Performance Evaluation of Sediment Control Practices

Abstract

Sediment is a major pollutant of waterways, increasing turbidity, carrying other harmful pollutants, and reducing flow capacity, which can lead to loss of aquatic life and increased risk of flooding. Construction projects, due to their earth-disturbing nature, are one of the leading causes of sediment runoff. To protect water bodies and natural areas, the United States Environmental Protection Agency requires construction projects with disturbed land within 50 ft (15 m) of a water of the United States to provide and maintain a 50 ft (15 m) vegetated buffer or provide equivalent sediment controls. Despite this requirement, there is little guidance on the effectiveness of vegetated buffers in removing sediment or how sediment barriers can aid shorter buffers or replace buffers. A modeling methodology was developed that evaluated the sediment removal capabilities of 50 ft (15 m) vegetated buffers; 11,664 buffers with conditions local to Nebraska were evaluated and ranged from 18.5% to 99.5% sediment capture with an average of 92.6%. All analyzed properties of the vegetated buffers were found to have an impact on the sediment capture capabilities of the buffers. To evaluate alternative sediment control practices, a large-scale testing apparatus at the Auburn University – Stormwater Research Facility was used to evaluate Nebraska Department of Transportation standard sediment barrier installations of silt fences, slash mulch berms, and wattles; all standard installations experienced structural inefficiencies that led to them being ineffective at capturing sediment under standard or excessive impoundment conditions. Modified installations, resulting in most feasible and effective installations, were evaluated to improve upon structural inefficiencies of the standard installations. A silt fence installation that employs a 6 in. (15.2 cm) offset trench to prevent undermining, reduced post spacing in areas of increased impoundment to prevent fabric sag, and a dewatering board with overflow weir to allow for timely dewatering and the creation of an emergency spillway to protect the installation from excessive impoundment was developed through iterative testing of modifications. This most feasible and effective silt fence installation treated stormwater in turbidity through the process of sedimentation within the impoundment formed and captured an average of 85.5% of introduced sediment upstream of the practice. A slash mulch berm installation with a reduced profile to use less material than the standard and compacted in three lifts to facilitate more impoundment was developed; this installation captured an average of 73.5% of introduced sediment upstream of the practice but was also able to capture sediment within the practice, leading to a total average sediment capture of the installation of 98.9%. A straw wattle installation was developed that used sod staples to facilitate ground contact and non-destructive teepee staking at joints with an increased overlap of wattles to ensure consistent impoundment capabilities. The most feasible and effective straw wattle installation allowed for the overtopping of wattles, allowing for maximum impoundment to be reached, rather than flow bypass or undermining; this installation captured an average of 80.8% of sediment upstream and facilitated a maximum impoundment depth of 7 in. (17.8 cm). An excelsior wattle installation using the same method was tested to allow for a comparison of material; the excelsior wattle had higher flow-through rates, which led to less impoundment facilitated, less sediment capture at 75.3%, and discharge that was higher in turbidity and total suspended solids. These findings indicate that 50 ft (15 m) vegetated buffers can be highly effective in capturing sediment, especially in buffers with shallower slopes experiencing runoff with larger average size soil. Large-scale testing of sediment barrier standard installations indicated a need for modifications to increase the protection of downstream areas. However, even with these modifications, most sediment barrier installations failed to match the sediment capture capabilities of 50 ft (15 m) vegetated buffers.