Silt fences have long been a key component for controlling construction stormwater runoff; nonetheless, many silt fence installations fail to perform in the field as intended. Silt fences are temporary sediment control measures used to retain sediment by impounding runoff and allowing for sedimentation on-site, while simultaneously discharging stormwater runoff at a controlled rate. This study evaluated the performance of eight alternative configurations of the Alabama Department of Transportation (ALDOT) standard wire-backed, nonwoven silt fence. Standard installation parameters associated with the ALDOT silt fence include (1) 32-in. (81.3-cm) high fence, (2) 0.95 lb/ft (1.4 kg/m) support T-post, and (3) 10 ft (3.0 m) T-post spacing. Throughout the series of configurations tested, these standard parameters were varied individually and jointly in efforts to improve overall performance. Variations to the standard parameters include (1) 24-in. (61.0-cm) high fence, (2) 1.25 lb/ft (1.9 kg/m) support T-post, (3) 5 ft (1.5 m) T-post spacing, and (4) trench offsetting. Performance analyses were conducted on each configuration and results were evaluated to determine the best overall configuration to enhance the in-field performance of the ALDOT silt fence. Ultimately, the offset 24 in. (61.0 cm) fence with 1.25 lb/ft (1.9 kg/m) T-post spaced 5 ft (1.5 m) on-center resulted in the best overall improvement, retaining an average of 93% of sediment and deflection of only 0.18 ft (0.004 m) over the course of three simulated storm events.
Sediment barriers (SB) are devices typically installed along the perimeters of construction sites to intercept, capture, and contain pollutant discharge. Effective SBs minimize sediment transportation off-site by temporarily impounding stormwater and facilitating sediment capture upstream of the installation. Full-scale experiments were conducted on common, innovative, and manufactured SB practices used within the construction industry to better understand their performance. These practices consisted of two manufactured silt fence systems, three sediment retention barrier practices, and three manufactured sediment barrier products. Installation details for each practice were analyzed and amendments made to provide the most effective installations. Test observations indicated that a major failure mode of innovative and manufactured SB practices was flow bypass due to undermining. Performance-based comparisons of sediment retention rates, maximum impoundment depths, effluent flow rates, and treatment efficiencies were determined for each practice. Longevity tests were also conducted to evaluate characteristic changes over iterative storm events. Overall performance evaluations indicate practices that achieve impoundment depths greater than 1 ft (0.3 m) have consistent sediment capture rates of at least 90%. More importantly, impoundment depths greater than 1.5 ft (0.46 m) do not facilitate improved sediment capture rates. These observations suggest that optimized sediment capture is achieved when a SB practice has an effective upstream impoundment depth between 1 and 1.5 ft (0.3 and 0.46 m). Additionally, impoundment depths within this optimal zone reduce impoundment surface turbidity up to 60% when compared with turbidity levels along the bottom of the impoundment.
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