The narrow shelf concept: couplings and fluxes

The continental shelf domain has the nearshore zone as land-ward boundary while the shelf slope plays the role of sea-ward boundary. Shelf dynamics are, therefore, influenced by terrestrial and marine processes and are an important component for any integrated assessment of the coastal zone. The quantitative influence of nearshore/slope processes is, however, seldom considered and many continental shelves are considered wide (e.g. parts of the North Sea coast) or narrow (e.g. parts of the Mediterranean coast) based on intuition and implicit knowledge. No objective and quantitative assessment of the relevance of such processes in controlling the resulting shelf dynamics has been presented so far. The main aim of this paper is to examine the mechanisms isolating and linking the shelf to its neighbouring domains (viz. nearshore and slope) and, from this, to obtain a natural definition of the shelf category. The resulting critical shelf width, dividing shelves controlled by border processes from those controlled by their internal dynamics, turns out to depend on the considered process and its associated time/space scales. The paper will, thus, start by presenting the main mechanisms isolating shelf and slope, viz. the geostrophic and potential vorticity constraint. This will be followed by a discussion of the main processes linking shelf and slope, viz. non-linearities, boundary layer and unsteadiness effects and stratification. The same approach will be applied to the analysis of shelf and nearshore links, highlighting the distinction between isolating and coupling mechanisms. From this a natural evaluation of the critical shelf width will emerge, allowing a hierarchical classification of the various shelf width assessments and illustrating the relative importance of the different processes involved for the Ebro delta coastal sea (Spanish Mediterranean coast). The performed theoretical analysis, although based on field observations from the Ebro delta site, is also expected to be of use for other shelf environments, where there is a need to distinguish between the competing influences of nearshore and slope boundaries. (C) 2002 Elsevier Science Ltd. All rights reserved.