SAND SUSPENSION AND TRANSPORT ON THE MIDDELKERKE BANK (SOUTHERN NORTH-SEA) BY STORMS AND TIDAL CURRENTS

Estimates have been made of the suspended sand transport at two sites on the Middelkerke Bank, in the southern North Sea, from suspended san d profiles and current meter measurements over a period of approximate ly 40 days. Sand resuspension was mainly due to waves while transport was dominated by a few hours when large waves coincided with peak floo d currents. Soulsby's relationships for the stress under combined curr ents and waves were found to be poor predictors for the observed near- bed concentrations; the mean stress, tau(m), predicting 45% of the var iance while the maximum stress, tau(max), predicted just 15%, and over estimate the effects of the waves. When the influence of the stress du e to the waves is reduced, the variance explained increases to 67%. Th e sand transport rate on the steep slope of the bank was 10 times that of the southern end, and was up-slope at 25 degrees to the bank axis, in the direction of the major axis of the tidal ellipse. The transpor t on the steep slope was mainly in the size range 100-140 mu m which d id not occur in any significant proportion in samples of the sea bed a t that site but was advected from deeper water to the southeast. Exclu ding this finer component the transport rates of coarser sand (>200 mu m) 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 t he continual growth of the sand bank due to asymmetrical transport by the tidal currents alone. The transport rates are consistent with a ti me-scale of 10(2)-10(3) years for the formation of the Middelkerke Ban k. (C) 1998 Elsevier Science B.V. All rights reserved.