Abstract

In this presentation the inorganic salt marsh sedimentation processes are examined through the use of (i) intensive hydrodynamic and sediment resuspension experiments and (ii) utilization of long-term (decadal scale) data on suspended sediment concentration.

A laser diffraction system (LISST), optical backscatter sensors (OBS) and acoustic Doppler velocimeters (ADV) were used to measure suspended sediment concentration and particle settling velocity variability throughout various tidal cycles for spring and neap conditions. . During neap tides, sediment in the tidal creeks is primarily in the form of flocs with a settling velocity ranging between 0.02 - 0.20mm/s and a mean particle size of 25 to 75 microns. This population of sediment is enriched by both larger flocs and larger individual grain particles, during spring tides. The results indicate that in the tidal creek, erosion can occur only during the ebb stage of the spring tides, while during the flood tides the tidal creeks do not experience any significant erosion. On the contrary, they might act as temporary depositories for the small flocs. Sedimentation on the marsh surface occurs predominantly in the form of flocs, which seems to have the same settling velocities (0.24 mm/s) both during neap and spring tides. This indicates that the neap - spring tidal cycle is the basic controlling marsh sedimentation through sediment availability for deposition and inundation time. Spring tides provide more sediment and more time for the flocs to be deposited and thus contribute more to marsh accumulation.

The above described basic process is modulated by various episodic events such as rainfall and seasonal variability. Analysis of long-term data showed that low water rainfall is responsible for elevated suspended sediment concentration in the channels due to the increased erosion of the exposed marsh surface and creekbanks. These low-tide rainfall events are not erosional periods but rather appear to be associated with periods of sediment redistribution within the marsh system. Sediment import during periods of rainfall is either not affected or can be increased by approximately 50% when compared to sediment import corresponding to periods without local rainfall.

Furthermore, a seasonal modulation of suspended sediment concentrations is observed with highest average concentrations in the summer when water temperature and biological activity is at a maximum. Despite this increased sediment availability in summer, the fall season is responsible for the majority of sediment imported annually (55%). This occurs because mean sea level is greatest during these times and leads to increased times of marsh inundation. Therefore, the frequency and duration of marsh inundation is more important than sediment availability in controlling surface accumulation within micro and meso-tidal marsh systems.

Biographical Sketch

Dr. Voulgaris is an Associate Professor at the Marine Science Program and Department of Geological Sciences at the Univeristy of South Carolina In Columbia, SC. He got his BS in Geology and PhD in Oceanography at the University of Southampton, UK. He was a post-doctoral fellow at WHOI from 1997 to 1999, and from there he went
to South Carolina. His research interests are in coastal processes and sediment dynamics with particular emphasis on the interaction of bottom boundary layer and sediment resuspension.

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