Flood Routing in Compound Channels Using Diffusion models

Due to large differences in hydraulic characteristics of main channel and floodplains, unsteady flow mechanisms in compound open channels is very complex. In these types of channel sections, the main channel velocity is considerably higher than that of floodplains and this leads to high momentum transfers from main channel to floodplains. Available unsteady mathematical models for compound channels have such limitations that need to be modified. In this paper, to take this interaction effect into account in the flood routing computations, the combined depth-averaged Shiono and Knight model (1991) with Saint-Venant equations in diffusion form, have been solved numerically using the finite difference method. For evaluating the accuracy of this model, the results have been compared with two benchmark unsteady mathematical models for the homogeneous and heterogeneous compound channels. The finite element DAMBRK and RFMFEM models selected in this paper are based on full dynamic and diffusion wave solutions, respectively. The results showed that in comparison to the selected models, the present proposed model have considerably shorter execution time, i.e. about 10% that of DAMBRK model. Furthermore, compared to the experimental data of Treske (1988) in a straight compound open channel, the results showed that the computed peak outflow discharge differs from the observed one by a factor of 3.5%. Generally, the most important features of the proposed model are the small execution time and good performance in both homo and heterogeneous compound channels.