TIDAL RECTIFICATION ON DRYING ESTUARINE SANDBANKS

Drying estuarine sandbanks experience only that part of the tidal cycl e around high water. In a partially progressive tidal wave, this means that the duration of the flood over the sandbank will be greater than that of the ebb: a process of tidal rectification. In this paper, we propose the hypothesis that this leads to floods-directed tidal residu al currents over drying sandbanks. The hypothesis is tested by observa tion and a 2-D hydrodynamical model in the Conwy estuary, a vertically well-mixed macrotidal estuary in North Wales. The observations includ e tide gauge data, tidal cycle boat surveys, and fixed current meter d ata. The data show weak flood-directed residual currents over a drying sandbank and much stronger ebb-directed residuals in the channels alo ng the sides of the sandbank. The model reproduces the observations in the vicinity of the sandbank and shows that the tidal rectification m echanism produces a general pattern of residual circulation in the est uary, with flood-directed flow in the drying areas and ebb-directed fl ow in the channels. The flood residuals are most marked near the estua ry mouth where the tidal wave is most progressive in nature. The main application of this mechanism is believed to be in the transport of be dload sediment The flood-directed residuals will tend to move the tops of the sandbanks upstream.