Mobility of Large Sediment Features In the Southern North Sea

Abstract

Abstract: This paper examines areas of seabed instability along the east coast of England and Scotland. It demonstrates the existence of sediment circulation patterns which slowly evolve. When spoil deposits are injected in such systems they are seen to accelerate the process. However, an understanding of such circulations may permit the system being used for natural storage of spoil where a build up of a shoal can be tolerated. Mobility of Large Sediment features in the Southern North Sea: The east coast of U.K. is characterised by the existence of large sandbodies which can attain lengths of 25 miles, and may be 8 - 9 miles wide. It is also a coastal area which has known intensive navigation for several centuries and some surveys acceptable by present day standards have been made for almost 200 years. These hydrographic surveys were and still are subjective to a degree, in the sense that the surveyors experience and local knowledge make a significant difference to the quality of the survey. They contain a deliberate bias towards better consistency in the depth ranges of 3 to 6 fathoms which for over a century has been the warning depth zone for shipping or in deeper water show a marked preference for the shoaler soundings. During the last decade navigational attention has shifted dramatically to the 10-15 fathom depth range because the size of ships has increased enormously. The consequence has been a vastly increased need for new hydrographic surveys in areas where little or no historical knowledge exists of either the shape of the seabed, or its stability. In existing surveys in areas which have frequently been examined the information is not as reliable as at shoaler levels. The adequacy of available means to delineate the seabed accurately is the subject of another paper, being presented at this Conference (1), whereas the present paper deals with the question of relative stability of the seabed and the consistency of the movement to which it is subjected. The first example is a channel system in the Outer Thames Estuary (fig. 1, 2) called the Edinburgh Channels, which for a period of about a century traversed a major sandbank, the Long Sand, at a point where it is some 5 miles wide, which until recently, provided access by major shipping to the Port of London by a shorter route than the one now in use. The historical development of these channels is shown in Fig. 3 for the period 1839 to 1962. Initially a shallow cut is seen almost to cross the bank from SE to NW, but of more interest at this stage is the growth of two converging "spits", one from Knock John bank eastward, and the other from Long Sand westward. The sequence shows it took about 60 years for the convergence to build a new bank about 3 miles long by a mile wide, and at one point over 40 feet in thickness when referred to the 1839 surface.