Effects of Design Changes on Sediment Retention Basin Efficiency

Abstract

Sediment pollution from construction sites has been of increasing concern since the impacts on nearby streams can be severe. Controlling erosion is the most effective approach to reducing sediment loads, but construction sites typically have large areas of exposed soil during the active phase of clearing and grading. As a result, sediment traps and basins are required to capture eroded sediment on most of these sites. The purpose of this research was to determine the trapping efficiencies of sediment basins of various designs installed on active construction sites. Five traps and one basin were monitored in the Piedmont of North Carolina, all on highway construction sites except one trap on a private development. Automatic samplers were installed to measure flow and to obtain representative samples during storm events. The basins were surveyed after storms to determine the change in volume after repeated surveys. Trapping efficiency was calculated from the sediment accumulation within the traps or basin and the amount of sediment discharged, the sum of which was the total sediment entering the device. Particle size distribution in the sediment deposits was also determined. Two standard traps with rock outlets were found to have 37% and 46% trapping efficiencies after three storm events. A standard trap with silt fence baffles was found to have 45% and 36% efficiency rates during two time periods. Two additional traps, which had been sized for a 25-year storm event, instead of the standard 10-year event, had retention efficiencies of 96% and 99%. A sediment basin with porous baffles and a skimmer outlet had a retention efficiency of 99.8%. One standard trap had particle size distributions for sand, silt, and clay of 34%, 36%, and 30% while a standard trap with a permanent pool had particle size distributions of 55%, 25%, and 20%. The standard trap with silt fence baffles had a distribution of 36%, 50%, and 14%. The 25-year traps had distributions of 75%, 18%, and 7%; and 55%, 20%, and 25%, respectively. The skimmer basin had a distribution of 62%, 28%, and 10%. The higher proportion of sand in the more efficient devices suggests that the less efficient traps are releasing significant amount of sand-size sediment. Larger basins and surface outlets clearly provide greater sediment trapping on construction sites.