Bypassing of sand over sand waves and through a sand wave field in the central region of the Southern North Sea
Smith, D.B. (1988). Bypassing of sand over sand waves and through a sand wave field in the central region of the Southern North Sea, in: de Boer, P.L. et al. (Ed.) Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. pp. 39-50
In: de Boer, P.L. et al. (Ed.) (1988). Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. D. Reidel Publishing: Dordrecht. ISBN 90-277-2622-1. ix, 530 pp., meer
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| Abstract |
A sand wave field lies unconformably in the middle of a slightly undulating, current swept lag gravel pavement between Sandettie and South Falls banks in the central region of the Southern Bight of the Southern North Sea. In cross section, moving from north east to south west, this sand wave field undergoes a transition from large, relatively straight crested solitary sand waves with megaripples and smaller sand waves between them to smaller sand waves and finally a field of megaripples that in turn gives way to ephemeral sand ribbons. The low angle northern 'upstream' slopes of the large sand waves are typically straight or slightly convex with gradients of between 1 : 18 and I : 13 (3.3° - 4.4°). In contrast, the southwestern lee slopes are very steep. Gradients of between approximately 1 : 1.2 and 1 : 1.4 (35° - 40°) are typical. Samples taken from the sand wave flanks, together with side scan sonar recordings show that the low angle slopes are armoured with a surface layer of sandy, whole and broken shells containing fine chalk and flint gravel. The very steepness of the lee slopes indicates that they too are at least in part comprised of this coarse sediment and that the sand waves are not moribund but maintained by an asymmetry in the sediment transporting capacity of the prevailing tidal currents. Sand carried southwestward across the lag gravel surface that lies to the north and northeast of the sand wave field is transported up and over successive large sand waves in the sand wave field. Once across the larger sand waves the sand accumulates in the almost flat intervening troughs as patches of megaripples or, deeper into the sand wave field, as small sand waves. The large sand waves with their very steep 'downstream' slopes appear to function as a series of largely unidirectional locks that successively allow sand to be passed on through the sand wave field while the small sand waves between them form temporary sand storage buffers within the system. Beyond the last of the large sand waves sand is eventually lost from the sand wave field onto the succeeding lag gravel pavement through a sequence of small sand waves, megaripples and finally sand ribbons. The larger sand waves manifest the ultimate or penultimate stage in the degeneration of a sand wave field in a basin where reserves of sand have become depleted. |
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