FACTORS AFFECTING SEDIMENT BASIN EFFICIENCY ON HIGHWAY CONSTRUCTION SITES IN THE PIEDMONT REGION OF NORTH CAROLINA

Sediment basins are a commonly used best management practice on North Carolina Department of Transportation (NCDOT) highway construction sites for capturing entrained sediment during stormwater runoff events. The NCDOT currently uses a sediment basin design that has a volume proportional to the disturbed catchment area and a surface area proportional to the 10-year peak runoff discharge from the catchment. This project monitored four active NCDOT sediment basins in the Piedmont region of North Carolina to evaluate several hydrologic and land use parameters for their ability to explain the variability of turbidity and total suspended solids (TSS) entering the basins as well as the efficiency of the basins with regard to these two parameters. TSS ranged from 270 to 53,000 mg L-1 entering the basins and from 87 to 70,600 mg L-1 leaving the basins. Turbidity ranged from 198 to 29,000 nephelometric turbidity units (NTU) entering the basins and from 27 to 28,700 NTU leaving the basins. There was no clear relationship between the stages of development or site activities and the turbidity and TSS entering the basins. Peak flow, which is used in the design of NCDOT sediment basins, was positively correlated with inlet turbidity and TSS. Additionally, the performance of the basins to reduce turbidity and TSS did not correlate to storm characteristics. The diversion ditches leading to the basins appeared to be a major source of sediment and likely contributed to the high variability in turbidity, TSS, and basin performance.