Evaluation of Erosion and Sediment Control Technologies and Development of Design Tools

Abstract

The rise in urbanization has increased pollutant loading in vulnerable receiving water bodies, attributable to impervious surfaces and construction sites. Impervious surfaces increase runoff volume and velocity and promote the accumulation of pollutants in stormwater runoff. Likewise, earthmoving activities across construction sites leave bare soil susceptible to erosive forces. Under the United States Environmental Protection Agency’s National Pollutant Discharge Elimination System (NPDES), stormwater control measures can be used for both active construction and post-construction applications through the Construction General Permit and municipal separate storm sewer system permit. With these, the use of stormwater control measures is necessary to manage stormwater runoff before it enters waterways. Sediment basins are used on construction sites to capture and detain sediment-laden runoff, allowing sedimentation to occur. Floating surface skimmers are used to dewater sediment basins by discharging from the water surface, the least turbid section of the water column. There is also an opportunity for their use in post-construction detention basins. Throughout this study, thirteen floating surface skimmers, eight designed for sediment basins and five designed for post-construction detention basins, were evaluated. Testing took place in the skimmer evaluation apparatus at the Auburn University-Stormwater Research Facility. Each skimmer was assessed across several configurations for its hydraulic performance and discharge rates. For each configuration, models were developed using various methods to predict flow rates across depths below 7 ft (2.1 m). The sediment basin skimmers were found to have average flow rates ranging as high as 92,585 ft3/d (2,622 m3/d) on the 8 in. (20.3 cm) skimmer set to 100% open to as low as 981 ft3/d (28 m3/d) on the 1.5 in. (3.8 cm) skimmer set to 50% open. Likewise, the maximum observed flow rates for the post-construction skimmers ranged as high as 2.984 ft3/s (0.085 m3/s) on the 8 in. (20.3 cm) skimmer, 100% open, unvented configuration, to as low as 0.045 ft3/s (0.001 m3/s) on the 1.5 in. (3.8 cm) skimmer, 50% open. Once all flow rate models were developed, two skimmer sizing tools were constructed for the sediment basin skimmers and the post-construction skimmers: an engineer’s calculator and an estimator’s calculator. The engineer’s calculator describes the stage-storage relationship as the skimmer dewaters a given basin. This calculator provides flow rate information and dewatering time. The estimator’s calculator provides the user with a skimmer and associated characteristics based on the basin volume and time. The user can enter a known basin volume or calculate volume from geometric parameters. Roadside conveyance channels are used to direct runoff away from the road’s right-of-way and to protect the integrity of the road. Low-volume roads in rural areas often lack adequate stabilization measures, leading to erosion and damage to infrastructure and the surrounding environment. An outline of various channel stabilization practices was developed to provide a foundation for future testing efforts. Design concepts were typically described using the permissible tractive force method, as recommended by leading existing roadside channel design guidance. Stabilization practices highlighted throughout this study include bare channels, grass linings, rolled erosion control products, geosynthetics, riprap, concrete linings, and check dams. Using the design concepts reviewed for each practice, a preliminary draft of a channel stabilization design tool was developed. This design tool consists of two main parts: determining peak flow rates and designing an appropriate channel armoring practice. Peak flow rates are computed using site-specific characteristics, such as rainfall type and land cover, with the TR-55 method. Designing a channel stabilization practice is based on permissible shear stress concepts. The user may select a bare soil, vegetated lining, vegetated TRM, or unvegetated TRM condition to determine an appropriate stabilization technique using channel geometry parameters. These findings indicate that hydraulic performance and associated flow rates vary across different skimmer designs and configurations. Thus, users can appropriately size skimmers to meet their needs based on experimental results. Furthermore, design concepts outlined for various roadside channel stabilization measures provide a foundation for future testing efforts and implementable design guidance.