The Zwin is a unique nature reserve at the border between the Netherlands and Belgium. A tidal channel connects it to the North Sea. Through this channel seawater flows into the Zwin twice a day, resulting in an intertidal area of mud flats and vegetated platforms. Due to small differences in height, certain areas are more or less frequently submerged, which leads to an extraordinary diversity in flora and fauna.
Unfortunately, the Zwin suffers from the continuous net import of marine sediment (sand), which is currently jeopardizing the unique character of the area. In fact, the excess accretion of sediment has led to (i) a decrease of the tidal prism (the volume of water that flows in and out of the Zwin each tidal cycle) and (ii) a local increase of the bed level. The latter causes a decrease in the portion of the area that is frequently submerged by tides, thereby challenging the currently present diversity in vegetation and fauna.
Up-to-date, several anthropogenic measures have been taken to inhibit the infilling of the area and to avoid the loss of diversity in flora and fauna, such as the placement of a sediment catchment in front of the tidal channel, and the relocation of the channel by dredging. Unfortunately, these measures are found to be effective only for a limited period of time and need to be re-executed periodically.
In order to counteract the ongoing infilling of the intertidal area in a more structural way and to obtain a larger area with intertidal characteristics, currently, a significant excavation and enlargement of the intertidal region is scheduled (IMDC, 2006). This measure is advocated by considering empirical stability criteria introduced by Bruun and Gerritsen (1960), which were inspired by field observations. In particular, based on empirical evidence, Bruun and Gerritsen (1960) discussed that the stability of a tidal inlet increases when the ratio between the tidal prism and the yearly total amount of along-shore sediment transport in front of the inlet becomes larger. Hence, excavating and enlarging the Zwin tidal basin, which leads to a significant increase in the tidal prism, is expected to structurally inhibit the gradual infilling of the intertidal region.
The aim of this contribution is to support the motivation to the planned measures, which were based on empirical relations (IMDC, 2006), by evaluating the influence of an increase in the tidal prism on the stability of a tidal inlet, considering basic physical principles (e.g. Escoffier, 1940).