分汊型河床的形成与演变——以长江中下游为例

A STUDY ON THE FORMATON AND EVOLUTION OF BRAIDED CHANNEL WITH STABLE ISLAND——TAKING THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER AS AN EXAMPLE

本文根据多年的地质、地貌,来水来沙等考察资料,河道变迁的历史记载以及实验研究成果,分析了分汊型河床的两个必要条件及三方面的演变特征,最后用河流力图使自己的能量损耗率达于最小的假说对汊河的形成和演变作了解释。

Taking the middle and lower reaches of the Yangtze River as a typical example, this paper deals with the problem about the formation and evolution of braided,channel with stable island.Two fundational requirements are necessary for the formation of braided channel with stable island. 1. Sediments are deposited in the centre of a river, 2. Deposited sediments become some srable deposit bodies with a cetrain shape.In order to attain these requirements it is necessary that various conditions of the river itself, including geological, geomorphological and boundary conditions, coming water and coming sediments should be combined in a certain way, In the middle and lower reaches of the Yangtze River these conditions are as follows.1. The geomorphological condition with alternate wide and narrow seetions. This brings about the difference of stream power along the river and then the difference of depositing places. Besides, the free shift of the channel in lateral direction is also confined.2. Different geologic structures along the river. It is discovered from the data of natural conditions and experimental researches that crustal subsidence favours the widening of the channel and the formation of central bars or islands, while crustal uplift cuts down the channel and forms straight and single channel.3. The moderate erodibility of the channel boundary makes the channel moderately widened and sediments deposited in the centre of the river.4. For the properties of the coming sediments there must be a suitable proportion between the rolling and suspended materials, i.e. the former occupies 13.7% and the latter 25.7%. This makes the deposited sediments become a stable island. From a lot of historical records and real measured material, we obtainthe evolutional law of the braided channel with stable island as follo’ws.1. At first a lot of piecemeal central bars are formed in the river, then they gradually combine with each other, develop and become islands, and then move down-stream; or side bars are formed and cut by the flow and become central bars, and then the above sequence goes on repeatedly.Thus is the general law of the formation and development of the braided channel with stable island.2. After the braided channel with stable island is formed, the status of every branching channel changes alternatively itself, some develop and others weaken or even die off, then new branching channel is formed again.3. The period in which the alternation between the main channel and the branching, channel is performed is relatively long, from decades to hundreds of years, so the braided channel possesses relatively stable property.The internal-mechanism of the formation and evolution of braided channel with stable island can be explained by the hypothesis that a river has a tendency to minimize its rate of energy dissipation. Depending on the concrete conditions of a river and its changes, it can adjust itself to achieve this purpose. The concrete conditions of the Yangtze river are just the case to force the channel to become a braided one with stable island by means of automatic adjustment.