A mathematical model for routing unsteady flows through compound channels is described. The new model, RUFICC, accounts for flood plain contributions to system conveyance and also for lateral momentum transfer between adjacent deep and shallow zones of compound flow fields. In the modeling approach composite channel sections are divided into representative deep (main channel) and shallow (flood plain) zones. Continuity and momentum equations are written separately for the different sub-sections and summed to yield the composite section equations. The one-dimensional model equations, which are a modified form of the St. Venant equations, are solved using a four-point implicit finite-difference scheme. The initial model validation tests reported herein used laboratory data from a European study on unsteady flow in open channels. For the various model flood events simulated a good agreement was obtained between predicted and observed stage and discharge hydrographs. RUFICC's performance was compared to those of two frequently-used conventional models, namely: the Off-Channel Storage (OCS) and the Separate Channels (SC) models. A better agreement was achieved between RUFICC's simulated hydrographs and the observed data. Applying the two conventional models resulted in a marked delay in the fall of the recession curve of the simulated hydrographs. The OCS method also underestimated stages and discharges by more than 25, especially for fairly high flood plain flows.
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