HYDRODYNAMIC ASSESSMENT VARIOUS TYPES OF BAFFLES IN A SEDIMENT RETENTION POND.

We assessed the relative improvement to the sediment trapping effectiveness of a permanent-pool sediment retention pond due to the installation of baffles composed of different materials commonly used on construction sites. A suite of experiments was performed at the Sediment and Erosion Control Research and Education Facility (SECREF) at North Carolina State University in which an acoustic Doppler velocimeter was used to record steady-state flow velocity and signal-to-noise ratio data. The data was gathered at 25 grid points at two depths within the pond for three different fixed input flow rates. The free flow maximum mean velocity in the pond, averaged over all input flow rates, was reduced by roughly 75% due to the presence of baffles composed of jute germination biotextile backed by coir fiber. Baffles made from a standard tree-protection fence, folded and tied together into three layers to reduce pore size, reduced the free flow maximum mean velocity by 65%, while baffles made of standard silt fabric fence reduced the free flow maximum mean velocity by 55%. A similar trend in the reduction of the signal-to-noise ratio along the length of the pond confirmed that the jute/coir baffles most effectively reduced the concentration of turbulent density fronts over that of the tree-protection fence or silt fence baffles, or free flow. In addition, analysis of the transverse velocity variance and vertical velocity gradients for each experiment further demonstrated that the jute/coir baffles most effectively diffused inflow momentum along the width and depth of the pond. The results of our analysis were used to calculate a sediment trapping efficiency based on Stokes settling. The minimum grain size captured would range from 30 to 42 microns with jute/coir baffles, compared to 68 to 86 microns for free flow. [ABSTRACT FROM AUTHOR]