Long-term morphological modelling of tidal inlet systems

Abstract

A rise in the global mean temperature induced by climate change is expected to have a large impact on ecosystems in all regions of the world. One of the threats is accelerated sea level rise (SLR). This may induce the loss of intertidal areas in tidal inlet systems. The long-term morphological response of tidal inlet systems can be modelled using reduced complexity model ASMITA (Aggregated Scale Morphological Interaction between Tidal inlets and the Adjacent coast). The ASMITA model simulates morphological development on an aggregated spatial and temporal scale by imposing a morphological equilibrium condition. As such the model is fast, allowing for multiple long-term simulations. The model is physics-based and the parameters can be related to field values.

Currently, salt marshes are not implemented in ASMITA. However, salt marshes could be of importance to the morphological development in tidal inlet systems. Moreover, it is relevant to assess the resilience of the ecologically important salt marshes by themselves. The aim of this research is to implement salt marshes in ASMITA to assess their influence on the rest of the tidal inlet system and to gain insight into the long-term morphological response of salt marshes to accelerated SLR.

Salt marsh development is governed by horizontal and vertical processes. The marsh height increases by capturing mineral sediment and by the accumulation of plant biomass. Autocompaction and deep subsidence lead to a decrease in marsh height. The implementation of salt marshes in ASMITA relies solely on the input of mineral sediment. At the marsh edge, generally, a cyclic behaviour of sedimentation and cliff erosion occurs. Due to the high degree of spatial aggregation, cliff erosion is excluded from the model extension. The governing processes for salt marsh development in the ASMITA model extension are mineral sedimentation, sediment availability and relative SLR.

The spatial and temporal aggregation of governing processes for salt marsh development are included in the aggregated advection-diffusion equation and model parameters for the horizontal & vertical exchange of sediment, and sediment availability. Data analysis on hydrodynamic conditions and salt marsh development was conducted for the derivation and calibration of these model parameters. To verify the salt marsh implementation, three ASMITA models were created. A one-element salt marsh model consisting of only a salt marsh element, and two different multiple elements models, which contain the ebb-tidal delta, channels, tidal flats and salt marshes.

It can be concluded that ASMITA can model the mineral sedimentation on a salt marsh but depicts a large sensitivity to the parameter setting, particularly for the sediment concentration. Based on the chosen parameter configuration, the Oosterkwelder salt marsh is preserved when subjected to SLR rates below 16 mm/year.

The ASMITA salt marsh extension can be employed to obtain an expeditious first impression of long-term morphological salt marsh development. However, due to the lack of incorporation of detailed processes, the model should not be employed for in-depth analyses of salt marsh development. The interaction between the salt marsh element and the remaining tidal inlet system components requires further model improvements.