EFFECT RESONANCE ON MORPHOLOGY OF TIDAL CHANNELS

Abstract

Tidal basins with a length of the order of 1/4 of a wave length of the dominant tide, susceptible to resonance, occur on many sites on the world. This paper will show, that these basins can become morphologically unstable. The most primitive schematization possible has been investigated: a prismatic channel, closed at one end. Under some circumstances (for which dimensionless criteria are given), small increments in depth intensify the vertical tidal amplitude at the landward side of the basin, which in turn triggers increased erosion of the basin. Knowledge on this subject can lead to redirect sand fluxes in the basin by taking small measures just in time. Starting from the assumption of a power-law sand transport formula, it is found, that for prismatic channels, morphodynamic instabilities occur when a small increment dh of depth generates a larger value2 of ZJh, where 2^ is the amplitude of the vertical tide at the landward end of the channel. This criterium can be translated in relative length of the basin and in bottom roughness (fig.2).