The impact of spatial representation on flood hazard assessment: a comparison between 1D, quasi-2D and fully 2D hydrodynamic models of Rio Marano (Rimini)

Four hydrodynamic numerical models were constructed and compared for a case study along the Marano stream (Rimini) using the software HEC-RAS . The models include a 1D model, with extended cross-sections to represent the floodplain flow; a 1D model, with floodplain represented as hydrostatic Storage Areas; a coupled 1D/2D model, with a 1D representation of the main channel and a 2D representation of the floodplain inundation, and a fully 2D model, with both main channel and floodplains represented through a two-dimensional hydrodynamic numerical scheme. First, 1D steady flow simulations were performed to get a conservative estimate of the maximum water levels along the stream. Second, unsteady flow simulations were performed with all four hydraulic models in order to assess flood attenuation associated with the routing. Third, the floodplain inundation dynamics resulting from each model were compared. The results of the steady flow simulations showed that six bridges are inadequate and most of the natural floodplains of the stream are inundated proportionally to the flood discharge considered. Concerning the unsteady flow simulations, each model returned different results in terms of flood attenuation and floodplain inundation dynamics. The 1D model resulted inadequate for modeling channel-floodplain interactions and floodplain inundation dynamics. The 1D model with Storage Areas resulted to be suitable for assessing the flood attenuation induced by the introduction of levees separating the floodplain from the main channel. Regarding the coupled 1D/2D model, the results showed that the elevation profile of the structure coupling 1D and 2D flow areas has a significant impact on modelling results. The 2D model returned the most detailed information regarding flood propagation in both main channel and floodplains.