边界条件对水库下游河床演变的影响——以汉江丹江口水库下游河道为例

THE EFFECT OF BOUNDARY CONDITIONS UPON THE CHANNEL PROCESS DOWNSTREAM THE DAN JIANG KOU RESERVOIR

通过野外考察和室内自然模型试验,本文探讨边界条件在水库下游再造床过程中的作用。从土壤物理力学性质与水流作用力的对比关系出发,建立了河床边界抗冲性的定量指标;采用多元回归分析的方法,建立了调整之后的河床形态与边界条件、水动力条件之间的经验方程;还运用判别分析的方法,得到了区分水库下游河床演变趋势的判别函数。

Due to the construction of Dan-Jiang-kou Dam and the reservoir regulation, the discharge downstream the dam has been approaching to a constant, and the sediment load has not been an independent variable any more.In this situation the effect of boundary condtitons upon the channel process is much bigger than that before the construction of the dam.Contrasting the soil mechanics properties of the bank and bed materials with the flow shear stress, the writer of this paper has established the follow-ing indices. 1) the bank resistance index Kbank= 2) the bedresistance index K. 3 )the relative resistance index, Kr =In addition, we have proposed some other indices about boundary conditions and flow conditions .They are. 4 ) the height from the bank top to the bed,H0; 5 ) the lower floodplain restraint index, 6 ) the circulation in-tensity index, 7 ) the angle between the thalweg line and the bank,θ. In the above-mentioned indices, M is the percentage of content of silt-clay in river bank material, D the median grain diameter of bed material,m bank slope coefficient, H the average flow depth, B0 the channel width at mid-discharge, B’0 the channel width when lower floodplain is submerged, J the channel gradient, Q the water discharge,R the bend iradius and r the specificgravity of water.By multi-regression analysis between the adjusted width depth ratioθB/h and the above-mentioned indices, we have.In spite of the discharge which is flattened and does not flood the floodplain any more, there are still two process tendencies, one is that bank widening is predominant, and the other is that bed cutting-down dominates. The two tendencies depend on the boundary conditions .U sing the discrimination analysis method, we have gained a discriminating function which decides the process tendencies.Y = 0.7044-0.058ln -0.216ln - 0.176lH0 + 0.028lnθif Y>0, the channel will be mainly widened; if Y<0, the channel will be mainly cut down.It is also indicated in this paper that the manner of channel gradient regulation has been controlled by the variation of boundary conditions downstream the reservoir, and so has been the tendency that the river channel becomes more sinuous.And the corresponding quantitative relations are given in this study.