*WINNER* Performance Evaluation of Vegetated Swales for Highway Runoff Reduction Based on Site Characteristics and Meteorological Influences

Abstract

Vegetated swales are stormwater control structures designed to reduce peak runoff flow volumes, and promote infiltration and evapotranspiration. Their historical use in highways was solely to convey runoff into a downstream outlet in an inexpensive way. In recent years, due to new stormwater regulations, the use of vegetated swales in highway systems has shifted from just flow conveyance to the reduction of total flows. This transition, however, is inhibited due to the lack of knowledge on the actual performance of existing swale structures within highway systems. Therefore, this project aims to evaluate the stormwater reduction potential of two existing vegetated swales located on State Route 111 and Interstate 40, in Putnam County, Tennessee, based on their individual site characteristics, and meteorological influences. The site characteristics selected for the study include the soil infiltration rates, initial soil moisture content, site geometry, and the land cover. As for the meteorological factors, rainfall intensity, rainfall volume, antecedent dry period, and evapotranspiration rates were selected to gauge the performance of the reduction potential of these two swales. Preliminary data from the site located on State Route 111 has exhibited reduction potentials of up to 75% for small rain events (< 0.35 in), but only after long dry periods (> 6 days). When there is little to no dry period (< 1 day), the reduction potential of the swale is minimal.