黄淮海平原河流的纵剖面凹度特征

A STUDY OF LONGITUDINAL PROFILE CONCAVITY OFRIVERS IN THE NORTH CHINA PLAIN

本文以黄淮海平原为例,对于冲积河流纵剖面凹度的形成机理及其与不同河型的关系,进行了初步探讨。

It has been suggested in this paper that the profile concavity of rivers can be expressed bythe exponent in the power function describing the relation of elevation fall to river length, and by using this index the formation cause of channel profile concavity of alluvial rivers has been dealt with based on data from the rivers in the North China Plain. The results obtained are summarized as follows. 1. Profile concavity of alluvial rivers reflects the manner in which the potential energy available is distributed along river length in a downstream direction. As the downstream variation of energy expenditure is controlled by the river's relative load (namely, the sediment load in relation to the water discharge), a close relation can be expected to exist between river profile concavity and the sediment concertration p at the controlling station where the river enters the plain, as well as the relative change rate of water discharge downstream (Q2~Q1)/Qt. For rivers in the North China Plain, the regression equation can be established as 2. Profile concavity of rivers in the Ncrth China. Plain shows a close relation with their channel pattern characteristics, notably, the river with a longer reach of wandering braided channel generally has a lower profile concavity, while the river with the meandering reach appearing not far from where it enters the plain has a high profile concavity. In addition, it has been found that the higher the river's profile concavity, the more stable its channel will be. 3. As a measure expressing the manner in which energy is distributed along the river length, profile concavity reflects the material migration character in the river basin. As demonstrated by this study, rivers with a high intensity of material migration from their drain age basin have a low profile concavity for the lower reaches in the plain. 4. The river channel changes of the lower Yellow River downstream from the Sanmenxia Dam indicate that scour by clear water released from the reservoir led to an increased profile concavity, but after the reservoir has been operated for flood retention, the profile concavity started to decrease and returned to its original low level in a few years. 5. By using the conclusions reached in this study, a new explanation may be given concerning the formation of climatico-genetic river terraces. During the period when the climate is relatively dry, the river tends to form a channel with low profile concavity. If climate turns to be humid, a more concave-shaped channel will occur. This means that downcut of river bed will take place, with the maximum depth of downcut in the middle reaches, and a decreasing downcut depth towards both the head and the mouth of the river. Then a terrace is formed, with its relative height varying in the same way along river length.