Efficiency of distributed flood mitigation measures at watershed scale
Diagnostic d'efficacité à l'échelle du bassin versant d'un aménagement visant à laminer les crues
Résumé
To test the above approach, a tool reproducing the physical processes at watershed scale was needed. The tool is composed of a rainfall generator (stochastic),a hydrological model (GIS) and a hydraulic model (1 D). The rainfall generator (TBM) feeds spatially distributed rainfall fields into the distributed hydrological model (MARINE), which in turn simulates lateral surface run-offs. The surface run-offs are then injected into a hydraulic model (MAGE), which routes the flow and can also simulate scenarios with the presence of hydraulic structures. The developed tool was then calibrated for a virtual watershed generated from realistic data. The spatial aspect is of importance, since the response of a watershed is highly sensitive to this parameter. Protection of dispersed zones of interest against flooding inside a watershed needs careful consideration. Uncoordinated flood defence strategies, render certain areas vulnerable while assuring protection to a given zone. Hence decentralised flood mitigation measures are solicited. The mitigation measures proposed here involves dispersed retentions zones while being efficient at catchment scale are eco-friendly and comply with sustainable development. Dry dams are ideal structures which attenuate flood peaks without rupturing the normal river flow regime and are activated only in case of high flows. Our study analyses the impact of dispersed retention measures along the drainage network on the watershed dynamics. For a chosen set of intense events input into the tool, a flood frequency distribution is calculated with the simulated outputs. The statistical data calculated give the likelihood of various discharges as a function of recurrence interval or exceedence probability. In the next step, hydraulic structures are introduced and their influence on the dynamics of the watershed is then simulated and then analysed at different locations. To quantify the efficiency of mitigation measures, the frequency distribution obtained with hydraulic structures is compared with frequency distribution without any mitigation measures. The attenuation achieved with different mitigation configurations are noted and their respective efficiency is compared. The tool serves as a means to test the influence of rainfall distribution, mitigation measures, strategies and also helps in identifying efficiency indicators. Thus isolating potential applicable solutions at watershed scale and serve as a decision-support system.