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The risk of levee overtopping has increased because of the frequency of severe rain storm due to climate change. Flow depth over levees is a design parameter that must be considered, but standards have not been agreed on. In this study, the mechanisms of levee overtopping are investigated by analyzing the water surface profile in an open channel with side-overflow using a 2D shallow water model on an unstructured triangular mesh system. Scenarios in which the channel capacity decreases downstream were considered; the first when the channel slope decreases and the second a “Y-shape” confluence of two river channels. The computation for the slope changing channel exhibited the following results: (1) levee overtopping occurred not only in the downstream channel but also in the upstream channel due to the backwater effect, (2) the overflow depth was much smaller than the value predicted for normal flow without side-overflow, and (3) the water stage at the levee crown was lower than the transversely averaged one around the slope change section due to the 2-D flow effect. The computational results for the confluent channel had the tendencies common to those for the slope changing channel, but further showed: (4) the overflow depth anomaly appeared at the inner corner of the confluence with an increase in the confluent angle, which suggested that the delta area between the confluent channels could be used as a detention basin by armoring the short reach at the corner.

Characteristics of water surface profiles of open channel flow with levee overtopping