The effectiveness of sediment control devices was studied on a large NC DOT project to determine the effects of different designs and conditions. Flow and sediment content of water exiting six different traps and basins were measured and the amount of sediment trapped estimated from periodic surveys. Sediment trapping and discharges strongly suggested that commonly used designs are relatively ineffective. The three devices with rock dam outlets had sediment retention of <57% of sediment entering the traps and discharged up to 20 t ac-1 during up to 12 months of monitoring. In contrast, the skimmer basins with surface outlets, stable sides and inlets, and porous baffles, retained more that 90% of sediment entering them, as long as they were properly maintained. While the skimmer basins retained most of the sediment entering it, the discharges were still relatively turbid and contained considerable suspended solids. The skimmer basin which was monitored longest (one year) had average turbidity of 891 nephelpmetric turbidity units (NTU) and total suspended solids (TSS) of 537 mg L-1. It is likely that the remaining suspended materials are very fine and will not settle by gravity alone under typical retention times. The more efficient designs tended to retain more of the coarse fraction entering the basins than the less efficient designs. The improvement in sediment retention with the design improvements will significantly reduce the impacts of land disturbances from construction activity on water quality in nearby streams.
