A PHENOMENOLOGICAL STUDY ON TIDAL VARIATION OF TURBIDITY IN THE SOUTH PASSAGE OF THE CHANGJIANG ESTUARY
长江口浑浊带含沙量的潮流变化及其成因分析

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.