Sediment exchanges across the coastal margins of NW Europe

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Abstract
This contribution serves as an introduction to the ‘Coastal Margins as Sediment Traps’ thematic set. One of the most basic questions which faces those concerned with sediment transport in the marine environment is whether the coastal margin is a net source or sink for sediment. The coastal margin is taken to mean the coast itself with its cliffs, bays and estuaries, and the immediate coastal zone stretching a few hundred metres to a few kilometres off shore. It is not clear to what extent the coasts of NW Europe have recovered from the last period of depressed sea level. Uncertainty exists as to whether sediment lost from the coastal margin is deposited in offshore sinks, whether sediment originally laid down on the continental shelves is returning to depositional sites located principally in bays and estuaries, or more particularly whether all coastal margins are subject to the same balance of processes. The reason for this lack of knowledge is clear. The calculation of the sediment budget of an estuary is difficult. The calculation of the budget for a whole coastal sector, including the continental shelf fronting it, is even more problematical. For entire seas our capacity for reliable quantification at the present time is negligible. A perspective on this issue can be obtained by considering the situation in a parallel discipline. Oceanographers have been studying the circulation of our shallow seas for 70–80 years, latterly with co-operative experiments using tens of simultaneously deployed, self-recording, current meters and the mathematical models