Mainland Seaside Salt Marsh Response and Resilience to Sea-Level Rise on The Eastern Shore of Virginia, USA

Sea-level rise is a major threat to salt marsh persistence on the Eastern Shore of Virginia, an area experiencing relatively rapid rates of rising sea-levels. Salt marshes respond both vertically and laterally to persist and function as sea-levels rise. Salt marshes of differing geomorphologies may be responding to rising sea levels in different ways and at different rates due to variations in factors like protection from high-energy lagoonal events and wave energy. There is an increased interest in understanding how quickly mainland seaside salt marshes on the Eastern Shore of Virginia are changing in both elevation and area, and there is an interest in models that assess the resilience of salt marshes to rising sea-levels. The overarching goals of this study were to understand better how mainland seaside salt marshes on the Eastern Shore of Virginia respond to rising sea levels and to evaluate how resilient these marshes are to sea-level rise.
Between 2002-2017, migration and edge erosion were measured in three mainland geomorphic marsh types (headland, valley, hammock) and were used to assess the rate and spatial extent of marsh change for the Eastern Shore of Virginia. Using ArcGIS, it was found that all marsh types increased in spatial extent; increases were greatest for the valley type (0.58 ha ± 0.31 ha or + 0.32% per year). Measured rates of migration (headland > valley > hammock) and erosion (headland > hammock > valley) for each geomorphic type were averaged and applied to obtain changes in these same marsh types at the regional scale. At this scale, valley marsh area increased (82.5 ha or 5.5 ha a-1) more than the other two marsh types combined. This analysis demonstrates the critical influence that geomorphic type has on lateral marsh response to sea-level rise, and the reliance of Eastern Shore of Virginia salt marshes on marsh migration to persist and function.
Elevations were measured between 1999 and 2019 through Real Time Kinematic surveys in nine mainland seaside salt marshes on the Eastern Shore of Virginia. Sites were classified as headland, valley, or hammock marsh types. The rates of elevation change were almost uniformly negative across all sites (-14.7 ± 1.2 mm yr-1 , mean ± SE). Elevation change rates differed among sites and among marshes based on geomorphic classification (hammock > valley > headland). The nearly uniformly negative rates of elevation change found here indicate that, perhaps, salt marshes on the Eastern Shore of Virginia rely primarily on lateral responses to sea-level rise to maintain area rather than vertical responses.
Using multiple indicators of marsh resilience, The Marsh Resilience to Sea-Level Rise (MARS) model was created to assess the resilience of these coastal wetlands to sea-level rise. The model was applied to nine salt marsh sites on the Eastern Shore of Virginia. Resilience scores were on a scale of 1 to 5, 1 indicating low resilience and 5 indicating high resilience. The model resilience scores suggested that nine study sites uniformly had low relative resilience to sea-level rise, ranging from -5.51 to 3.26, with an average index score of 0.06 ± 0.41. Mean resilience index scores at sixteen National Estuarine Research Reserve sites ranged from 1.06 to 4.1 with an average index score of 2.47 ± 0.24 (Raposa et al. 2016). The results of this study suggest that Eastern Shore of Virginia mainland salt marshes may be some of the least resilient marshes to SLR in the coastal United States. Further improvements to the MARS model should be made as critical processes for marsh persistence, such as marsh migration into uplands, that tend to vary either by marsh or by marsh geomorphic type are not included in this model as it stands.