The projects combining protection against floods and floodplain reconnection are very complex and it is quite difficult to show their impact on flood mitigation. Using hydrological and hydraulics modelling, engineer and scientists are able to assess the hydrological efficiency of natural flood mitigation. This modelling is necessary based on a fine topographic knowledge on the whole floodplain. Digital elevation models with a very good altimetric accuracy are now available thanks to airborne laser surveys. In addition, in order to highlight the dynamic storage on floodplain, unsteady flow simulations must be perform. They require hydrographs as input, but for a given peak flow frequency many shapes of hydrographs could be chosen. We use a stochastic model for hourly rainfall, coupled with a conceptual rainfall-runoff model, to compute many hydrographs at different basin scales. We choose the hydrographs which keep the same frequency from upstream to downstream. At any place along the river, each flood event has the same peak flow frequency. By comparing different scenarios, we can assess the hydrological efficiency of re-entering river waters onto floodplains. For mountainous river, floodplain reconnection is not a good solution to improve flood mitigation, but this solution keep a lot of interests to preserve the water quality and floodplain's biodiversity.