Fluvial processes and morphological response in the Yellow and Weihe Rivers to closure and operation of Sanmenxia Dam

摘要

The fluvial and morphological processes induced by impoundment of the Sanmenxia Reservoir and relevant human activities on the Yellow River and its tributaries are complex. The long-term annual sediment load of the Yellow River was 1.6 billion tons, ranking first of all the world's rivers. In 1960, Sanmenxia Dam began filling. Sediment transport in the river then was greatly disturbed and a new cycle of the fluvial processes was induced. First, the dam caused not only anticipated sedimentation in the reservoir, but also serious sedimentation in the largest tributary of the river (the Weihe River). The response of fluvial process to the dam closure varies in space and time. Second, the downstream reaches of the dam experienced erosion and resiltation, changes of river pattern, and development of meanders. Moreover, the downstream reaches of the dam have experienced more and more water diversion, which has induced readjustment of the longitudinal profile of the river. The study reveals that sedimentation in the Sanmenxia Reservoir enhanced the bed elevation at Tongguan, where the Weihe River flows into the Yellow River. The rising Tongguan's elevation caused retrogressive siltation waves in the Weihe River, which propagated at a speed of about 10 km/yr. An equilibrium sedimentation model is proposed, which agrees well with the data of sedimentation in the Weihe River. In the reaches below the dam the river changes from braided to wandering, or from wandering-braided to wandering-meandering. The discharge released to the downstream reaches has been regulated by the reservoir and it decreases along the course because the quantity of water diversions is more than the inflow from tributaries. The reduction in discharge causes readjustment of the longitudinal bed profile. By using the minimum stream power theory, we prove that the riverbed profile is developing toward an "S-shape" profile.