Suitability of 1D models for hydraulic hazard assessment on meandering floodplains

"On October 23, 2007, the European Parliament approved the ‘Floods Directive’ 2007\/60\/CE (FD), aimed to regulate the procedures for hydraulic risk assessment. One of its most innovative aspects consists in the characterization of flood hazard by multi-scenario hydraulic analyses, based on the estimation of flow velocities and water depths all over the flooded areas. In principle, the accurate evaluation of these parameters would require a complete analysis of the watershed hydraulics, based on two- or even three-dimensional modeling; the latter, however, may hardly be applied to river-scale problems due to its huge computational and input data requirements, so 1D and 2D analyses represent the practical alternatives for fluvial hydraulic investigation. Within this perspective, the systematic comparison between these two modeling approaches provides a valuable help to the user in the confident choice of a tradeoff between complexity and accuracy. The present work discusses a comprehensive comparison between the results of 1D and 2D approaches in the analysis of floods in meandering channels. In particular, both numerical models were applied to a typical river morphology which may be represented by a few shape parameters (e.g. sinuosity, depth\/width ratio, cross-section shape and so on). Conformingly to the common praxis for hydraulic risk assessments, steady flow conditions have been assumed. From the computational point of view, normal-flow framework is considered for the 1D analysis, whereas two-dimensional modeling is performed using CCHE2D, a finite element depth-averaged model developed at NCCHE, the University of Mississippi .. Various channels with different shape parameters (namely sinuosity and depth\/width ratio) are considered, according to their typical values for natural rivers. For each combination of shape parameters, several simulations are performed to produce in-bank and overbank flows. The models results are compared in terms of flow velocity, free surface elevation, shear stress distribution and computed stage-discharge hydrograph. Finally, the values of the minimum freeboard (defined as the minimum distance between water surface and the top of river bank) across the channel bend are evaluated and compared."