Effects of hypsometry on the morphodynamic stability of single and multiple tidal inlet systems

The evolution of the cross-sectional areas of tidal inlets draining the same back barrier lagoon is investigated with a simple model. The main new aspect is that hypsometry (cross-sectional area of the inlet and the wetted surface of the lagoon vary during the tidal cycle) is taken into account to determine the morphodynamic stability of the inlets. Results are presented for both a single and a double inlet system. In the double inlet system two inlets drain the same back barrier basin, which is divided into two sub-basins by a topographic high. vection of the depth-averaged along-channel momentum results in an extra term in the momentum equation. The joint presence of hypsometric terms and this new term instigates a shift of the stable equilibrium cross-section to smaller values. The sole effect of the hypsometry of the inlet cross-section is a larger amplitude of the tidal current, causing a shift of the stable equilibrium cross-section to higher values. Similar conclusions are found in the case of a double inlet system. For this configuration also multiple stable morphodynamic equilibria are found, which strongly depend on the cross-sectional area above the topographic high. It further turns out that the morphodynamic equilibria are sensitive to changes in parameters that break the symmetry of the system, viz. the ratio of the surface areas of the back barrier basins, the ratio of the inlet lengths, the phase difference of the tidal wave and the cross-channel bottom slope of one of the inlets.