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Woods Hole Coastal and Marine Science Center > Coastal Model Applications and Field Measurements > Research Activities > Sediment Supply to Wetland Complexes

Coastal Model Applications and Field Measurements

Sediment Supply to Wetland Complexes

Anthropogenic and climatic forces have modified the geomorphology of tidal wetlands over a range of timescales. Changes in land-use, sediment supply, river flow, storminess, and sea level alter the layout of tidal channels, intertidal flats, and marsh plains; these elements define wetland complexes. Diagnostically, measurements of net sediment fluxes through tidal channels are high-temporal resolution, spatially integrated quantities that indicate 1) whether a complex is stable over seasonal timescales, and 2) what mechanisms are leading to that state. We estimated sediment fluxes through tidal channels draining wetland complexes on the Blackwater and Transquaking Rivers, Maryland, USA. While the Blackwater complex has experienced decades of degradation and been largely converted to open water, the Transquaking complex has persisted as an expansive, vegetated marsh. The measured net export at the Blackwater complex (1.0 kg/s or 0.56 kg/m2/y over the landward marsh area) was caused by northwesterly winds, which exported water and sediment on the subtidal timescale; tidally forced net fluxes were weak and precluded landward transport of suspended-sediment from potential seaward sources. Though wind forcing also exported sediment at the Transquaking complex, strong tidal forcing and proximity to a turbidity maximum led to an import of sediment (0.031 kg/s or 0.70 kg/m2/y). This resulted in a spatially averaged accretion of 3.9 mm/y, equaling the regional relative sea-level rise. Our results suggest that in areas where seaward sediment supply is dominant, seaward wetlands may be more capable of withstanding sea-level rise over the short term than landward wetlands. We developed a conceptual model to determine a complex’s tendency towards stability or instability based on sediment source, wetland channel location, and transport mechanisms. We illustrated the model with the two wetlands from this study, as well as two additional wetlands in San Francisco Bay. Wetlands with a reliable portfolio of sources and transport mechanisms appear better suited to offset natural and anthropogenic loss. We have repeated these types of measurements at Pt. Mugu, California; Seal Beach NWR, California; and Rachel Carson NWR, Maine over the last year. Those data are currently being processed and interpreted to infer wetland stability.

Drawings of four marshes

Illustrations of conceptual model for four wetland complexes. All but the Blackwater complex have a diverse portfolio of external sediment sources, mobilization mechanisms, and advection mechanisms.

 

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