Date of Award
Doctor of Philosophy
Zhou, Jianpeng (Jim)
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.
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