The US Army Corps of Engineers Hydrologic Engineering Center recently released version of the River Analysis System (HEC-RAS) has added two-dimensional (2D) modeling capabilities to a decade old one-dimensional (1D) model dating back to HEC-2 developed in the 1970s. Several recent studies have indicated that 2D flood modeling is preferable in urban environments to better account for the complex topography caused by infrastructure. The newest version of HEC-RAS also allows users to simulate unsteady flow using either the Saint Venant equations or the diffusion wave (DW) equations using an implicit finite volume algorithm. The Saint Venant solution allows for turbulence and Coriolis effects to be accounted for with momentum additions. While applicable to a wider range of flood problems, the Saint Venant solution is slower and inherently less stable than the DW approach. We evaluate the similarities and differences between both 1D and 2D solution techniques using the lower Provo River in Utah as a prototypical urban river. Furthermore, since the 2D version of HEC-RAS is relatively new, we compared the HEC-RAS simulations to the 2D sedimentation and river hydraulics (SRH-2D) model developed by the U.S. Bureau of Reclamation. The method uses high resolution light detection and ranging imagery to determine floodplain topography and cross-section information for the channel properties. While no single river reach can adequately answer the question of whether 2D flood modeling produces superior results compared to 1D solutions, in this study the 1D unsteady flow model struggled to predict meandering stream phenomenon particularly because it was difficult to identify active flow versus storage areas as a function of flow depth. We conclude that temporal variations in most complex flow regimes will not be well modeled in 1D and that 2D modeling will produce superior results.
HEC-RAS, LiDAR, mesh generation, overbank storage, SRH-2D
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