This paper presents a study that aims to evaluate the causes for flooding frequency increase and to assess the effectiveness of its mitigation. The research was conducted next to the city of Lyon (France) in a periurban catchment showing a rapid urban development. A semi-distributed hydrological model was fitted in the 1990s. Simulation of floods using land cover from the 1970s in combination with rainfall time series from the 1990s reveals no effects on large floods, but a sensitive increase in frequency and magnitude of small floods. Rainfall regimes evolution between the 1970s and the 1990s seems to be the major influencing factor to explain the flooding frequency increase in the downstream part of this per urban catchment. Two dry reservoirs are expected to mitigate the present 100-years flood to protect this area. Simulations indicate that they would be partly inefficient both in the present and in the future.