Study On Sediment Transport And Sediment Transport In The Lower Weihe River

The Weihe River, an important tributary of the Yellow River, is one of the major sources of water and sediment of the Yellow River. As a sandy river, it attracts a wide range of attention of scholars. Since the construction of Sanmenxia Reservoir, corresponding with the rise of the Tongguan Elevation, Weihe River appeared different degree of changes in sedimentation, especially in the lower reach. This maked the Weihe River’s water and sediment problem more significant. Gradually but steadily in recent years, the Tongguan Station’s elevation is getting plain and the river channel returns to a equilibrium state relatively. But problems of water and sediment disproportion and water use increment still exist. Finding out the new balance point of utilization of water resources and the water demand for sediment transportation under the new condition has become a fresh research interest. Concluding the formal research findings, along with the basic knowledges of sediment dynamics and river bed evolution, this paper aims to study the water demand for sediment transportation and the channel scale for water and sediment transportation in the future, under the current condition of Tongguan Station’s elevation and the specific character of Weihe River’s runoff and sediment load. The contexts and main conclusions of this paper are as followings:1. Based on the analysis of the boundary condition, the characteristics of water and sediment, the regional distribution of runoff and sediment and the temporal proportion of water and sediment from 1950～2009, the findings are:1) The downstream of Weihe River is of water shortage and rich sediment, characterized as different sources of water and sediment. The interannual variability of water and sediment is larger, with a general decrement of water and sediment input from 1960s and an obvious decrement from 2000. The flow and sediment concentrates in the flood season, with the sediment proportion in the flood time of Xianyang station, Huaxian station and Zhangjiashan station up to 97%.2) The sediment input from Jinghe river causes the unbalance of water and sediment relationship of the lower reach of Weihe river. The yearly-averaged and wet seasonly-averaged incoming sediment coefficient of Huaxian staition is 0.217kg/s.m6 and 0.178kg/s.m6, which an order of magnitude larger than that in Huayuankou station. The relationship of water and sediment went worst during 1986-2002, is appearing better during wet seasons and is becoming worse during dry seasons since 2003.3) The flow of 0～500m3/s appeared most of the time in the lower reach of Weihe river, and the flow of 0～1000m3/s is the main one for water and sediment transportation. The amount of sediment transported by the flow of 0～500m3/s is increasing since 2003.2. The sedimentation processes and characteristics of channel shrinkage are also analyzed in this paper,which can be concluded as:1) Since the operation of Sanmenxia Reservoir, the lower reach of Weihe river suffered from severe sedimentation of about 1.286 billion m3 totally. The sedimentation was very significantly before 1973 with the rapid increment of the Tongguan elevation. The channel was stable relatively from 1973 to 1985. Due to the obviously decreasing of runoff from 1986 to 2002, the lower reach of Weihe river repeated sedimentation with the rising of Tongguan elevatioin. Since 2003, with the adjustment of the operation of Sanmenxia reservoir to open discharge during wet seasons and encountering the favorable runoff and sedimen in 2003 and 2005, the channel is getting in the state of slight erosion.2) The river channel was shrinkage before 2003 in generally, which showed as the reduction of the flow capacity of the main channel, the constriction of the cross-section and the rising of thalweg, and so on. The treand of channel shrinkage mitigates since 2003, with the bankfull discharge grows to about 2500m3/s.3. After the conclusion of the traditional methods to calculate the water demand for sediment transportation, three methods based on data analysis, scour and silting correction and non-equilibrium sediment transport theory are proposed in this paper to calculate the water demant in the lower reach of the Weihe river. It was found that: 1) the annually flow for sediment transport at Huaxian station is about 5.24 billion m3 the runoff of about 0.42 billion m3 was not used to transport sediment during 1973 to 2009.2) the runoffs of about 6.02 billion m3,6.51 billion m3, and 7.00 billion m3 at Huaxian station are necessary if the sedimentation amount decreases to 10 million tons,5 milliom tons and the balance of scouring and silting, respectively.4. A mathematical model is used to predict the sedimentation processes and the variation the channel scale for water and sedment transport under three different incoming water and sediment conditions. Computing results show:1) the lower reach of Weihe river will also be silting of about 0.19 billion ton in the next 10 years and 1.30 billion ton to 1.43 billion ton in the next 60 years accumutally.2) The main channel scales of about 2300mJ/s and the water for sediment transportation in the next 10 years between Lintong Station and Huaxian Station will be 2300m3/s and 2700m3/s can be maintained in the reachs of Lintong to Huaxian and downstream Huaxian, respectively. However, the bankfull discharge of the lower reach of Weihe river will be less than 2000m3/s in the coming 60 years.