United States Department of Transportation
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Evaluating sediment capture rates for different sediment basin designs.
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2007-08-01
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Edition:Final report; July 2005-June 2007.
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Abstract: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.
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