Recent variation in channel erosion efficiency of the Lower Yellow river with different channel patterns

The operation of the Xiaolangdi Reservoir has exerted prominent effects on the processes of sediment transport and channel evolution in the Lower Yellow River (LYR) since 2000. The investigation into the spatio-temporal adjustment characteristics of channel evolution is of significance to the management of the LYR covering different channel-pattern reaches of braided, transitional and meandering. An indicator of channel erosion efficiency was firstly defined as the ratio of annual channel evolution volume to annual incoming water volume in a specified study reach. Furthermore, the values of channel erosion efficiency in different reaches of the LYR were calculated using the measured hydrological data and cross-sectional profiles during the post-dam stage. Calculated results show that the channel erosion efficiency in the LYR was exhibiting a decline tendency at both temporal and spatial scales, with the mean value of the whole reach decreasing from 6.0 (2000-2007) to 2.7 (2008-2020) and that of the braided reach four times higher than the meandering reach. Main factors influencing the channel erosion efficiency were investigated, covering the incoming flow-sediment regime, riverbed composition and cross-sectional geometry. The temporal decreases in these factors, including incoming sediment coefficient, bed material supply and geomorphic coefficient, resulted in the temporal decline in channel erosion efficiency. The discrepancies in channel erosion efficiency of different reaches were mainly ascribable to the variations in cross-sectional geometry and riverbed composition. The geomorphic coefficient in the braided reach decreased from 19.7 to 9.0 m- 1/2, and meanwhile the percentages of fine and medium grain sizes in the riverbed reduced by around 50% from 2000 to 2020, which led to the most dramatic variation in channel erosion efficiency and the different trajectory of channel evolution. In addition, the seldom varied channel slope played a minor role in the temporal variation of channel erosion efficiency, but partly accounted for the spatial reduction along the LYR. This study provides an illustrative example for the response of a large alluvial river with different channel patterns to the varying boundary conditions caused by upstream damming.