How does damming on fine-grained rivers counterintuitively raise the flood risk of downstream channels?

When listing the benefits of river dams, flood control is often among the top. The benefit of flood risk reduction is generally attributed to two aspects: (1) dams attenuate flood waves reducing the peak value of the flood hydrograph, and (2) the overall incised and enlarged channel downstream provides enhanced water conveyance. However, it is often challenging to comprehensively evaluate the flood risk following dam closures, due to constantly evolving channel morphology, which renders the prediction of floodwater conveyance difficult. In this talk, we first provide a thorough evaluation of floodwater conveyance considering multiple aspects of channel morphologic change. We show that channel bed incision and widening induced by clear water flows released by the dam is accompanied by sediment coarsening, which facilitates the development of larger dunes. These larger bed roughness features increase flow resistance and reduce velocity relative to pre-dam conditions. As a result, although the flood stage is lowered for small floods due to bed incision, counterintuitively, the flood stage downstream of a dam can be amplified for moderate and large floods because of increased bed roughness and dune-related flow resistance. Our model indicates the underlying mechanism for such flood stage amplification can occur in >80% of fine-grained rivers (bed grain size < 2 mm). Specifically referring to the dammed lower Yellow River, China, we show that the flood wave attenuation of the dam under normal flood-control operation might not provide an overall reduction in flood risk as compared to the pre-dam condition, because the amplified flow stage can surpass the benefit of the flood wave attenuation produced by the dam. Our study suggests the need to reconsider flood control strategies in such rivers worldwide.