Coastal setting determines tidal marsh sustainability with accelerating sea-level rise

There is an increasing concern over how accelerated rates of sea-level rise (SLR) will impact tidal marsh ecosystems. The present study evaluates the potential impacts of SLR on marsh sustainability using the Tidal Marsh Model (TMM) with the addition of a new vegetation algorithm within the SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) framework. This new functionality contributes to an improved understanding of how vegetation affects the mean flow velocity and turbulence, and consequently, the sedimentation processes. Using two SLR scenarios (intermediate and extreme SLR rates), we projected the changes in marsh extent over the next 50 years in two representative marsh systems within a subestuary of Chesapeake Bay. Each study site has marshes associated with different physical settings and anthropogenic components: Carter Creek (developed, high topography) vs. Taskinas Creek (natural, low topography, steep banks). Carter Creek experienced a net marsh loss of 7.3% and 60% in the intermediate and extreme SLR scenario, respectively. In some places, due to the local geomorphic settings, marshes were able to migrate inland and offset part of the total loss, whereas marsh transgression was truncated near development and hardened shoreline structures. In Taskinas Creek, marshes are associated with natural lands with steep upland slopes (inhibitor for marsh transgression due to SLR). Marsh net decline was 23.1% (intermediate SLR scenario), and 89.6% (extreme SLR scenario). Marsh transgression was not substantial in this site, suggesting that marsh loss can be primarily attributed to upland bank conditions which prevented marsh migration with accelerated SLR rates. The enhanced TMM provides the highly-resolved simulations of multi-scale processes needed to inform restoration, strategic planning, and monitoring activities to support marsh sustainability in an evolving system.