This study will investigate sediment dynamics on a salt marsh surface both experimentally and mathematically. We have selected a mainland fringing salt marsh for the study in the Phillips Creek area near Brownsville, located on the Atlantic side of the Delmarva Peninsula on the Eastern Shore of Virginia. It was found practical to focus the work on the low salt marsh environment, because tidal flooding here occurs on a regular basis and the vegetation (short or tall form of Spartina alterniflora) is very uniform. The daily flooding provides the opportunity to conduct repeated measurements and allows for a reasonable spatial resolution using one set of instruments to measure currents and suspended sediments. The following is a description of the conceptual framework of the study as the project is still in its preliminary stages.
The sediment observed in suspension during the flooding of a tidal salt marsh is very fine grained (in the silt or clay range). This material has a low settling velocity, and typically the settling time is greater than the actual time of inundation during a tidal cycle. Kastler (1993) measured sediment accumulation on sediment plates during regular tidal cycles which she attributed to mobilization on the marsh surface and Pb-210 dating of sediment cores taken on the LTER site on the Virginia Eastern shore indicate deposition of 1 to 2 millimeters per year (Kastler, 1993) (also see Kastler and Robinson in this compilation). In addition, several studies have, described the existence of a concentration gradient across the marsh surface, with high suspended sediment concentrations near the banks of a tidal channel and low suspended sediment concentrations in the interior of the marsh indicative of deposition (Stumpf, 1983 and Wang et al., 1993). Wang et al., (1993) also observed that the concentration of sediment in the water running off the marsh on the falling tide is lower than the concentration of sediment in the water flooding the marsh on the rising tide. Previously described mechanisms of deposition other than settling include interception by plants and pelletization by filter feeders (Stumpf, 1983), but deposition during seepage into the marsh may also be a contributing process.
One way to resolve the mechanisms of sediment deposition is by combining a mathematical description of the physical processes of flow and sediment transport with measurements made in the field. By measuring sediment concentration along with current velocity and water surface elevation, the dominant sediment transporting processes on the marsh surface can be addressed. Once an accurate description of the sediment transport and the flow field has been obtained, sediment and water budgets can be calculated, and rates of deposition can be determined using the information gained from the sediment budget. The deposition will represent the net contribution of the various mechanisms of deposition such as settling, interception by plants, consumption by filter feeders and vertical seepage. The process of interception by plants and consumption of sediment by filter feeders are processes that must be quantified empirically, whereas we hope to be able to quantify settling and seepage processes mathematically. The suspended sediment measurements will be used along with the current velocity and water level measurements to test and constrain a model for flow and sediment transport on a salt marsh surface that we plan to formulate.
To describe sediment movement on a marsh surface experimentally, both the vertical distribution and the spatial distribution of suspended sediment concentration will be measured. The vertical suspended sediment distribution allows for accurate determination of the volume of sediment transported at a given location, and the spatial distribution provides information on areas of sediment deposition and erosion. The study by Wang et al. (1993) indicated that sediment deposition typically occured in the vicinity of the banks of a tidal creek, and the measurements will be focused in this region. It is intended to measure concentration profiles along transects perpendicular to the creek bank and on both the rising and the falling limb of the tidal cycle. The velocity field in the study area will also be characterized both as it varies spatially and as it varies temporarily with changing tide. In addition velocity will be measured as a function of flow depth because the boundary shear stress is proportional to the slope of the vertical velocity profile and the site will be equipped with a water level recorder.
To resolve the variation of both velocity and sediment concentration over the tidal cycle, it is necessary to record these parameters every few minutes and it would not be possible to obtain such a resolution using a mechanical instrument. It has therefore been decided to measure turbidity, which is proportional to sediment concentration, using an optical backscatter instrument (OBS), and to measure velocity using an acoustic current meter. Both these instruments are electronic and thus allow for collecting a continuous record. Because an optical sampler is an indirect measurement of sediment concentration, it is necessary both to calibrate the instrument against known sediment concentrations and to take water samples (perhaps one every hour over the tidal cycle) to determine the actual size of sediment in suspension and to determine the relative contributions of organic and mineral material to the total concentration of suspended material.