Predicting future marsh conditions requires models that capture how both punctuated events and gradual physical and biological processes shape marshes. In Newport Bay, CA marshes can keep pace with sea level rise if sea level rise (SLR) rates rise slowly. To make decisions on if and how to protect an existing marsh, or to acquire new land for the marsh to migrate into as sea levels rise requires good predictions on if the marsh will remain healthy in the future. These predictions come from being able to simulate the gradual accumulation of sediment on the marsh surface (i.e., accretion) under SLR, and this calls for an understanding of: (a) physical processes acting over the time scale of storm events, and (b) biophysical processes acting over time scales longer than storm events. This study addresses this gap in understanding by linking two established models with complementary strengths (i.e., WARMER and Delft-3D), and comparing future simulations of a marsh in Newport Bay, CA.

Figure 1. Image taken from Brand et al. 2022. Spatial distribution of contoured habitat elevations in 2100 from Delft3D (left panels), WARMER (middle panels) and Delft3D-WARMER (right panels) for low (top panels) and high (bottom panels) sea-level rise scenarios.

Prior to this project, episodic inputs of sediment from creeks and streams and the spatial distribution of deposition was not captured by existing marsh models for long-term predictions of how a marsh will evolve.  Accounting for these inputs is a very important consideration when anticipating future change in habitat distributions and suitability for sensitive species. This approach is similar to the Hydro-MEM model, a coupled physical- and biological model that has been run for much of the Gulf of Mexico and is expanding to include the southeast U.S. However, to date, Hydro-MEM does not have the sediment transport and morphological components of the Delft-3D model.

This work draws attention to the role that sediment supplies from creeks can play in shaping the spatial distribution of marsh evolution under SLR. The team then applied the model to Newport Bay, and results suggest sediment supplies can keep pace with rising sea level under lower rates of SLR to support a diverse mix of marsh habitats, but higher rates of SLR point to increasing inundation. In Newport Bay, large storm events that don’t come very often (>20 year return period) are the primary driver of sediment deposition. These punctuated events shape the future marsh topography and marsh growth and sediment compaction will continue to gradually act upon the new surface elevation over time.

This advancement represents a very important contribution towards approaches that capture punctuated and gradual physical and biological processes in a real world fashion, giving us the ability to understand the role of these processes and how we can better manage our coastline. This study combines research teams from multiple projects, including two projects supported by the NCCOS Effects of Seal Level Rise (ESLR) program: Codevelopment of Modeling Tools to Manage Sediment for Sustainable and Resilient Coastal Lowland Habitat in Southern California and Marshes on the Margins: Developing Tidal Wetlands Adaptation Strategies in Southern California

Citation: Brand, M. W., Buffington, K., Rogers, J.B., Thorne, K., Stein, E. D., & Sanders, B. F. (2022). Multi-decadal simulation of marsh topography under sea level rise and episodic sediment loads. Journal of Geophysical Research: Earth Surface, 127, e2021JF006526. https://doi.org/10.1029/2021JF006526