Effect of roughened strips on flood propagation: Assessment on representative virtual cases and validation

The interaction between the main channel and floodplains is one of the most challenging topics in open channel hydraulics. In particular, a better understanding of these interactions is needed to better assess flood control strategies using flood expansion in the floodplain. The aims of this paper are: (1) to investigate the effects of floodplain Roughened Strips for reducing and delaying flood peaks, (2) to compare the results of models (one- and two-dimensional hydraulic models) to find out to what extent the 1D model can be applied. First, we assessed the effect of roughened strips on the propagation of flood hydrographs with/without roughened strips installed in four compound channel configurations. The channels differed in width and slope. For every simulation, the regime remained subcritical. To quantify the effects of the roughened strips, appropriate indicators such as relative attenuation and relative delay of peak, are proposed. For simple symmetrical arrangement of roughened strips, the results of 1D and 2D models are quite similar: the effects are proportional to the roughened area and the relative indicator values are almost constant with the flood return period. But in the case of more complex arrangements, it appeared that a 2D model should be used to obtain accurate results. Finally, a test was carried out on the river Hogneau. A simple interpolation method was devised to estimate a priori effects on flood mitigation of Roughened Strips implementation, using the results of simulations on the compound channels. It yielded indicators of the same order of magnitude as the results of the complete modelling.