Study On Impact Of Tongguan Riverbed Elevation On Fluvial Processes Of The Lower Weihe River By Numerical Model

A series of problems caused by the increasing riverbed elevation at Tongguan have been concerned by many departments along the upstream and downstream of the Sanmenxia dam since its construction in 1960. Recently, these problems are again concerned because of the planning of the Weihe river basin comprehensive regulation and the catastrophe caused by the small flood on the Weihe river in 2003. Accordingly, it is urgent and important to study the effect of different Tongguan riverbed elevations on aggradation and degradation as well as the flood elevation of the lower Weihe river.Firstly, past and present achievements are briefly reviewed in this paper. Secondly, based on practical field data, it is systematically analyzed and studied that the process and mechanism of Tongguan riverbed elevation as well as aggradation and degradation of the lower Weihe river bed before and after the construction of the Sanmenxia dam. By the analysis and study, it is showed that: 1) the lower Weihe river main channel was at a balanceable state between aggradation and degradation and floodplain of the lower Weihe river is a tiny aggradation state before the construction of the Sanmenxia dam; 2) the severeaggradation of the lower Weihe river bed is mainly caused by the increasing riverbed elevation at Tongguan after the construction of the Sanmenxia dam; 3) the main reason leading to the increasing riverbed elevation at Tongguan is the unreasonable operation of the Sanmenxia reservoir; 4) the rate of the riverbed aggradation at Tongguan reach and the lower Weihe river reach are speeded by adverse condition of incoming water and sediment recently. Thirdly, several problems are discussed, such as the calculation of the silting velocity of sediment, the sediment transport capacity, the coefficient of sediment resume saturation, etc. On the basis of these discussions, the mathematical model is improved. Lastly, the improved mathematical model of sediment transport is tested by practical field data of 1969~2001. It is showed that the calculated volume of silted and scoured sediment is well in accordance with the practical measured volume in the lower Weihe river. Finally, by using of the mathematical model, it is calculated and predicted the effect of the different Tongguan riverbed elevations as well as the different conditions of incoming water and sediment on the variance of the flood elevation and the tend of aggradation and degradation of the lower Weihe river bed. It is answered quantitively the lessening extent of sediment silt and the decreasing extent of the flood elevation of the lower Weihe river after 14 years on two different incoming water and sediment conditions with three different Tongguan elevations, respectively 328m, 327m and 326m. These results supply important references to the planning of flood control and comprehensive regulations of the Weihe river basin.