Abstract

A permanent contour current, which moves sand northward along the upper 550 m of the slope west of Scotland, forms sand waves up to 2 m high. This current may occasionally sweep across the outer shelf and into the North Sea. Preliminary results from a numerical model have been used by R. Flather to investigate the distribution of extreme currents associated with tides and surges. The maximum computed surge currents from 16 storms exceed 60 cm/s on much of the outer shelf in depths of 100 to 150 m and exceed 100 cm/s in some parts of the North Sea. Tidal, surge and wave oscillatory currents are strong enough, particularly when combined, to move sand on most parts of the shelf.

An attempt is made to determine areas dominated by tides, waves and surges in Northwest European seas, whilst accepting that much of the shelf will be affected by considerable variation in flow conditions. Most of the larger bed forms found in tide-dominated areas (R. Belderson, this meeting) can be found in surge-dominated areas. However, large, sharp-crested, symmetrical sand waves are exclusive to tidal areas.

Tabular sand patches are found only on those parts of the open shelf where the modelled values of extreme surge current and tidal current are less than the threshold velocity for sand transport. They are usually symmetrical, with relatively steep edges (up to 15°), very broad flat tops and a height of 2 to 3 m. They consist mainly of fine sand, but some of them are known to be intermittently buried by mud. Bioturbation is common. These neglected bed forms have been described from a number of wave-dominated shelves. As they are one of the few bed forms from Holocene shelves that appear to be indicative of wave dominance, it is important to discover their internal structure. Apart from a low sand supply, they have many of the characteristics of environments where hummocky cross-stratification has been predicted.