Evaluation of Construction Stormwater Detention Measures Using Full-Scale Techniques

Abstract

Sediment is a dominant pollutant in waterbodies due to the heightened turbidity, transport of other harmful pollutants, negative impacts on ecosystems, and reduction of flow capacity. Construction projects, especially during their earthwork phase, are one of the largest contributors of sediment-laden runoff. The United States Environmental Protection Agency and often many state regulatory agencies mandate that every project obtains a construction general permit which involves completing a Stormwater Pollution Prevention Plan. Detention practices are one of the common methods used for sediment control when employing a Stormwater Pollution Prevention Plan, but detention practices come in various forms and sizes. A methodology using simulated hydrologic and erosive conditions typical to Nebraska was developed to evaluate the performance of Nebraska Department of Transportation (DOT) silt trap and sediment trap practices. A full-scale testing apparatus was constructed at the Auburn University - Stormwater Research Facility. Through the analysis of sediment capture and water quality, the most feasible and effective installation of each practice was identified to provide design guidance for practice selection. Silt traps can serve a contributory drainage area up to 1 ac (0.4 ha) while the sediment trap can serve an area from 1 ac (0.4 ha) to 5 ac (2.0 ha). Under a flow rate of 0.5 ft3/s with a sediment introduction rate of 36.6 lb/min and a flow rate of 1.0 ft3/s with a sediment introduction rate of 49.0 lb/min respectively, both Nebraska DOT standard installations presented opportunity for enhancement. The silt trap standard was evaluated in conjunction with an erosion check, various silt fence installations, rock checks, an earth check, and a slash mulch berm. The installation with the most overall improvement was the modified silt fence that created a V-shape of wire-backed, non-woven geotextile with an 18 in. (45.7 cm) weir cut out of the center. For the 2 modified silt fence, the resulting sediment retention was the highest achieved in testing at 95.9%, the highest impoundment at 35 ft (10.6 m) the TSS reduction from inflow to downstream was 89.1%, and the turbidity reduction from inflow to downstream was 67.7%. The reduced post spacing – from a standard 5 ft (1.5 m) to 3 ft (0.9 m) – used to create the fence shape and the wire backing with the geotextile increased structural abilities to withstand flow. The weir allowed for timely dewatering and protected from excessive impoundment. The installation had the highest sediment capture at 95.9% and the longest impoundment at 35 ft (10.7 m) for sedimentation to occur. The slash mulch berm had the second highest sediment capture at 94.6% and the most improved quality of water leaving the installation, 91.4% TSS reduction and 69.1% turbidity reduction. While slash mulch was a high performing installation, the issue of mulch availability must be considered for project implementation. In the silt trap testing, the low porosity silt fence failed, indicating a need for structural improvement. A series of clean water tests were conducted on various configurations of the low porosity silt fence to find the structural MFE-I. Factors tested were fence height, post material, post spacing, and fence shape. The best performing installation, based on least amount of post deflection, was adapted from the modified silt fence yet utilized the low porosity fence geotextile. The other high performing installation was a reduced fence height, from 30 in. (76.2 cm) to 24 in. (61.0 cm) and reduced post spacing, from 5.0 ft (1.5 m) to 2.5 ft (0.76 m). The sediment trap installation with most overall improvement was the installation with three wire-supported, coir baffles and a 1.5 in. (3.8 cm) surface skimmer. The combination of elements provided evident turbulence reduction and controlled dewatering to restore the available storage volume. The baffle and skimmer installation improved TSS from nearly 5,000 mg/L inflow average to under 500 mg/L downstream TSS and turbidity from around 1,600 NTU inflow average to around 700 NTU 3 despite having high background turbidity. The combination installation also had the second highest sediment capture at 91.1%, 0.8% below the highest sediment capture. The results of full-scale testing indicate that the standard installation of the silt trap and sediment trap have opportunity for improvements through modifications. The silt trap increased sediment capture and improved water quality when it was paired with a modified silt fence while the sediment trap addressed various shortcomings, resulting in improved performance through the use of coir baffles and a skimmer. The most feasible and effective installation of the silt trap and sediment trap provided enhanced sediment capture and downstream water quality. Since the silt trap was found to perform better in conjunction with another practice, especially the modified silt fence, the recommendation is to always implement a silt trap with a ditch check. The structural, clean water testing indicated that fence with similar characteristics to the modified silt fence would improve the low porosity silt fence as would a reduced post spacing and reduced fence height installation. Since the sediment trap was shown to perform well in water quality and sediment capture on its own, the recommended baffles and skimmer advance the performance by dissipating incoming flow and enhancing downstream water quality while restoring storage volume for subsequent storm events.