Study On Water And Sediment Characteristics Of Branching Reach With Bend By Experimental Generalized Model

Most of braided channels are formed in the extended lower reach of narrowmountainous river with bend, where the beach risks are the difficulty of waterwayregulation. It is necessary to master the water and sand characteristics and river bedevolution regularity, which are crucial premises of regulating channel well.In this article, several of material is collected, the upper reach of the Yangtze Riveris made as the prototype, changeover portion of bending、widening and braided riverreach are classified from the hydraulic geometry relation point of view,which is dividedinto “convex shore type” and “concave bank type” according as the relative positions ofmain channel during dry season, which is also divided between primary braided riverand secondary braided river according as the period and the relative positions ofdeposition.The incoming runoff of ZhuTuo hydrological station in1978years isregarded as a typical hydrological process in mountainous rivers, which is the basis ofthe incoming runoff and sediment、the lasted time for generalized model. so thetypical “S” type and extended generalized model with bend in the mountainous streamhas been established. Both of physical generalized model test and numerical modelcalculation of Caiyuanba river as the natural river are the important methods for theanalysis.The influences of the boundary conditions including plane form、cross-section shapes on the water and sand characteristics and river bed evolutionregularity are studied.Firstly, through flow test and sediment test on the fixed bed as the main researchmeans, in some incoming runoff conditions and changeover portion boundaryconditions such as straight equal-width single channel, extended as2.3times singlechannel, extended as2.3times compound channels,.The influences of the boundaryconditions on the flow are analyzed synthetically.The experiment shows that transversegradient in the extended compound channel is distributed in the form of non-fullsymmetry and get maximum in the mid-position of channel bend.The transversegradient of export is bigger than that of entrance in the channel bend. The maximum oflongitudinal river slope in concave bank appears around export of the channel bend,which in convex bank is around entrance of the channel bend. The maximum relativescale parameter（？）B_max of marginal bank below the concave bank ofchangeover portion is built based on the experience summed up by our predecessors according to single channel. The corresponding expressions of transverse gradient andlongitudinal river slope have been fitted.Secondly, through the movable-bed model test of sediment deposition, the riverbed evolution regularity impacted by extended compound channel has been studied Thesub-slot of left bank is formed in the high water period; the major trough of right bank isalso formed in the low water period; the chur or mid-channel bar is formed. The aboveevolutionary process in the changeover reach has been showed vividly. The experimentshows that the position of major trough in the low water period is more close to the rightbank when the river width is more broadened.Finally, through2-D flow numerical model, Caiyuanba River is regarded as atypical extended reach with bend in mountain river and is calculated. The results ofcalculation are compared with the conclusion of generalized model test.It shows thatthe water and sand characteristics and the river bed evolution regularity in the naturalriver and generalized model keep agreement basically.In this article, the conclusion would be directly used in waterway regulation of theupper reach of the Yangtze River, which can also be applied to the mountainous rivershaving similar conditions and exist the great reference value in river bed evolution andwaterway regulation.