Modeling Sea Level Rise Impact on Estuarine Morphodynamics

Abstract

Estuaries are dynamic environments where riverine fresh water meets tidally forced marine saline water. They encompass unique ecosystems and provide natural buffers that protect economic livelihoods. Many estuaries face continuous pressure from urbanization, altered river flows and sediment supply, subsidence, and sea level rise (SLR).

Estuarine beds are dynamic, influenced by marine (waves, tides, salinity, sea level) and fluvial (discharges, sediment load) forces. Morphological development is key to sustainable estuarine ecosystems and human activities. SLR will trigger a long-term (century-and-beyond scale) morphodynamic adaptation that will significantly impact the future state of estuarine systems. There is an urgent need for an enhanced understanding of relevant morphodynamic processes and the development of skillful forecast tools to assess SLR impact.

This research aims to assess SLR impact on the long-term morphological development of the estuarine environment, focusing on intertidal area by applying a process-based, numerical model (Delft3D) to predict morphodynamic behaviour. Fundamental studies on idealized estuaries are performed to determine governing processes and model parameter settings. In particular, these include wind wave dynamics, sediment properties, and grid resolution. Further, the modeling approach is applied in two case studies, namely San Pablo Bay (USA) and the Western Scheldt Estuary (Netherlands). Hindcasts are validated against unique datasets covering a period from decades to 1.5 centuries, while forecasts cover a century timescale under various SLR scenarios.

Model results show that morphodynamic adaptation lags behind SLR. Despite accretion under SLR, intertidal areas decay. The adaptation time lag and intertidal area loss mainly depend on SLR rate and magnitude along with sediment supply and sediment properties. Extreme SLR scenarios (1.67 m and 3.02 m over a century) can result in a 91% and 54% loss of intertidal area for San Pablo Bay and the Western Scheldt, respectively.

The results of this study advance understanding of the SLR impact on the morphological evolution of estuaries. It also shows that, despite their complexity, process-based models are reliable and valuable tools for performing morphological forecasts. Delft3D-type models can be used in future studies to investigate potential adaptation measures or to determine and quantify parameters for more aggregated models.