Geometry of tidal inlet systems: A key factor for the net sediment transport in tidal inlets
The construction in 1932 of the ‘Afsluitdijk’, which closed off the Zuiderzee from the Dutch Wadden Sea, decreased the length of the back-barrier basin connected to the Texel and Vlie inlets from ~110 km to ~45 km. It was observed that after this closure, a large volume of sand was transported from the North Sea into the Wadden Sea.
These observations raised the question how the net sediment transport in a tidal inlet depends on the length of a back-barrier basin. To answer this question, a one-dimensional analytical model that computes the water motion in a tidal inlet that is connected to a back-barrier basin was constructed. Besides the externally forced semi-diurnal lunar tide, residual currents and quarter-diurnal tidal currents are generated in the model by nonlinear interactions (e.g. the bottom friction depends on the time-varying water depth). The joint action of these currents and the imposed semidiurnal tide causes net sediment transport.
In this study, it was found that both the magnitude and the direction of the net sediment transport vary with the length of the back-barrier basin. By mimicking the situation in Texel-Vlie inlet system, it was shown that the closure of the Zuiderzee changed the direction of the net sediment transport in the inlet that results from the interaction of the semi- and quarter-diurnal tidal currents. Before the closure, this process caused a net export of sediment, while after the closure it caused a net import. Thus, the explicit dependence of the net sediment transport on the length of a back-barrier basin provides a possible explanation for the sediment import that was observed.
Wim Ridderinkhof, PhD student Coastal and Shelf Sea Dynamics at IMAU