Tidal basins are connected to the outer sea by one or more tidal inlets. In this presentation, we focus on so-called double inlet systems, i.e., tidal inlet systems with two connecting channels. A typical example is the Marsdiep-Vlie system, located in the Dutch Wadden Sea. Using an idealized morphodynamic model, we aim at directly finding morphodynamic equilibria of double inlet systems and assess their stability. For unstable configurations, the linear stability mechanism will be presented.@en

The existence of morphodynamic equilibria of double-inlet systems is investigated using a cross-sectionally averaged morphodynamic model. The number of possible equilibria and their stability strongly depend on the forcing conditions and geometry considered. This is illustrated by considering a rectangular double-inlet system forced by M₂ tidal constituents only. Depending on the M₂ amplitudes and phases at both entrances, no equilibrium, one equilibrium or multiple morphodynamic equilibria may exist. In case no equilibrium is found, the minimum water depth becomes zero somewhere in the system, reducing the double-inlet system to two single-inlet systems. In the other cases, the location of the minimum water depth and the direction of the tidally-averaged sediment transport, as well as their actual values, depend strongly on the M₂ tidal characteristics. Such parameter sensitivity is also observed when including the residual and M₄ forcing contributions to the water motion, and when allowing for width variations. This suggests that, when considering a specific system, the number and stability of morphodynamic equilibria, as well as the characteristics of these quantities, can only be assessed by investigating that specific system in detail. As an example, the Marsdiep-Vlie inlet system in the Dutch Wadden Sea is considered. It is found that, by using parameter values and a geometry characteristic for this system, the water motion and bathymetry in morphodynamic equilibrium are qualitatively reproduced. Also the direction and order of magnitude of the tidally-averaged suspended sediment transport compare well with those obtained from a high-complexity numerical model.

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The tidally averaged sediment transport is important for the stability of the tidal system, since this transport has to vanish in morphodynamic equilibrium. Hence net import (export) of sediment at the seaward boundary indicates that the basin’s bathymetry is still evolving in time. In the literature, analytical expressions have been derived to quantify the contribution of various tidal constituents to the tidally averaged sediment transport, such as in Van de Kreeke and Robaczewska (1993) or in the appendices of de Swart and Zimmerman (2009). In the present study, the tidal constituents will be calculated using a 2D-idealized model, developed by the authors, and the results of the analytically obtained transports will be compared to those obtained numerically. @en