黄河河口河床响应河口延伸的调整极限及其机制

在三角洲建造过程中河口不断延伸，黄河口河道相应发生调整。分析观测资料发现，河床可调整要素宽深比、比降、河床质组成的调整都存在一个极限值，并在河口河道行水晚期的淤积中保持基本不变。研究认为河床质调整极限是由来沙组成及河床质细化机制决定的。但是，河槽的宽深比不能进一步减小，以保持在更小比降下的输沙平衡，并非宽深比所对应的河槽形态已是所谓最大输沙形态，而是由于大流量过程和小流量过程不能适应对方所塑造的河槽形态，相互破坏对方塑造的窄深河槽。所以出现了虽然断面缩窄可加大输沙力，而河槽却不向这个方向调整，这样一个表面上违背自然规律的现象。解决这对矛盾对河口河道治理有着重要意义。

Thresholds and mechanisms of channel adjustment in the Yellow River mouth

Using channel geometric surveys and hydrological records, this paper analyzed the processes and mechanisms of channel adjustment in the Yellow River mouth responding to the prolongation of the river delta. It was found that with the extension of the river mouth the bankfull width-depth ratio, bed material size, and slope decrease gradually in the young and mature stages of a mouth channel but reach a lower limit or threshold in the old stage. The occurrence of the lower limit of bed material size is attributed to the grain size composition of incoming sediment and the fining mechanism. The reason for the existence of a limit of bankfull cross-sectional shape is that the large flows can fully transport the sediment load they are carrying. The channel formed in the large flows is unfit to the low flows, resulting in siltation in the channel in the low flows. On the other hand, the channel filled up in the low flows may lead to siltation in some high flows which may transport sediment carried by the river through the channel before it was remolded by the low flows. In this way, the channel slope is kept around a lower limit by sediment accumulation in the channel throughout the old stage. It seems that it can be a good way to manage the mouth channel by reducing the natural limit of width-depth ratio of the channel through regulating the flows, or separating high flows from low flows, or narrowing the channel using channel training works.