Modelling the equilibrium hypsometry of back-barrier tidal flats in the German Wadden Sea (southern North Sea)

Abstract

Hypsometry is the distribution of volume or horizontal surface area with respect to elevation. Some observations show contradictory scale-dependent characteristics of tidal flat hypsometries in back-barrier tidal basin environments, and traditional theory explains the flattening hypsometry by relating concave-up hypsometries in low tidal range basins to the dominating influence of wind waves rather than tidal currents. In order to investigate these two problems, two series of numerical modelling exercises were carried out using schematized rectangular back-barrier tidal basins roughly corresponding to the tidal basins found in the German Wadden Sea. The results show that, in the equilibrium states of tidal basins, hypsometries of back-barrier tidal flats are dependent on the basin scale and tidal range. Thus, large basin areas and low tidal ranges favour strong concave-up hypsometries, whereas small basin areas and high tidal ranges favour less concave-up hypsometries. Because wind waves were excluded in the model, these relationships are purely associated with tidal-current action. In addition to the traditional theory, which emphasises the relative importance of wind waves, the flattening of hypsometric profile shapes can, therefore, also be interpreted as a response to the relative area of intertidal areas or channels in the tidal basins. Thus, if tidal channels have relatively large areas, the development of tidal flats between channels is markedly restrained, the tidal flats being prevented from growing upward to higher elevations. As a result, strong concave-up hypsometries are formed that are associated with relatively large areas of low tidal flats and small areas of high tidal flats. As basins with large areas and low tidal ranges have large relative channel areas, this results in pronounced concave-up hypsometries, and vice versa.