Evolution of meanders with hyperconcentrated flow in the loess areas

Abstract

In the loess areas, especially on the Loess Plateau in China, most of the rivers discharging hyperconcentrated flows have developed a typical meandering pattern. Based on the data of thirteen meandering rivers on the Loess Plateau and the Yangchangzi River, the properties of water flow and sediment load of these rivers, the effects of variations of flow and sediment load on channel cross sectional geometry and sinuosity, are analyzed in the paper. The data include the daily flow and sediment load and the geometry of channel cross sections recorded from 1976 to 1980 as well as the plan of channel measured on the landscape maps with a scale of 1 : 50 000 or 1 : 100 000. From these data we extrast parameters reflecting the characteristics of flow and sediment load, channel geometry, and sedimentation in channel and channel adjustment thereupon. Follows are the results of analyses. 1) As the rivers with low sediment concentration, the hyperconcentrated flow dominated rivers have a narrower and more sinuous channel if the fluctuation in water discharge is low. 2) With the increase of sediment concentration, the channel cross sectional geometry is usually widened in the case of the rivers with low sediment concentration. Since a narrow channel is one of the requisite conditions as well as the result of sediment transport of hyperconcentrated flows, the channels become narrow initially as the sediment concentration of hyperconcentrated flow increases. However, if the sediment concentration is larger than a certain value, the energy consumption of sediment transport is lowered down thenceforth with the increase of sediment concentration, and the flow becomes unsaturated and has a higher channel making capacity, so the channel is widened. Revealed by the instantaneous hydrological records, the transition takes place at the sediment concentration of about 400 kg/m3. 3) Both excessive and low intensity of sedimentation in the channel is unfavorable to development of meanders. In the case of the rivers with a low sediment concentration, the energy consumption per unit distance increases with the enlargement of slope and discharge and so the capacity of channel making of the flows; therefore, the channel sinuosity increases initially and decreases later. Regarding the hyperconcentrated flow dominated rivers, the energy consumption of sediment transport is lowered and the channel making capacity as well as magnitude of sedimentation are enlarged accompanying with the increase of sediment concentration, so an initial increase of channel sinuosity is followed by a decrease also. Due to the difference of the mechanism of channel sinuosity adjustment from that of cross section changes, the existence of the asychronism in the adjustment of the two factors is reasonable.

依据中国黄土分布区多沙曲流的水沙资料、河床平面形态和河床大断面测量资料分析认为: 多沙河流来水变率影响河床断面形态和曲流发育; 由于窄深的河槽是高含沙水流输沙的必要条件和结果, 因此随着含沙量的增加, 多沙河流河床断面形态首先出现变窄深的趋势, 随着含沙量的进一步增加, 水流造床能力增大, 河槽会向宽浅方向调整; 一定的河床冲淤强度有利于曲流的发育, 随着含沙量的增加, 多沙河流河床曲率存在先增加后减小的规律。