Physical and numerical modeling of a river confluence reshaping

The paper refers to the study about the appropriate actions to undertake in order to avoid or to reduce the flooding of an urban area close to a river confluence located in the northwest of Italy. This area presents many buildings, directly faced the rivers, that are affected by inundation on occasion of flood events. According to the Froude similarity, an undistorted scale of 1:40 physical model was constructed in order to estimate the water depths in the area during these events. The scale model showed a lack of conveyance of the rivers with the reference discharge. With the aim of a better design of the river reshaping actions, a two-dimensional numerical model was implemented and calibrated according to the water levels detected on the physical model. The numerical model, which solves the shallow water equations adopting a finite volume approach, was developed by the Civil Engineering Department of University of Parma. As a first step a digital elevation model (DEM) of the scale model was generated by means of a photogrammetrical technique and used as the basis for the numerical modeling of the river bathymetry. By means of a Kriging interpolator, the computational domain was discretized with square cells of 1 cm. The calibration of the numerical model was performed in steady-state using the data measured on the scale model, in particular the water levels of 149 points. The calibration parameter was the Manning roughness coefficient; after some attempts, the best results were obtained as the value leading to the minimum mean square difference between observed and predicted water levels. The calibrated numerical model was modified in order to simulate a river bed reshaping obtained from the removal of an existent weir downstream and the lowering of the central portion of the river bed for a stretch upstream the weir. After an evaluation of the effects of different river modifications and the identification of the most efficient reshaping, the new river configuration was tested on the scale model. The results obtained from the modified physical model were used to validate the numerical one. The water surface elevations, evaluated by the two-dimensional mathematical model, were in a good agreement with the ones measured on the scale model. The present study confirms that a two-dimensional numerical model can be used as a support or even as a substitute for a physical model in the evaluation of the different strategies useful for the reduction of inundation damages involving complex river reaches.