Morphodynamics of De Slufter

Abstract

De Slufter is a small tidal inlet located on Texel, The Netherlands. From 8 September to 17 October 2008, a field work was carried out in De Slufter measuring the morphological development of the mouth area and water levels and flow velocities in the main channel. The aim of this report is to study the interaction between basin morphology and the hydrodynamics of the system and its implications for the long term stability and evolution of De Slufter. In a stable inlet there is a balance between the import of sediment during the flood phase and the export of sediment during the ebb phase, by which the inlet remains connected to the sea. The tide is however deformed when it enters the inlet. The most important mechanisms causing this deformation include tidal truncation and a hypsometry effect. Tidal deformation results in duration asymmetries, causing differences in peak flood- and ebb-velocities. A deformed velocity signal leads to a net sediment transport. This determines the stability of the inlet. Not much is known about the stability of small tidal inlets. This is related to the distinct morphology of De Slufter, with its small amount of intertidal flats and large surface area between 1.1-1.5 m +NAP. Analyzing yearly aerial photographs, a channel migration cycle of 3-7 years is observed in the mouth area. Migration occurs from the southwest to the northeast and the channel curvature increases at the end of the cycle. The cycle ends with channel relocation by man. During the initial calm weather period of the field campaign, no channel migration was observed. During the two storm events a channel migration was observed in the most seaward section of the channel. The narrowest section of the channel migrated approximately 10 metres. The curvature of the channel also increased and the channel slightly changed its orientation. Most channel profiles reveal a storm-induced widening of the channel. This resulted in an increase of the cross-sectional area up to 22.1% in the seaward section of the channel. This enlargement was accompanied by slightly lower peak ebb- and flood-velocities. During fair weather conditions in the final phase of the field work, equilibrium was partly restored by decreasing the cross section again. Migration of the channel is possibly related to the combined effect of wave-impact, the longshore current and a circulation pattern developing during storms. This circulation imports water over the beach flat, due to submergence and overwash, and exports it again through the channel. The accretion on the beach flat and retreat of the beach face as a result of storm-conditions are probably related to the same circulation pattern. The storm events enlarged inlet cross sections. During fair weather conditions a large tidal deformation was measured in De Slufter. The amplitudes of M2 and M4 and the M4/M2-ratio all decreased compared to the North Sea. A phase relationship (2M2-M4)sea of 270-360 degrees was measured, indicating a longer ebb than flood duration. Compared to the North Sea (where the tide is already slightly deformed) ebb-duration in De Slufter increases with 53.5 minutes. This asymmetry is caused by a truncation of the tide. The channel bed has an elevation of -0.5 m NAP at the mouth, while the mean low water is -0.89m NAP. This indicates that most of the times during ebb, drainage still occurs while the tide at the North Sea already starts to rise. Tidal asymmetry is further caused by the hypsometry of the basin. At water levels over 1.1 m +NAP, the surface area covered with water increases rapidly. This mechanism causes large differences between normal and storm conditions. During normal conditions water is confined to the channel. The wet surface area (Axy) increases proportional with the cross-sectional area (Ac). Peak flood velocities are slightly larger than peak ebb velocities. Differences in ebb- and flood-velocities are very small. During storm events Axy increases much faster than Ac. Peak ebb velocities are twice as large as peak flood velocities. Based on the measured velocity signal, the net bed load transport appears to occur in the ebb-direction. The net suspended load transport occurs in the flood-direction, because the duration of the high water slack is much larger than the duration of the low water slack. A simple model was used to simulate flow velocities. The order of magnitude and phases of the simulated velocities agree with measurements, but deviations arise due to inaccuracy of the measurements and over-simplicity of the model. Flood velocities during normal conditions are overestimated, while ebb velocities during storm conditions are underestimated. Based on modeled bed load transport ratios and slack durations, the system appears to be ebb-dominant, both during normal and storm conditions. The system appears to be in equilibrium. This also follows from the fact that the system already exists for 150 years and from several (semi)-empirical relationships derived for stable inlets all over the world. The present configuration of De Slufter in its stable situation is however artificially maintained, because the channel is relocated every 3-7 years.