Highways and roadways are the major source of stormwater runoff due to their prevalence and large non-permeable surface area. Best Management Practices (BMPs) such as bioswale provide effective on-site management and control of stormwater runoff from linear infrastructure such as highways. Many factors affect the performance of bioswales for stormwater volume reduction. The ratio of the installed BMP area to its service drainage area, characteristics of precipitation and the amount of sediment build-up over the surface of the BMP area are among the most important factors. Earlier studies have indicated that volume reductions in stormwater runoff from bioswale application range from 50% to 94%. However, the reported research lacks adequate information for a full understanding of how bioswales perform under various conditions. Consequently, additional systematic and in-depth research to better understand and the potential of bioswales as a method of controlling stormwater runoff is indicated. This research examined the effect of the following factors on bioswale performance: the ratio of the BMP area to the service drainage area, precipitation amounts and intensity, and sediment build-up. Hydraulic and hydrological processes were developed and analyzed through conceptual and physical models using appropriate governing equations including the Green-Ampt method. Field study of discrete rainfall events was conducted to collect information to calibrate and validate the numerical models. The field study tested various bioswale conditions with different levels of sediment accumulation. It also considered expected soil loss in the study area using the Universal Soil Loss Equation (USLE) method. In addition to field study, extensive simulations were conducted considering various contributing areas, rainfall depth and intensity, and sediment accumulation. These variables were manipulated to evaluate their effect on runoff volume reduction. Findings indicate that, for a given rainfall depth and duration, increasing the ratio of the BMP area to the service drainage area from 4% to16% results in increased bioswale efficiency ranging from 84% to 99%. The results revealed that input flowrate to the bioswale ranged from 0.04 to 4.7 in./min. depending on the rainfall intensity and soil type in the area. The runoff reduction performance of a newly constructed bioswale ranged from 44% for the highest input flowrate to 99% for the lowest input flowrate rainfall events. On the low end of rainfall volume/intensity, a 4% increase in the BMP area ratio results in a 34% improvement in efficiency (50% to 84%). On the high end of rainfall volume/intensity, a 16% increase in the area ratio results in only a 5% increase in efficiency (94% to 99%). Results also show that sediment accumulation has a substantial negative effect on infiltration rate. The observed efficiency of a bioswale in runoff reduction ranged from 13% to 100%. According to the USLE, the expected amount of soil loss occurring in the right-of-way area of a highway is approximately 1 ton/acre annually. The research revealed that for a given rainfall depth, duration, and area ratio; increasing the amount of sediment accumulation from 0 lbs./sq. ft. (equivalent to a newly constructed bioswale) to 2.7 lbs./sq. ft. (equivalent to a 10-year old bioswale) results in a 52% reduction in the runoff effectiveness of the bioswale sub-catchment from 98% to 46%. Finally, the physical model and associated governing equations were analyzed to describe the process of each studied factor. These results can be used for further study where the sediment accumulation rates differ from those modeled in this research.

The focus of this research was to assess the performance of bioswales in mitigating and treating stormwater runoff from highways and to identify critical parameters that influence the load of pollutants from the drainage area. These bioswales are located in Philadelphia and are part of a project initiated by the Pennsylvania Department of Transportation to upgrade a major roadway (Interstate 95) running through the area. The work included sampling and laboratory analysis of runoff water from 9 storm events to characterize concentrations of contaminants coming from the highway and going in to the bioswales. For one storm event, sampling of vadose-zone and ponded water was included to assess how contaminants move or are retained within the bioswale. The various contaminants include solids, nutrients and metals, which have all been shown to be parameters of concern when dealing with stormwater runoff from highways. In addition, a simulated runoff test was performed to assess the potential risk of a very large storm in mobilizing contaminants within the bioswale. Stepwise linear regression in IBM SPSS was used to analyze the runoff data collected. Characteristics of the rainfall (antecedent dry period, total rainfall, rainfall intensity) were selected as potential explanatory variables to predict contaminant concentration or load. Results of the runoff characterization showed contaminant concentrations that fell within range of literature values from a similar drainage area. Estimated annual loads of contaminants were also in range of what has been observed for highway runoff. Vadose-zone and ponded water sampling showed removal of ammonia, total phosphorus and chemical oxygen demand and build-up of nitrate, total nitrogen and TKN. The build-up was likely due to lack of ion interaction with soil particles, which caused the contaminants to remain in the water. Simulated runoff testing showed no potential for contaminant mobilization within the bioswale but did indicate potential areas of contaminant buildup via observation of a dye tracer. Stepwise linear regressions performed in SPSS showed total rainfall as the most significant predictor of suspended solid, nitrate and total phosphorus load in the bioswales. Results also indicate that there are significant differences between the loads observed for the two bioswales monitored. / Civil Engineering