Long-Term Effects of Water Diversions on the Longitudinal Flow and Bed Profiles

Abstract

Water diversions along alluvial rivers cause water and sediment loss, and thereby affect morphological development. Assuming spatially continuous diversions along a constant-width channel, previous studies suggest a longitudinally convex bed at the equilibrium state. However, the validity of a convex bed profile for practically discrete diversions in natural channels of longitudinally varying width remains to be justified. Moreover, such equilibrium analysis does not reveal the morphological time scale (MTS) associated with water diversions. To solve these issues, a general theoretical framework is proposed for predicting the equilibrium state of the fluvial system, which is applicable to both continuous and discrete water diversions in a longitudinally width-varying channel. Numerical experiments complement the MTS studies for water diversions. The effects of diversion intensity, diversion placement (discrete and continuous), and diversion scheme (pure water and water-sediment mixture) are also systematically studied. The present work confirms the previous findings that water diversions lead to a decrease of the equilibrium depth with respect to natural conditions and a convex bed in a constant-width channel. Moreover, it reveals that in a widening channel a convex bed also develops under water diversions, whereas convex, concave, or quasi-linear beds may occur in a narrowing channel. Nonmonotonic beds may develop in a strongly narrowing channel, depending on the diversion schemes. On large spatial scales, diversion placement is less important for the equilibrium development. The MTS for water diversions and natural development are very similar and large, indicating considerable influences of water diversions on river morphology. The present work advances the understanding of the long-term effect of water diversions on river evolution.