EVALUATION OF INFILTRATION SWALE MEDIA USING SMALL- AND INTERMEDIATE-SCALE TESTING TECHNIQUES

Abstract

When impervious surfaces such as paved roadways are constructed, the volume of water infiltrating into native soil decreases, leading to an increase in surface water runoff. This phenomenon results in higher peak flows, elevated erosion rates, and the transport of total suspended solids and pollutants. Hydrocarbons and other pollutants from paved surfaces find their way into receiving water bodies, posing environmental challenges. Regulations mandate water runoff control to minimize erosion and prevent sediment deposition. Low impact development (LID) practices aim to maintain the pre-development hydrological cycle through processes including evapotranspiration, infiltration, water reuse, and filtration. The Alabama Department of Transportation (ALDOT) relies on implementing infiltration swales, a type of LID practice, alongside roadways to manage water runoff quantity. These practices function by promoting surface water runoff to enter through an engineered media within roadside channels. By having a high permeability rate, the media serves to promote groundwater infiltration. Currently, ALDOT infiltration swale media is made up of a matrix consisting of topsoil, sand, and No. 57 stone wrapped with geotextile. Infiltration swales have been used throughout the state by ALDOT, however, their performance has not been evaluated and thus research is needed to understand how this standard media performs and to optimize its performance. The purpose of this research was to design a methodology for evaluating and optimizing the performance of infiltration swale media. Testing methodologies and apparatuses were developed to assess their capacity to infiltrate water on a small and intermediate scale. Three types of apparatuses were built for this research: a permeameters structure, consisting of 18 permeameters with a diameter of 6 in. (15.2 cm) and a length of 3.0 ft (0.9 m), a clear infiltrometers structure, consisting of six infiltrometers with a diameter of 6 in. (15.2 cm) and a length of 3.0 ft (0.9 m), and an infiltration swale chamber, monitored by a moisture content system, with internal dimensions measuring 8.0 ft (2.4 m) in length, 2.5 ft (0.8 m) in width, and 4.0 ft (1.2 m) in height. Constant head permeability tests conducted on the permeameters revealed that the current ALDOT infiltration swale media design yields a very low permeability ranging from 0.0017 in./min (0.0043 cm/min) to 0.019 in./min (0.0495 cm/min). This is attributed to the low permeability of the topsoil, which yielded 0.002 in./min (0.004 cm/min). As a result, designs containing topsoil as the top layer could not achieve the minimum infiltration rate of 1.0 ft/day (0.38 m/day) required by the Alabama LID Manual. To improve the infiltration rate through the topsoil layer, alternatives with amended materials were investigated. Several mixtures of amended topsoil, consisting of topsoil and pine bark fines at different proportions, underwent falling head infiltration rate tests. The amended topsoil mixture containing 80% topsoil and 20% pine bark was selected as the top layer for future alternative designs because it yielded an average infiltration rate under falling head conditions of 5.6 ft/day (1.6 m/day), 8.8 times higher than topsoil alone, which yielded 0.63 ft/day (0.19 m/day). Throughout the process, the testing methodology to evaluate the performance of infiltration swale media design in the infiltrometers was refined to establish a consistent testing regimen comprising three constant head infiltration tests lasting six hours each, followed by three falling head infiltration tests. Constant head infiltration tests simulated the prolonged use of infiltration swale media, providing insights into their long-term performance. Falling head infiltration tests allowed for understanding the time required by the designs to infiltrate the ponding water, enabling comparisons of their performances with the minimum required infiltration rate of 1 ft/day (0.38 m/day). Initially, five infiltration swale media designs were proposed and subjected to this testing regimen. In an iterative cycle of evaluation and improvement, the results of previous tests were analyzed to identify causes of low performance and potential enhancement options. During this testing and optimization process, it was evident that designs including a geotextile layer wrapped up around the No. 57 stone exhibited a continuous decrease in their infiltration rate due to the gradual clogging of geotextile pores by sand particles. This cycle of evaluation and improvement was iteratively repeated until finally achieving the F3 design, composed of 6 in. (15.2 cm) height of amended topsoil (80% topsoil and 20% pine bark fines by weight), 10 in. (25.4 cm) height of field sand, 6 in. (15.2 cm) height of pea gravel, and 9 in. (22.9 cm) height of #57 stone. The F3 design exhibited a performance of 13.73 ft/day (4.18 m/day) in constant head infiltration tests, 15.1 times higher than the 0.91 ft/day (0.28 m/day) obtained by the ALDOT standard matrix, and 11.66 ft/day (3.55 m/day) in falling head infiltration tests, 37.61 times higher than the 0.31 ft/day (0.09 m/day) obtained by the ALDOT standard matrix. Finally, the ALDOT and the F3 design were tested in the infiltration swale chamber under constant and falling head conditions. The F3 design yielded 87.06 ft/day (26.54 m/day) in constant head conditions, 13.37 times higher than the 6.51 ft/day (1.98 m/day) yielded by the ALDOT design, and 75.79 ft/day (23.20 m/day) in falling head conditions, 15.28 times higher than the 4.96 ft/day (1.51 m/day) yielded by the ALDOT standard matrix. The tests conducted in the infiltration swale chamber were monitored by a moisture content system, showing that the F3 design has a drying rate 111 times higher than the ALDOT design. The results of this research showed that with the F3 design, infiltration swales will achieve higher infiltration rates in the short and long term, as well as superior drying rates, leading to a larger available storage volume after each rainfall event. The F3 design and the ALDOT design will be evaluated on a field-scale by the Auburn Stormwater team, and the results will be compared with those obtained in this research.