Presented here are flow simulations of a network of natural rivers flanked by one or two large and complex floodplains with impermeable groynes and bridge embankments using one- and quasi-two-dimensional dynamic flow models. The effects of the large floodplain storage capacity, momentum transfer interaction and apparent shear stress at the vertical interface between the main channel and floodplain on the flow-simulated discharge and water depth values could be well explained. The two models were tested and validated in the Arakawa River basin, Kanto Region, Japan. The simulated results show that the large floodplain storage capacity greatly affected the flow discharge and water depth results and cannot be neglected. The quasi-two-dimensional river flow model was used in a flow simulation of a compound channel with complex floodplains with groynes and gave more acceptable results. In the simulated case, the average reduction in flood discharge using floodplain groynes was about 7–15%. Thus, floodplain groynes can be effective for flood protection and attenuation.

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