PHYSICAL EFFECTS OF REEF RUGOSITY ON THREE DIMENSIONAL HYDRODYNAMICS AND SEDIMENTATION OVER TRANSPLANTED OYSTER REEFS IN A SHALLOW TIDAL CREEK SYSTEM

The purpose of this study is to identify and quantify the physical effects that reef rugosity has on both three dimensional flow dynamics and sedimentation over constructed oyster reefs in a shallow tidal creek system. Benthic bivalves are keystone species and play a crucial role in nutrient cycling within their ecosystem, both increasing productivity and improving water quality. In recent years, the viability of using transplanted oyster reefs to mitigate anthropogenic effects on water quality has been investigated. For this study, oyster reefs were constructed in a shallow tidal creek system in New Hanover Co., North Carolina, U.S.A., and were altered with respect to rugosity, a measure of reef surface roughness. Three dimensional hydrodynamics over the reefs were measured using a Nortek Aquadopp® 3-D current profiler, and sediment data is being collected using sediment traps placed along the reefs’ surface. Hydrodynamic flow data and sedimentation rates were compared with respect to reef rugosity.

The study area was sub-divided into three separate sites, all of which received the same treatments. At each site, both higher and lower rugosity reefs were constructed from dead shell material (culch). The constructed reefs have an approximately circular base with a diameter of 2 m, and a height of approximately 0.4 m. Rugosity of the reefs was altered by adjusting the attitude and direction of the shells along the surface of the reef. Sediment traps were placed within the surface of the constructed reefs in order to measure sedimentation rates and to conduct sediment analyses. Sediment traps were deployed three times for approximately one month durations. Preliminary results indicate that sediment retention ranges from 0.094 g day^-1 to 1.043 g day^-1, with lower rates on the higher rugosity reefs. The Aquadopp® was deployed over the reefs, in a downward looking manner, attached to a portable frame, which will be set on the substrate. Preliminary data show that the shape of the vertical profile and extent of turbulence near the reef surface differs with rugosity. Although both data collection and analyses are ongoing, initial findings indicate that turbulent flow over the reefs is proportional to reef rugosity and that sediment retention rates are inversely proportional to reef rugosity.