Analysis of Flood and Dry Threshold Definition in Two-Dimensional Hydrodynamic Flood Modeling Tools

Extreme hydrological events are more and more frequent due to the ongoing climatic variations. Indeed, flash flood events are more intense and drought periods are increasing significantly. In the meantime, cities are expanding worldwide, concentrating in vulnerable areas, population, and economical assets. With the concern to protect their growing urbanized areas, municipalities tend to use new decision support tools in order to forecast extreme events. These tools include numerical modeling tools which represent the reality by simulating hydrodynamic processes. When flooding on dry land is the issue, modeling objectives clearly focus on the fluid dynamics, thus on the determination of the flooding front. This means that the characterization of the flood extent and its dynamics are directly related to the model definition of wetting and drying areas. With the goal of improving the accuracy of the modeled results, numerical flood and dry transitions should be closer to the reality. The French Riviera is typically an area of strong economic expansion. Furthermore, with its Mediterranean climate, this location faces flash floods and drought periods. In order to prevent the consequences of these extreme events, the local authorities are managing two projects within the French Riviera related to (1) fluvial modeling on the lower Var river catchment and (2) urban stormwater management in subcatchments of Nice city. For partnership purposes, the two-dimensional (2D) hydraulic modeling tools Mike 21 and Mike 21FM (DHI) are used for both research works. These hydraulic software tools have been developed to model 2D free surface flow using the 2D Shallow Water Equations (SWEs) and require the definition of flood and dry thresholds to compute the simulated results. In both cases, issues regarding the choice of the thresholds’ values have been encountered leading to model instabilities or impacting significantly the results. Only few publications are dealing with this topic in the literature and there is a clear need for more investigation. This chapter presents the method followed to answer the question: how to choose the correct value for flooding and drying depth parameters in Mike 21 and in Mike 21FM software tools in case of urban flood, fluvial flood, and drought event? Several scenarios will be tested on theoretical cases and the results will be analyzed in order to conclude on the impact of the flood and dry thresholds definition on the model results.