Stage–discharge relationship in an erodible compound channel with overbank floods

Due to environmental changes and anthropogenic activities, a dramatically diminished upstream sediment supply can promote riverbed erosion and alter the stage–discharge relationship in a channel with overbank flows, which can impact flood control. It is important to understand how the stage–discharge relationship in an erodible compound channel is affected and how to predict this relationship. In this study, we performed laboratory experiments in a straight compound channel with a mobile main channel under clear water scour. Stage–discharge relationships were measured before the destruction of the armor layer and after the formation of a new armor layer at the riverbed surface. The results indicated that the impact of riverbed erosion on the stage–discharge relationship cannot be ignored. Under the same discharge, the flow depth in the main channel increased, while the flow depth in the floodplains decreased after the new armor layer was formed relative to the case before armor layer destruction. Considering the impacts of the erosion depth and the variation in the bed resistance, we proposed a new method for predicting the stage–discharge relationship in an erodible compound channel after riverbed erosion. The predicted stage–discharge relationships agreed with the measurements, indicating that the proposed method could be used to precisely predict the stage–discharge relationship after riverbed erosion occurrence. The comparison of the results of the proposed method with and without considering the erosion depth supported the idea that the impact of riverbed erosion on the stage–discharge relationship must be included. Finally, the proposed method was further used to examine the influence of floodplain encroachment on flood control and riverbed erosion. Floodplain encroachment produced a larger flood flow depth on floodplains and a higher mean velocity in the main channel, suggesting that floodplain encroachment should be avoided to eliminate possible increases in flood disasters and riverbed erosion risks.