Long-term Morphologic response of tidal inlet systems to changes in basin area.

Abstract

Along the majority of the worlds shoreline, tidal inlet systems represent a common morphological feature. Like in the Wadden Sea, which forms one of the major intertidal areas on the Earth, they are found along barrier islands coasts and they play a significant role in the sediment budget of the coastal zone. Tidal inlets are highly dynamic systems that evolve under a constant interaction of forcing and morphological adjustment. In order to model and predict their long-term morphological evolution, the analysis is carried out at high level of aggregation. In this aggregated scale perspective, three morphological elements can be distinguished within a tidal inlet system, namely the ebb-tidal delta, the channel and the flat. Field observations reveal that a relationship exists between the elements state (viz. wet/dry volume) and the hydrodynamic conditions (tidal prism). ASMITA (Aggregated Scale Morphological interaction between a Tidal inlet and the Adjacent coast) concept is built on the existence of an equilibrium state. This behaviour oriented model, describes the evolution of tidal inlet systems towards the equilibrium in terms of exchange of sediment, which depends on the need and the availability of sediment. In the present study, ASMITA is used to simulate the long-term morphological evolution of tidal systems in response to changes in basin area. The size and the shape of the basin area plays an important role in the morphological development of tidal inlet systems as it determines largely the propagation of the tidal wave and, hence, it influences the tide-residual sediment transport. In the original formulation of ASMITA the basin area is considered time-invariant and therefore acting as a fixed morphometric condition on the morphological development of tidal inlet system. To evaluate the influence of a temporal change in basin area, the model implementation has been extended with the area varying as function of time. Three different scenarios are chosen in this report in order to assess the morphological evolution of tidal inlet systems: -asymptotic increase/decrease of basin area -periodic area change -abrupt change Since the basin area is the sum of the channel and the flat area, the change in the area can be ascribed to a combination of relative change in the flat area and in the channel area. Only two possible combinations are considered in the present study: an immediate response of the channel area (Ac/Af=const.), and no response of the channel area (Ac=const.). The analysis of the long-term morphological response to changes in basin areas is carried out on two adjacent basins of the Dutch Wadden Sea, viz. Vlie and Borndiep. The two basins respond to the required characteristics for this study, as they have wide different initial size of the basin area, and as their intertidal area has a rather different extension. This provides to draw insight into the dependence of the morphological behaviour on the size of the basin and on the influence of the channel area. The morphological evolution of Vlie and Borndiep is then simulated in response to the defined scenarios. The tidal system are seen to follow the imposed disturbance even in the case of a sudden increase of the basin area with 50% A sensitivity analysis on the system time scale is carried out on the basis of the analysis of the morphological behaviour of the systems under abrupt changes of the area. From this analysis, the time-scale appears to be influenced mainly by the size of the basin (the bigger being the slower), and by the imposed disturbance. The channel area was observed to play a significant role only on the response of Vlie, whereas Borndiep was observed to be flat dominated.