According to the clear water discharge period of Sanmenxia Reservoir and Xiaolangdi Reservoir, the bed sand D50 in the lower Yellow River will be coarsened from 0. 1mm to 0.2 ~ 0.3 mm, and the river resistance n will be increased from 0.0 1 to 0.03 ~ 0.04. When the hyperconcentrated flood composed of extremely fine sand comes, the shear force acting on the bed surface will increase due to the sudden increase of flow.
The main reason for the increase of flood peak and flood volume in the reach above Huayuankou is that the riverbed shape is flattened from sand ridges and sand waves with great resistance in the early stage, the resistance is greatly reduced, and the increase of flow velocity leads to the decrease of river water storage. Because of the specific gravity of water flow when the flood with high sediment concentration comes, it will inevitably squeeze the clear water in the river; Thereby increasing the discharge flow.
Taking the river flow of 1500 m3/s as an example, it is assumed that when the value of Manning coefficient n suddenly drops from 0.03 to 0.0 15, the average velocity of water will increase from 1m/s to 2m/s, and the corresponding water passing area will also drop from 1500m2 to 750m.
The history of harnessing the lower reaches of the Yellow River;
Over the past 70 years, the Yellow River management system has changed from single to comprehensive, from the initial emphasis on downstream flood control to the whole basin system management; In order to adapt to the new situation of increasing river-related behaviors, increasing intensity and expanding control objects and elements, the comprehensive goal of "no dike breach, continuous smooth river course, no excessive pollution and no elevation of river bed" has been gradually established, and a comprehensive support system for maintaining the healthy life of the Yellow River has been constructed.
The winding Yellow River, which runs through ancient times and nourishes everything, is the mother river of the Chinese nation. At the same time, the Yellow River was once a river in danger, which was famous for its good siltation, easy migration and frequent floods. In its initial development process, the Chinese nation not only benefited from the nourishment and moisture of the Yellow River, but also suffered from the flood of the Yellow River.
The working people and the sages of river management throughout the ages have made arduous practical explorations to control the flood and drought disasters in the Yellow River. However, due to the complexity, productivity and social system constraints of the Yellow River, or lax river management, or ineffective management measures, the situation of the Yellow River has never fundamentally changed.