Improvements in Stormwater Detention Technologies through Large-Scale Testing Techniques

Abstract

Due to urbanization and a changing climate, stormwater management has become increasingly discussed, regulated, and researched. In many areas of the United States, one effect of climate change has taken form in more intense or frequent storms, yielding increased stormwater runoff events. There are unique challenges faced in each land development stage to preserve and protect water resources. If mismanaged, stormwater runoff can expedite overland and streambank erosion processes, resulting in profound environmental and economic implications. Stormwater detention systems are commonly implemented in construction and post-construction stormwater management to capture sediment and additional pollutants and attenuate discharge flow rates. This dissertation presents results from techniques used to evaluate the performance of stormwater detention technologies to provide science-based design guidance and solutions. This research includes existing erosion and sediment control technologies used in the construction industry, existing research, performance, and cost assessments. Specifically, this dissertation investigates sediment basins, which are considered a temporary sediment control practice typically employed onsite perimeters to detain sediment from stormwater runoff before discharge. Sediment basins are heralded for effective sediment capture; however, design and installation techniques vary nationwide. Researchers at the Auburn University - Stormwater Research Facility (AU-SRF) examined the performance of an in-channel sediment basin design, which uses existing roadside conveyance features to minimize the required footprint for installation. The traditional design detailed an earthen berm installed to the full channel height with an armored overflow spillway and a perforated riser pipe for primary dewatering. The large-scale testing described in this dissertation follows a 2018-2019 field-monitoring study of the in-channel sediment basin design on the construction of U.S. 30 in Tama County, IA. Field data indicated negligible turbidity and total suspended solids reduction when comparing inflow and discharge samples. In an effort to improve performance, several structural treatments and one chemical treatment were evaluated through large-scale, controlled flow and sediment introduction testing. Treatments included: (1) geotextile lining, (2) a floating surface skimmer, (3) porous flow baffles, (4) an upstream forebay, and (5) application of flocculant. Performance was categorized through water quality, quantity, and soil retention data. Sediment retention was reported as high as 96% by weight when an upstream forebay, geotextile lining, surface skimmer, and surface were used as a system, and 98% when flocculant was added to the skimmer. The sediment retention can be compared to 76% when only a geotextile liner was used. When flocculant was applied, turbidity reduction increased by 42%, and discharge turbidities were consistently below 100 NTU during dewatering periods. Flocculant reduced the captured D50 particle size by 400%, on average, indicating that flocculant aids in capturing the finest particles, which may decrease required storage volume and detention times in basins. In addition to experimental testing, a spreadsheet-based tool was developed to aid in the implementation of in-channel sediment basins and the structural and chemical components that enhanced sediment capture and turbidity reduction. In some cases, sediment basins become permanent installations to control post-development flows. As post-construction stormwater designs and performance face enhanced regulations and site footprints remain limited, technologies are being developed to design basins with a smaller footprint while still achieving hydrologic and water quantity goals. However, it is important to understand and achieve water quality and quantity goals to meet design standards and regulations. The final portion of this dissertation describes the development of a standardized test procedure to evaluate permanent outlet systems in post-construction basins in Alabama. A 2,790 ft3 (79.0m3), large-scale sediment basin was retrofitted with an impervious plastic liner to prevent infiltration during testing, emulating a post-construction dry-detention basin. Sediment was introduced at a rate ranging between 100 to 300 mg/L to simulate a 1.5 in. (3.8 cm) rainfall depth across a 0.6 ac (2.43 ha) contributing area made up of 90% impervious surfaces. The testing apparatus was calibrated using an Alabama native, Sandy Loam soil. After several calibration tests, an orifice size of 0.70 in. (1.78 cm) was selected for the dewatering cap. The target inflow sediment concentration was eventually achieved by introducing 35 lb (16 kg) of sieved soil throughout the 60-minute testing window. To ensure a consistent introduction rate, one volumetric pound of sediment was passed through the adjustable density over a 1.5 minute period until all 35 lb (16 kg) were introduced. According to mass balance equations, the control testing was providing at least 75% sediment capture during settling periods. In summary, the findings presented in this dissertation are expected to inform the performance, design, and implementation of stormwater detention practices, particularly on sites with limited footprints, to minimize detrimental downstream effects.