Performance Evaluation of Erosion and Sediment Control Practices

Abstract

Construction-generated stormwater runoff has the potential to negatively affect receiving water bodies downstream. Not only does eroded soil transport other pollutants, but sediment also remains suspended in water, where it can block sunlight and cause hypoxic conditions, resulting in a variety of other adverse environmental effects. Federal and state regulations stress the importance of erosion and sediment control on construction sites and mandate the implementation of effective stormwater pollution prevention plans. The regulations aim to prevent the impairment of receiving waterbodies by mandating the management of construction stormwater with appropriate design, implementation, and maintenance of erosion and sediment control practices. This thesis explores practical methods to enhance wattle impoundment abilities and application of flocculants on construction sites. This research evaluates construction stormwater treatment through (1) modification of a wattle’s encasement to improve impoundment abilities, (2) exploring possible flocculant detection methods for large-scale applications, and (3) conducting large-scale testing evaluations with flocculants to develop guidance on dosage and applications rates. Temporary erosion and sediment control practices seek to minimize and reduce the effluent turbidity from a construction site by preventing sediment detachment and capturing suspended sediment particles, respectively. A common method to protect against erosive forces and allow sediment to settle out of suspension is to impound water. Wattle ditch checks are a common erosion and sediment control practice for slowing down water in channels to reduce those erosive forces. Wattle popularity is correlated to their price, weight, application versatility, range of fill materials, and installation process. According to previous research, the hydraulic performance of wattles is predominantly determined by the fill material. This study assessed the impact of encasement material (e.g., netting, socking, etc.) on wattle hydraulic efficacy. In a two-phased process, two separate hydraulic flumes were used. Phase I evaluated configurations of wattle encasement fabric. In Phase II, select encasements were evaluated with 4.0 ft (1.2 m) long wattles with excelsior fill materials – heavy-duty synthetic plastic netting (control), polypropylene, polyester and polypropylene blend, and cotton woven encasements. The outcomes of each wattle test were normalized using ratios of impoundment length and depth. The results indicated that the percent open area (POA) had a direct relationship with the impoundment length and depth when the encasements were evaluated independently of the fill material; however, the encasement type had a greater effect on performance when the fill material was included. Using a cotton fabric increased the length and depth ratios of the impoundment by 30% and 24%, respectively, compared to a plastic netting encasement; these ratios increased to 52% and 44% when two additional cotton fabric layers surrounded the wattle. Even though implementing erosion and sediment control practices can be highly effective at capturing sediment from stormwater, fine sediment and clay particles are difficult to capture as they can take days to settle out of suspension. Often times, practitioners do not have options to increase the detention time for impounded water for the time necessary to capture those fine particles before another storm event. Flocculants are a chemical additive that can be added to sediment-laden water to facilitate the efficient capture of fine sediment particles. Flocculants function by agglomerating fine sediment particles to produce flocs, which are larger colloids that facilitate settlement. Thus, greatly reducing detention time necessary for capturing fine particles. While effective, there are concerns that improper use of flocculants could create potential pollution to downstream waterways and harm aquatic life. The effectiveness of flocculants used in a variety of applications, including stormwater management, has been studied; however, there is a knowledge gap regarding application techniques and dosage guidance for construction site applications. When exploring flocculant detection methods for large-scale applications, methods that were simple and easy to perform without extensive lab training, cost-effective, capable of working with sediment-ladened samples, produced reliable results in a short time, and were capable of quantifying concentration ranges above and below manufacturer’s recommended concentrations were periodized. A Cannon-Fenske Routine Viscometer and Brookfield Digital Viscometer were not sensitive enough to quantify the necessary concentration range needed, while a Laboratory Charge Analyzer could. Depending on the flocculant type and charge, the Laboratory Charge Analyzer could only accurately quantify low or high concentrations around the manufacturer’s recommendations. Quantifying soil settling velocity remained to be the most consistent method that met all criteria for predicting residual flocculant concentrations. Research found that large-scale applications require sample temperature and pH to be accounted for in the settling velocity prediction equations to generate reliable results as the time of year and vegetation can influence the temperature and pH of stormwater, which, in addition to flocculant concentration, all significantly influence soil settling velocities. Flocculant application placement and reapplication timing guidance on construction sites to achieve optimum dosing and mixing for granular and block form flocculants were determined by performing large-scale tests and using the soil settling velocity residual flocculant detection method that accounted for pH, temperature, and flocculant concentrations. Anionic granular polyacrylamide flocculant was spread across three wattles spaced over 43 ft (13 m) at a rate of 6.36 oz. (180 g) per wattle. Results found that during a 0.75 ft3/s (0.07 m3/s) flow event, the channel is initially dosed with 14 mg/L above the manufacturer’s recommendations and exponentially decreased over the first 25 minutes or 1,060 ft3 (30 m3) of flow to reach the recommended dosage of 5 mg/L. From this, it is recommended that reapplication of granular flocculant should be performed after 3600 ft3 (101.9 m3) of flow or 1.0 in. (2.54 cm) of runoff per acre (0.4 ha). Anionic block form polyacrylamide tests were conducted, and results indicated that six flocculant blocks provided optimum dosing for a flowrate of 1.80 ft3/s (0.05 m3/s) of channel flow in a 4 ft wide bottom channel; however, further analysis is necessary for accurate concentration predictions using block form flocculants. All flocculant applications evaluated indicated the need for at least one flocculant-free ditch check at the end of a channel to provide necessary mixing. Flocculants vary by manufacturer and are highly soil-dependent. Therefore results may vary based on the product manufacturer and soil type. Effective application of flocculants on construction sites is possible with the correct dosage, dosage delivery mechanism, and application. This study provides practitioners with a framework for establishing flocculant implementation that effectively treats construction stormwater. The findings of this study enable advances in flocculant usage in construction stormwater treatment through new and revised guidelines, as well as an increased understanding of the use of flocculants in the erosion and sediment control business.