Sediment Trapping by Five Different Sediment Detention Devices on Construction Sites

Abstract

Sediment pollution from construction sites has been of increasing concern, since the impacts on nearby streams can be severe. Five sediment trapping devices were monitored on construction sites in the Piedmont region of North Carolina to determine their trapping efficiency and the improvement in the water quality of their discharges. For each device, discharges were measured and sampled over periods of 5 to 13 months and the amount of trapped sediment was determined. Three of the devices were basins with rock outlets designed for 10-year recurrence storms with the following differences: one device was over excavated to have a 1 m standing pool, one device had silt fence baffles with weirs, and one device was open and fully drained. The fourth basin with a rock outlet was open and fully drained but sized for a 25-year storm. The fifth device was sized for a 25-year recurrence storm and had a floating surface outlet and solid riser spillways plus porous baffles within the basin. The three rock outlet basins that fully drained retained <40% of the sediment entering them regardless of their variations. However, the rock outlet basin with a 1 m standing pool retained 73% of the sediment for 16 of 17 storms. This could have been higher if the inlet and sides had been stabilized. The larger basin with surface outlets retained over 99% of the sediment it received until the floating outlet became mired in sediment, reducing the efficiency to 76%. Average discharge water quality was the highest for the standing pool and surface outlet designs, but all of the devices had very high turbidity and total suspended sediment (TSS) during peak flows. Maximum values for turbidity and TSS ranged from 16,000 to >30,000 nephelometric turbidity units (NTU) and from 20,000 to 168,000 mg L-1, respectively. There was a high correlation between turbidity and TSS among all the discharge samples. This study suggests that typical sediment traps are inadequate for retaining construction site sediment using current design criteria. It is possible, however, to have very effective sediment retention using recent advances in design.