ReviewModelled channel patterns in a schematized tidal inlet
Introduction
Barrier tidal inlet systems are typically found along sandy coastal planes world-wide (Glaeser, 1978). A series of barrier inlets forms the entrances to the Dutch Wadden Sea. These inlets are defined as mixed energy tide dominated systems (Pugh, 1987, Hayes, 1979). The inlets are defined as tidal inlets when the main channel through the strait is maintained by the tide (Escoffier, 1940). The inlet water motion and the morphology are strongly governed by offshore waves and tidal forcing. The present study focuses on one of the Wadden Sea inlets, the Ameland inlet, which experiences strong alongshore tidal currents and strong currents in the inlet (Ehlers, 1988). This tidal inlet has a westward oriented main inlet channel and an ebb-tidal delta. The objective of this study is to find the governing physical processes of these morphological features. Furthermore, the results are used to prove the conceptual hypotheses that describe such features.
Numerous approaches are found in the literature to explain the physical processes and the long-term behavior of tidal inlets. Empirical relationships show that equilibrium can be found between different morphological parameters: inlet cross-sectional area and basin tidal prism (O'Brien, 1969, Jarret, 1976, Eysink, 1990), inlet cross-sectional area and discharge (Kraus, 1998) and ebb-tidal delta volume and basin tidal prism (Walton and Adams, 1976). An equilibrium relation has been developed by Wang et al. (1999) based on Friedrichs and Aubrey (1988) and Dronkers (1998) by using the ratio of shoal volume to channel volume (Vs/Vc) and tidal amplitude to mean channel depth (a/h). This relation can be used to determine the flood- and ebb-dominancy of a tidal inlet. Eysink (1990) described the morphological adaptation of tidal inlets due to sudden changes, i.e. closure or reclamation work.
A number of conceptual hypotheses have been formulated to explain the morphological changes in the Dutch Wadden Sea inlets. Van Veen (1936) suggested that the preferential orientation of the main inlet channel to the west (i.e. towards the direction where the tide propagates from) is related to the greatest water level gradient caused over a tidal period. This concept is extended by Sha (1989). The westward orientation of the main inlet channel and the ebb-tidal delta occurs as a result of the interaction between the alongshore tidal currents and the inlet tidal currents. Weaker and rotational current fields are found to the east of the inlet and they are enhanced due to the presence of the ebb-tidal delta. Sha and Van Den Berg (1993) showed that the phase difference between the alongshore currents and the inlet currents determines the evolution of the main inlet channel and the ebb-tidal delta.
Process-based approaches to analyze long-term morphological changes have rapidly been developed over the past decades. De Vriend et al., 1993, Latteux, 1995 describe reduction techniques and selection of representative conditions respectively to investigate long-term morphodynamics. More recently Lesser et al., 2004, Roelvink, 2006 have recommended modelling long-term evolution by bed level updating at each hydrodynamic time step, while accelerating the evolution by a constant: ‘morphological factor’. The updated bathymetry is used in the subsequent time step of the hydrodynamic computation. This technique is used to implement a feedback mechanism in the long-term morphological models. Marciano et al. (2005) used this approach to evaluate the branching channel pattern in the Wadden Sea tidal inlets. Dissanayake and Roelvink (2007) described the effect of initial conditions on the long-term evolution of tidal inlets. Van der Wegen et al., 2006, Van der Wegen et al., 2008, Van der Wegen and Roelvink (2008) showed the long-term morphodynamic evolution of a tidal embayment. The impact of sea level rise on a schematized model domain is described by Dissanayake et al. (2008). These works motivated the use of this technique to simulate long-term morphological evolution in the present study.
Few process-based model studies have attempted to investigate the conceptual hypotheses. Van Leeuwen et al. (2003) used a process-based approach with a schematized model to investigate the concepts proposed by Sha (1989). However, numerous uncertainties still remain such as effect of model domain, transport formulations, direction and asymmetry of the tidal forcing. Therefore, a process-based approach which allows us to estimate the temporal and spatial evolution of morphological changes is employed in this study.
The present study enhances the understanding of the physical processes which govern the evolution of the main inlet channel and the ebb-tidal delta in a tide dominated environment, with dimensions similar to the Ameland inlet in the Dutch Wadden Sea. Moreover, the results generally prove the conceptual hypotheses which describe the morphological features in the Wadden Sea inlets.
Section snippets
The Ameland inlet
Ameland inlet is located between Terschelling (on the west) and Ameland (on the east) barrier islands in the Dutch Wadden Sea (Fig. 1). The physical processes in this area are governed by semidiurnal tide which has a mean tidal range about 2.0 m and propagates from West to East at a speed of about 15 m/s. The average significant wave height is about 1.1 m and the dominant direction is from northwest (Cheung et al., 2007). Therefore, this is a mixed energy tide dominated inlet according to the
Initial patterns
The short-term simulation results are used to understand the hydrodynamic and sediment transport characteristics. For this, we compute the horizontal and vertical tide, variation of M2 velocity ellipses, mean residual flow and transport patterns.
Fig. 8 shows the variation of alongshore and inlet tidal currents together with the offshore water level at a 17 m deep location 7 km seaward and in line with the inlet. The alongshore current is computed at the same point. The inlet point is selected in
Conclusions
This paper presents the results of numerical experiments using a process-based model on a schematized model domain, with dimensions similar to the Ameland inlet in the Dutch Wadden Sea. These model runs were carried out with different scenarios; inlet width, tidal asymmetry and direction, transport formulations and relative location of the basin; to investigate the governing parameters and the applicability of conceptual hypotheses in the evolution of the main inlet channel and the ebb-tidal
Acknowledgement
The work presented in this paper was carried out under the project ‘Sustainable development of North sea and Coast’ (DC-05.20) of the Delft Cluster research project dealing with sustainable use and development of low-lying deltaic areas in general and the translation of specialist knowledge to end users in particular.
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