Reports
Evaluation of sedimentation impacts on Natural Oyster Bay 8-10 from dike construction at the Poplar Island Environmental Restoration Project
2002, Halka, J.P., and Ortt, R.A., Jr.
File Reports, Coastal and Estuarine Geology, File Report 2002-01
Executive Summary
The shellfish bed sedimentation study was designed to determine if impacts to the adjacent mapped Natural Oyster Bar 8-10 (NOB 8-10) resulted from the construction of the containment dike surrounding the Poplar Island Environmental Restoration Project. The study was part of a number of efforts designed to insure that construction of the containment area resulted in minimal impacts to the surrounding areas. NOB 8-10 lies immediately to the west of the construction area adjacent to the west-northwest facing portion of the perimeter dike.
The initial study design addressed the following null hypothesis: “There is no increase in sedimentation rates on the charted oyster bar during the construction of the exterior dikes at Poplar Island when compared to sedimentation rates in reference areas unaffected by dike construction.” Three sites were selected from within NOB 8-10 and compared to three reference sites located in similar water depths and located well away from the construction area.
The initial study results determined that sediment mobility within both NOB 8-10 as well as the reference sites was quite high and temporally dynamic. The high mobility prevented the direct measurement of small amounts of sediment accumulation and the comparison of the study and reference areas. As a result of these observations, the study design was altered from addressing the null hypothesis to measuring greater thicknesses of sediment accumulation on NOB 8-10 itself. Measurements were made using a depth sounder and associated data was collected with a side-scan sonar unit. The sonar data were analyzed to determine bottom type across a wide area of the oyster bar and classify the bottom as sand, mud, shell, or sand covering shell.
Pre- and post-construction bathymetry, corrected for tidal differences, showed no distinctive patterns or trends of sediment accumulation. Had sand spread outward from the dike construction it would likely have formed a thicker layer close to the dike and thinned with increasing distance. No evidence of this was apparent when the pre- and post-construction bathymetric data were compared.
Post-construction side-scan sonar records suggested that a thin layer of sand covered limited areas of shell in close proximity to the dike. Where present, the sand was apparently a few centimeters thick. Thick enough to cover some objects interpreted as oyster shell on the pre-construction side-scan survey. Although, it cannot be definitively stated that this sand was attributable to dike construction, proximity to the dike suggests that the source of the sediment was related to construction activities.
A follow up side-scan sonar survey of NOB 8-10 conducted approximately one year later showed bottom conditions similar to those that were present in the previous surveys. There was no evidence of accumulation of additional sand over oyster shells on the bar. Nor was there evidence that the sand that was over the shell on the previous survey had been removed because of proximity to the Poplar Island Environmental Restoration Project.
