Offshore Tidal Sand‐Banks as Indicators of Net Sand Transport and as Potential Deposits

Modern tidal sand-banks are generally not quite longitudinal bedforms, but are aligned obliquely to the regional direction of peak tidal flow and to the resulting net sand transport direction by as much as 20°. The crest lines of most sand-banks are rotated in an anticlockwise sense with respect to the regional peak tidal flow direction. In such cases the local sand transport direction on the banks, as indicated by sand-waves, veers to the right on approaching the crest of the bank from either side. Where the regional net sand transport direction is known, the steeper side of the sand-bank is found to face obliquely down the transport path. Thus, as with sand-waves, the asymmetry of the bank may be used as an indicator of net sand transport direction, providing the bank's sense of offset with respect to the peak tidal flow is known. Sand-banks in areas where the peak speeds of ebb and flood tidal currents are equal tend to have symmetrical profiles. New predictions of net sand transport directions for the Southern Bight of the North Sea are in agreement with earlier predictions. The predicted direction in the entrance to the White Sea (U.S.S.R.) is northwards, in the Yellow Sea it is seaward out of Korea Bay but it is landward into the Approaches to Seoul. Actively maintained sand-banks are distinguished from moribund ones formed at lower sea-levels. The moribund sand-banks have gentler slopes and sand-waves are less frequently found upon them. With continuing transgression such moribund sand-banks could be wholly or partly preserved beneath a mud blanket. Any preserved internal structure will probably take the form of small-scale high-angle cross-stratification, whose dip is almost opposite to that of the regional net sand transport direction, as well as to the large-scale low-angle interfaces parallel to the steep face of the bank.