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The Potential for Substantial Improvements in Sediment and Turbidity Control

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Pp. 262-272 in Total Maximum Daily Load (TMDL) Environmental Regulations–II Proceedings of the 8-12 November 2003 Conference (Albuquerque, New Mexico USA), Publication Date 8 November 2003.  .(doi:10.13031/2013.15571)
Authors:   Richard A. McLaughlin

The systems currently used for sediment control are designed to trap coarser fractions of the sediment load instead of finer fractions, which move off site. We investigated the effects of various modifications compared to the standard sediment trap, which commonly consists of a 1 m deep pit with a gravel outlet, on sediment trapping and turbidity reduction. The tests consisted of simulated storm events in which sediment was added to flows of up to 10-30 L/s and directed to model sediment traps in the field. Two outlets were tested: standard gravel and skimmer. Baffles were tested including silt fence, jute/coconut, and tree protection fence tripled over. In addition, polyacrlyamide (PAM) logs of two formulations were tested in combination with the trap modifications. The effectiveness of grade control devices in channels and inlet protection practices were also evaluated. The skimmer outlet significantly improved sediment retention compared to the gravel outlet . Additional improvements by directing the outlet water into level spreaders brought retention to 99.9%. Turbidity was not reduced in the gravel outlet compared to the basin, but the skimmer outlet did significantly reduce turbidity by over 50%. The addition of PAM reduced turbidity further to <20% of the basin regardless of outlet type. It was possible to attain a 50 nephelometric turbidity units (NTU) turbidity level with PAM under our conditions of 400-800 NTU in the untreated test water. For inlet protection, both the silt fence and gravel/wire mesh systems were similar with about 90% retention. Among the grade control structures, the Georidge improved over time to be similar to the Triangular Silt Dike (>85% retention), while the typical rock dam lost retention over time until it was significantly less than the other two. They retained 77-91% of the sediment load overall.

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