Enviro-Days 1997
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Trine Christiansen
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Enviro-Days 1997
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Trine Christiansen
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The processes controlling sediment deposition on a vegetated tidal salt marsh are not well understood, yet rates of sediment deposition are of fundamental importance for march ecology and hydrology. On the Atlantic coast of the United States, relative sea level is rising at a rate of approximately 2 mm per year. It is therefore important to understand the mechanisms by which marshes are maintained at an elevation that is adequate for sustaining marsh ecology. The goal of this study is to develop a conceptual model for the physical mechnanisms controlling sediment deposition on a tidal salt marsh.
The subject of our field study is a mainland fringing salt marsh in Hog Island Lagoon on the Eastern Shore of Virginia. This marsh is vegetated with Spartina Alterniflora and the tidal range in the area is 1-1.5 meters. The water depth on the marsh surface reaches 1 meter on the highest tides. Dating of sediment cores using 210^Pb isotopes indicate deposition of 1 to 2 millimeters per year.
We have used optical back scatter instruments to measure suspended sediment concentrations along a transect perpendicular to the tidal creek. Sediment observed in suspension during the flooding tide is very fine grained (in the silt and clay range). Sediment is advected on to the marsh surface from an adjacent tidal creek. The advection velocities are very low (less than 1 cm/sec). Magnitude and direction changes depending on whether tide is rising or falling. The velocities tend to be greater on the falling tides. We think that resuspension of sediment from the marsh surface may occur on the falling tide when velocities are greatest. Concentrations are highest on the rising tide, and decrease to a constant low level on the falling tide. Concentrations also decrease with distance from the creek. It is expected that concentration levels and hence rate of deposition is related to to innundation duration and frequency.
Based on long-term records of tidal variation and wind speed and direction data, we have determined the types of weather patterns that produce prolonged innundation of the marsh surface and thus can be expected to enhance sediment deposition. It appears that North Easterly storms produce the highest water levels on the marsh and these storms are most common in the winter season between October and April.