Mechanism Of Riverbed Aggradation At Tongguan Reach And Channel Shrinkage At Lower Weihe River

Sanmenxia reservoir is the pioneer water conservancy project after liberation in China on the Yellow River, which belongs to Muddy River. Because of subject and object limitations at that time, and lack of enough study in the characteristics of incoming water and sediment of the Yellow River, there are some shortages in design and operation of Sanmenxia reservoir. Serious sediment silt in the reservoir not only made Tongguan riverbed rise rapidly from 323.4m to 328.1m after two years of operation of Sanmenxia reservoir, but also caused environment seriously deteriorated in the following drainage basins upstream of Tongguan reaches as the middle reaches of the Yellow River, the lower reaches of Weihe river and Beiluohe river. Because of the serious sediment silt, a series of serious disaster was brought about in these areas. Although Tongguan riverbed degraded at some period after two times of reconstruction of Sanmenxia project in 1960's, however since 1975, Tongguan riverbed rose like a wave with a rising tendency as a whole. Riverbed Agradation at Tongguan Reach caused the lower Weihe further deposited and shrinked, and brought drainage basin of the lower Weihe about big damage by small flood in August of 1992 and 1995, and July of 1996, and seriously interfere with the economic development of drainage basin of the lower Weihe, Shaanxi province.In this paper, the process and mechanism of the riverbed aggradations of Tongguan reach was analyzed synthetically based on field data by using of basic theory of river mechanics and basic principle of river bed evolution, and it was also numerically simulated by using of the hydrodynamic model of Xi' an University of Technology. From the above analysis and calculation, the following conclusions was drawn:1) Tongguan riverbed was at a balanceable state between erosion and silt or a tiny silting state before the construction of Sanmenxia reservoir.2) The main cause leading to Tongguan riverbed Aggradations is the operational ways of Sanmenxia reservoir at the period of the storing floodwater.3) At the period of the storing clear water and releasing muddy water, the average reservoir water level in non-flood season takes the first responsibility for riverbed aggradations at Tongguan reach, the average reservoir water level in flood season the second responsibility, and the incoming water condition the third responsibility. Although the silt amount at the reach of Tongguan and Guduo has heavy influence on the riverbed aggradations at Tongguan reach, it is a dependent factor that is controlled by the operation ways of Sanmenxia reservoir.4) Tongguan riverbed elevation is the first factor impacting on the riverbed deposition and channel shrinkage in the lower Weihe River, the runoff decrease the second factor.5) The effective and allopathic measure to resolve the problem of Tongguan riverbed rising is to improve and perfect the operation ways of Sanmenxia reservoir. That is to say, to lower the average reservoir water level in non-flood season and in flood season as possible as it can, and those will force Tongguan riverbed to be scoured and make Tongguan riverbed lowered.