Study On River Reconstruction For The Ningxia Reach Of The Yellow River

The Ningxia reach of the Yellow River is historically natural status and its main channel sways frequently. Flood control becomes an important problem that limits development of local economy and society. The government has invested much in reconstruction of the Ningxia reach since 1998, but many problems appeared in the reconstruction process. Therefore, it is rather necessary to study the problems related to reconstruction of the Ningxia reach.The Ningxia reach of the Yellow River is 397 km long, and the main reconstruction reach located in plain region is 266.7 km long, which is divided into three reaches: the first is unsteady biforked channel between Xiayanhe and Rencundu, the second is bend-type channel between Rencundu and Toudaodun, and the third sway-type channel between Toudaodun and Shizuishan. The upperstream bed of Rencundu is consist of cobblestones and gravels, and the downstream bed of Rencundu is consist of sand.River reconstruction scheme is determined based on reconstruction guidance, river features, evolution tendency, economic level, etc. By analyzing river evolution features, this paper proposes a reconstruction scheme: mainly guiding flow by engineering constaiction, properly strengthening main channel and weakening sub-channels, andcombining other assistant methods. It is shown that a slightly-bending reconstruction scheme is suitable for the reach between Rencundu and Shizuishan. By comparing several calculating methods, the following reconstruction flow rates are proposed: 2500 m3/s for the reach between Xiayanhe and Qingtongxia, and 2200 m3/s for the reach between Qingtongxia and Xiayanhe. Modelling tests show that largest radius of the river bend along the Ningxia reach is 4450 m, smallest radius is 1450 m, averaged radius is ', 3206 m, largest central angle is 96? smallest central angle 36?and an averaged angle 64.9? Theoretical calculation and modeling test show that depths of scoured pit near the T-type dam may reach -10-13 m for the downstream of Rencundu with sand bed. For thedeepest case, largest scoured pit depth may reach 20 m. Based on the present research result, some detailed measures and methods of reconstruction are proposed.