长江口浑浊带含沙量的潮流变化及其成因分析

实测数据显示在长江口南槽浑浊带内,近河床含沙量在很多情况下在转流后1—3h内出现峰值,在涨、落急流速最大时含沙量却迅速下降。本文利用实测数据计算了不问潮时的泥沙垂向紊动扩散系数,发现它与底层含沙量有类似的变化过程,结合近底层高浓度悬沙的迁移,认为转流后出现的含沙量峰值主要是由于憩流期沉降到河床的悬沙重新悬浮引起的。重新悬浮是浑浊带的重要成因。

A PHENOMENOLOGICAL STUDY ON TIDAL VARIATION OF TURBIDITY IN THE SOUTH PASSAGE OF THE CHANGJIANG ESTUARY

The characteristic variation pattern of the near bottom suspended sediment concentration has been identified for the first time during the hydrographic survey of the turbidity maximum in the South Passage of the Changjiang Estuary. In most of the monitoring periods (each generally exceeding 25 hours)there are peak values in one to three hours after the reversal of the tidal current, moreover the concentration decreases sharply thereafter. The origination of those peak values have been analysed based on the diffusion equation. The eddy diffusion coefficients Kz are calculated by using the field data. Meanwhile the vertical profiles of velocity have been fitted in calculation in order to get insight into its tidal variation. The Kz so calculated show a tidal variation course similar to that of the near bottom concentration,i, e. the maximum values occur in the midway of the current speed acceleration phase, It is indeed closely related to the tidal variations of the vertical gradient of velocity which have large initial values. On the other hand,the longitudinal section on the distribution of concentration shows that there is a foreward and backward movement of near bottom turbide water during a certain tidal period. The conclusion is that those peak values of concentration occurring soon after the slack-water are mainly caused by the strongest diffusion of the settled suspended materials due to the largest gradient of velocity at the same time. The resuspension takes place in relative short time since the amount of settled sediments seems to be limited. It disappears before the arrival of the full current,which restrains effectively the further increase of near bottom concentration and explains why the turbidity maximum in the Changjiang estuary is developed near the bottom.