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Multi-Scale Marsh Accretion Modelling

Coastal marsh exposed to episodic streamflow is "shaken, not stirred" with respect to suspended sediment, which calls for greater attention to event-scale physical processes in addition to long-term biophysical processes when projecting future marsh habitat and suitability for sensitive species under SLR, and when planning management measures such as sediment source controls and dredging.

This work, in collaboration with the USGS (Kevin Buffington and Karen Thorne) and the Southern California Coastal Water Research Project (Eric Stein, and Jennifer B. Rogers) uses a coupling of Delft3D and WARMER aimed at multi-decadal simulation of marsh accretion under SLR and episodic sediment loads. Delft3D resolves the spatial distribution of deposition/erosion during episodic storm events, and vertical height changes are aggregated annually for input to multi-decadal, spatially-distributed, biophysical accretion modeling using WARMER. In turn, WARMER accounts for the complex, nonlinear interaction between deposition, inundation frequency, organic matter generation, compaction, and so on.



The takeaway, shown in the graphic above, is that attention to event-scale physical processes substantially reshapes the spatial distribution of future projections of marsh accretion - with greater accretion on the marsh plain near sediment sources (red coloring) and less accretion near channels (blue coloring), compared to a model that assumes sediment is well mixed (e.g., by tides). In Newport Bay located in Orange County, California, we observe a ~40 cm difference in vertical accretion per century across horizontal distances less than 100 m by accounting for event-scale physical processes. This work also shows that Newport Bay has sufficient sediment supply to maintain similar marsh habitat distributions under low rates of SLR (40 cm per century), but will face significant inundation under high rates of SLR (110 cm per century).

Brand, M. W., Buffington, K., Rogers, J. B., Thorne, K., Stein, E. D., & Sanders, B. F. Multi‐decadal simulation of marsh topography under sea level rise and episodic sediment loads. Journal of Geophysical Research: Earth Surface, e2021JF006526.

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