Rapid and long-distance channel incision in the Lower Yellow River owing to upstream damming

摘要

The operation of the Xiaolangdi Reservoir has significantly altered the flow and sediment regime of the Yellow River, which has led to rapid and long-distance channel incision in the Lower Yellow River. Variations in the channel incision depth and stage-discharge relationships were identified in the braided, transitional and meandering reaches based on the cross-sectional profiles at 91 sedimentation sections and hydrological data at hydrometric stations. The results show that the cumulative reach-scale channel incision depths in the braided, transitional and meandering reaches in response to upstream damming were 3.0 m, 2.5 m and 2.3 m, respectively. In addition, water levels at given discharges dramatically decreased at three hydrometric stations in the different reaches. The previous 5-year average incoming sediment coefficients are the key fluvial factor influencing the cumulative channel incision depth in the Lower Yellow River. The cumulative bank retreat width increased with the cumulative channel incision depth in the braided reach, and a linear function between these parameters was established. However, bank retreat occurred when the channel incision depths reached critical values in the transitional and meandering reaches. Empirical equations were established between the cumulative channel incision depth and the relative bed roughness in the braided and transitional reaches, and they indicate that the riverbed coarsening process can mitigate channel incision to some extent.