Flood hazard assessment: comparison of 1D and 2D hydraulic models

Abstract

The 'Floods Directive' 2007/60/CE by the European Parliament requires the characterization of flood hazard by multi-scenario hydraulic analyses, based on the estimation of flow velocity and water depth or free-surface level over flooded areas. In principle, this evaluation demands a complete analysis of the watershed hydraulics, based on two- or even three-dimensional modelling. Since the latter, however, may hardly be applied at the watershed scale, 1D and 2D analyses represent the usual approaches to hydraulic risk mapping. The present work discusses a comprehensive comparison between 1D and 2D modelling of floods in meandering channels, aimed to guide practitioners in the confident choice of a tradeoff between complexity and accuracy. In particular, both numerical models were applied to a typical river morphology, represented by few shape parameters. Steady-flow conditions have been generally assumed; furthermore selected numerical experiments have been run under unsteady flow to investigate the effect of the time-varying hydrographs. Uniform-flow framework is considered for the 1D analysis, whereas 2D modelling is performed using the CCHE2D code developed at the National Center for Computational Hydroscience and Engineering, University of Mississippi. The models' results are compared in terms of computed stage–discharge hydrograph, shear stress distribution and free-surface elevation.