Morphodynamic evolution of double-inlet systems: a numerical model study
Ruijsscher, T.V. de
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A vast amount of tidal inlet systems worldwide are multiple inlet tidal systems. Numerical modelling studies have been, however, mainly focussed on single inlet tidal systems. Furthermore, some semi-analytical models suggest that multiple inlet tidal systems are not stable on geological timescales. In this thesis a numerical morphodynamic model is used to investigate whether double inlet tidal systems can subsist on longer timescales in an environment without waves, and what effect a different inlet spacing and artificial lowering of the bed have on sediment transport patterns and on the bed level profiles. It is found that when developing from an initially flat bottom, a channel-shoal network develops that has a similar structure as that observed in the Wadden Sea. The main channels in terms of net sediment transport are tilted downdrift in the basin, making the system more asymmetrical. After 185 yr of morphological evolution, the total eroded sediment and sediment transports in the basin reach a quasi-steady value, indicating the development of a steady double inlet system. However, a continuous import of sediment occurs, which is due to tidal asymmetry, but in the updrift inlet mainly due to the residual flow. In the basin a tidal watershed develops — separating the basin parts drained by each inlet — which is shifted downdrift as a result of the phase difference between the inlets. When the inlet spacing is increased, the main channels in terms of net sediment transport shift from a dominant direction away from the watershed to a dominant direction towards the watershed. The system imports sediment for all studied distances between the inlets, but for increasing inlet spacing, the import due to the residual flow increases, whereas the import due to tidal asymmetry decreases. Furthermore, the sediment transports of downdrift and updrift sub-system become more equal with increased inlet spacing, consequently the updrift sub-system becomes less dominant. Regarding the tidal watershed, its location shifts downdrift with increasing inlet spacing (due to the phase difference between the inlets), whereas it forms a less effective separation of the two sub-systems for a larger distance between the inlets. Finally, when the bed level is lowered artificially, almost no morphological changes occur in the proximity of the peak lowering, mainly caused by a decreased velocity due to mass continuity. The tidal prism in the updrift sub-basin increases, resulting in increased erosion of the updrift inlet. Counterintuitively, a lowering of the bed level causes a decreased sediment import.