River bed degradation in a compound channel after base level fall

The bed degradation of a compound channel induced by a base level fall can significantly impact the flood conveyance capability and stability of riverine structures. This study found that severe compound channel degradation is due to the coupled effects of flood discharge and base level fall. Laboratory experiments examined the impact of base level fall on the grain size distribution at the bed surface, flow discharge in the main channel and floodplains, and longitudinal profile of the bed elevation. Two riverbed degradation patterns of a compound channel after base level fall were clarified. Relative to the original bed slope, degradation pattern (I) had a higher bed slope, but degradation pattern (II) had a lower one. A calculation method was proposed to determine the critical conditions for identifying these two riverbed degradation types. Both experimental and published data confirmed that the proposed method is valid for identifying these two compound channel degradation patterns.