Optimization of Sediment Basin Configurations

Abstract

Highway construction projects typically involve the disturbance of existing on-site vegetation, leaving bare soil vulnerable to erosion from environmental factors such as rainfall and wind. Soil erosion caused by precipitation events is transported via stormwater runoff and is deposited into receiving waterways. To reduce downstream sedimentation in receiving waterbodies, stormwater regulations mandate on-site control of pollution produced by construction activities. These regulations require designing, installing, and maintaining erosion and sediment control practices on construction sites. One of the more commonly used sediment control practices in Alabama is sediment basins. Sediment basins are sediment control structures used to capture, detain, and remove sediment from stormwater runoff. While standardized sediment basin design guidance exists, further research is necessary to optimize designs and ensure that guidance is backed by engineering principles. This research focuses on understanding the performance of sediment basin designs in terms of their geometry and the effectiveness of various treatment elements. A performance-based testing methodology and apparatuses were developed in the Stormwater Lab at Auburn University to address this need. Three small-scale sediment basins were constructed for this project, each with a trapezoidal cross-section configuration. The standard and in-channel basins were designed with similar volume capacities, mimicking the size of a 3,600 ft3 basin designed to treat 1 acre of disturbance on a representative construction site in central Alabama. In contrast, the undersized basin was intended to simulate the previous design standard of 1,800 ft3, resulting in a volume capacity approximately half that of the standard and in-channel basins. Specifically, the total water volume up to the spillway for the standard, in-channel, and undersized basins was measured at 15.3 ft3 (0.43 m3), 13.50 ft3 (0.38 m3), and 6.99 ft3 (0.20 m3) respectively. Small-scale testing allowed a higher degree of control of sediment basin parameters and conditions to optimize settling conditions. Nine treatment methods were tested in the standard basin (2L:1W). This included (1) three coir baffles, (2) an open basin without energy dissipators, (3) increased flow path length using impervious barriers, (4) a single coir baffle (5) a level spreader forebay (6) a check dam constructed in the inflow channel with a single coir baffle, (7) a level spreader forebay with a single coir baffle, (8) no energy dissipators or skimmer, and (9) flocculant with a single coir baffle. This paper investigates the performance of the most feasible and effective installation (MFE-I) in the standard basin (2L:1W), then replicating the MFE-I within the in-channel and undersized basins. Results indicate that the undersized sediment basin was nearly as effective (73%) in capturing sediment than the standard geometry basin (87%) without the addition of flocculants. Research is ongoing and the next task is to identify the most efficient sediment basin geometry. This research aims to improve sediment basin efficiency by modifying small-scale basins and performing tests to improve geometric properties combined with additional treatment methods. Overall, sediment basin design parameters can be combined and modified to optimize sedimentation, including geometric designs and treatment methods used within the basin. Results indicate that the introduction of flocculant can significantly enhance sediment retention in undersized basins, surpassing the performance of standard-sized basins without flocculant treatment. In the case of the standard model basin, designed to mimic a capacity of 3,600 ft3/ac (252 m3/ha), the utilization of a skimmer and three coir baffles (ALDOT Standard) resulted in an overall sediment retention percentage of 88% within the basin. Conversely, the undersized basin, replicating the previous design standard of 1,800 ft3/ac (125 m3/ha), achieved a total sediment retention percentage of 91% by incorporating a single coir baffle and applying a 5 mg/L dosage of flocculant emulsion. The finding that the introduction of flocculant significantly enhances sediment retention in undersized basins suggests that incorporating flocculant treatment can be an effective strategy to improve sediment capture. Field practitioners and engineers can consider the use of flocculant emulsion in their sediment basin designs to achieve higher sediment retention percentages.