渭河下游河道调整过程中的复杂响应现象

COMPLEX RESPONSE IN THE CHANNEL ADJUSTMENT OF THE LOWER WEIHE RIVER TO THE CONSTRUCTION OF THE SANMENXIA RESERVOIR

应用系统复杂响应的原理研究三门峡水库上游渭河河道调整过程,表明上游河道河床形态对基面上升所作出的响应是复杂的,弯曲系数的变化为先减小而后增大,宽深比的变化是先增大后减小,比降由先减小而后增大,最后均趋于稳定。

Applying the concept of complex response in systems to the study of river channel adjustment upstream from a reservoir, this paper has deals with the river channel changes of the lower Weihe River after the construction of the Sanmeaxia Reservoir. The dam divided the river channel system into twosubsystems, namely, upstream and downstream reservoir subsystems. For the former, the most remarkable change in the inputs was a rise in base level, which led to the aggradation in the river channel..As a result, the river adjusted its channel geometry to accommodate the altered input conditions, in order to get a new equilibrium .Plotting the river channel sinuosity, the width-depth ratio and channel slope against time respectively, we can see that the process of adjustment was very complex. At the beginning, because the aggradation mainly concentrated within the channel other than on the floodplain, the height difference between the bed bottom and the surface of the floodplain reduced gradually .This made the river bed shallower and wider than it was before Consequently, the opportunities for occurrence of over-bank flow was increased, leading to a more frequent occurrence of neck-cutoff of river bends.This gave rise to a decrease in river channel sinuosity .As the adjustment went forth, an inverse tendency appeared.The increased frequency of over-Lank flood tended to speed up the aggradation on the floodplain, which resulted in a rise in the height of floodplain, and also a smaller width-depth ratio Then the frequency of overbank flood was reduced gradually, and the opportunities for occurrence of bend cutoffs became less frequent, leading to an increasing river channel sinuositytlf the duration of adjustment was sufficiently long, a new equilbrium in the subsystem would be achieved, and both the sinuosity and the width-depth ratio would tend to be a constant.In the process of complex response mentioned above,the changes in boundary material composition also played an important role, which has been explained in this paper. Hassed on the description and explanation of river adjustment of the lower Weihe River, a descriptive model was established to generalize the complex response in river channel adjustment after a dam is constructed downstream.