Tidal hydrodynamics and sediment transport processes in ‘de Slufter’, the Netherlands
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Small tidal inlets along sandy coasts are important features which hold a great variety of flora and fauna. National park ‘de Slufter’ at Texel, the Netherlands, is a small tidal inlet. Its natural dynamics and ecological values must be maintained in future. Taking into consideration that the Slufter is a temporal phenomenon, the stability of the Slufter is therefore an important issue. Incidentally, Rijkswaterstaat relocates the down drift migration channel to the south, but generally the natural dynamics in the Slufter have free play. For future management planning and maintenance strategies we have to expand our knowledge on the dynamic behavior of small tidal inlets. Contradictory to the relative long physical appearance of the Slufter (150 years), many tidal inlet models predict a short-term closure of the Slufter. This raises questions to coastal engineers. Therefore Utrecht University has launched a series of fieldworks at the Slufer. This research is a qualitative assessment on the tidal hydrodynamics and sediment transport at the Slufter. The goal is to identify the sediment transport trends (import or export) during the neap-spring cycle, based on measurements and sediment transport models. Net import or export patterns are important for the stability of the inlet. The main research question reads as follows: What is the influence of tides on the import and export of sediment in the Slufters main channel. In order to answer the research question field measurements were carried out in and around the mouth of the Slufter, in September and October 2008. Results show that during non-flooding conditions (water levels < 1.1 m + N.A.P.), tides are the fundamental transport mechanisms for water and sediment at the Slufter. During spring tide, tidal interaction with the Slufters’ basin hypsometry enhances flow asymmetry which becomes ebb dominant. Measured fluxes are quite equal for both inflow and outflow stages, but calculated sediment fluxes clearly indicate an export of sediment during spring tidal conditions. This difference can partly attributed to high measured suspended sediment concentrations during flood. During neap tide, both computed and measured sediment fluxes indicate an import of sediment. The flood dominance is a direct result of the external tide, which emerges in longer ebb durations resulting in higher maximum landward directed flow velocities. During spring tide, tidal interaction with the Slufters’ basin hypsometry enhances flow asymmetry which becomes ebb dominant. Measured and calculated fluxes clearly indicate an export of sediment during spring tidal conditions. Tidal flow asymmetry in the Slufter is mainly controlled by the geometry of the inletsbasin, rather than a differentiation between duration of the ebb and flood currents. During storm conditions the total basin of the Slufter is inundated, and both current field and suspended sediment concentration (SSC) greatly differ from calm condition. During these flooding conditions (HW > 1.1 m + N.A.P.), import of water mainly occurs over the beach flat, whereas export mainly occurs through the relative narrow tidal channel. During storms sediment fluxes in the main channel are significantly larger than during non-flooding conditions. A residual circulation flow pattern has been identified during the storm. Dominant transport in the Slufers main channel during a storm event is seaward directed. The effect of waves on sediment transport is mainly restricted to the offshore zone, where sediment entrainment is enhances due to wave dynamics.