Modeling Long-term Morphological Developments of Intertidal Flats

Abstract

Estuarine intertidal flats comprise valuable ecosystems and act as an important sediment source for the adjacent salt marsh systems. However, accelerating sea-level rise threatens the mudflats and associated ecosystems, where the mudflat accretion lag behind sea level rise. A reliable forecast on the morphological developments of the mudflat under sea-level rise scenarios is of vital importance to assess sea level rise impact on the estuarine system.

Different tools exist that can predict the long-term evolution of the mudflats, viz. Delft3D, ASMITA and the hybrid model (Delft3D-ASMITA). Since the hybrid model is newly developed, the comparison among the various approaches has not yet been available. However, it is significant to know if they can produce the same results.

The research aims to compare the three modeling approaches (Delft3D, ASMITA, and the hybrid model) based on a case study in South San Francisco Bay. This comparison will reveal the strengths and weaknesses of the three approaches as well as indications where the approaches may strengthen each other.

The research is conducted in three main phases. Phase 1 consists of the sensitivity analyses in Delft3D for the case of South San Francisco Bay; Phase 2 contains the calibration of one-element and multi-element ASMITA models to reproduce the Delft3D model; Phase 3 focuses on the potential to improve the simulation efficiency of Delft3D in the hybrid model.

Model result comparison shows that, after the calibration, the ASMITA and hybrid model can efficiently simulate the same cases as in Delft3D. However, the upper part (landwards end) of the mudflat is more sensitive to the water level changes in the hybrid model due to the different sediment transport computation modules. The power indicating the relation between the equilibrium and actual morphology as well as the reference level is the important calibration coefficient to adjust the steepness of a mudflat. It can be concluded that the Delft3D model is used as the foundation to calibrate ASMITA and the hybrid model, both of which can improve the simulation efficiency with simplifications, especially in the long-term morphological development.

The study provides clear insights into the comparisons among different modeling approaches in the case of the long-term morphological developments of mudflats by the impacts of sea-level rise. It is recommended to do further research on the configuration of a 2D model and the conduction of the combination of different modeling approaches in other similar cases to confirm the validation.