Sand suspension and transport on the Middelkerke Bank (southern North Sea) by storms and tidal currents

Abstract

Estimates have been made of the suspended sand transport at two sites on the Middelkerke Bank, in the southern North Sea, from suspended sand profiles and current meter measurements over a period of approximately 40 days. Sand resuspension was mainly due to waves while transport was dominated by a few hours when large waves coincided with peak flood currents. Soulsby's relationships for the stress under combined currents and waves were found to be poor predictors for the observed near-bed concentrations; the mean stress, τ m, predicting 45% of the variance while the maximum stress, τ max, predicted just 15%, and overestimate the effects of the waves. When the influence of the stress due to the waves is reduced, the variance explained increases to 67%. The sand transport rate on the steep slope of the bank was 10 times that of the southern end, and was up-slope at 25° to the bank axis, in the direction of the major axis of the tidal ellipse. The transport on the steep slope was mainly in the size range 100–140 μm which did not occur in any significant proportion in samples of the sea bed at that site but was advected from deeper water to the southeast. Excluding this finer component the transport rates of coarser sand (>200 μm) at the two sites were similar over the 40-day period. The up-slope transport during storms suggests that waves play an important part in the bank maintenance and are not simply the mechanism which prevents the continual growth of the sand bank due to asymmetrical transport by the tidal currents alone. The transport rates are consistent with a time-scale of 10 2–10 3 years for the formation of the Middelkerke Bank.