected small scale river basins ought to be carried out. Based on these studies conceptual meso-scale modelling approaches can be improved and validated in terms of reducing the uncertainty factor, which is inherent in all scenario calculations.