Measuring and modeling the impact of roadway runoff on a headwater tributary of the Cahaba River

Abstract

Stormwater runoff from highways has been a relevant focus of study both in terms of its characterization during construction phases as well as during the operational years. Highways have been thought to have adverse impacts on the water quality of nearby water bodies in terms of parameters such as solids, turbidity and metals, among others. This thesis presents results of a 24-month long monitoring of the Little Cahaba Creek (LCC), a perennial headwater tributary of the Cahaba River, located north of Trussville, AL. In this study, levels of nitrate, total nitrogen, total phosphorous, turbidity, and total suspended solids (TSS) were monitored and recorded upstream and downstream of Interstate-59 (I-59) on a biweekly basis. In addition to the biweekly samples taken at each site, two Water Quality Sondes were deployed at the upstream and downstream sites. The turbidity readings from these Sondes were converted to TSS using a turbidity-TSS relationship derived from samples collected in the stream in various flow conditions. The stream flow was continuously measured by two Area-Velocity Sensors each deployed at sites upstream and downstream from the crossing with I-59. The stream flow data, along with the rain gauge and TSS measurements, were entered into PC-Stormwater Management Model (PCSWMM), a decision support system and processing tool for EPA’s Stormwater Management Model (SWMM5). The LCC SWMM5 model was calibrated for various hydrological characteristics from 6/12/2014 to 12/31/2014. The model calibration detected the sensitive parameters: subcatchment flow length width, % impervious, Horton’s maximum and minimum infiltration rates, and channel roughness. Once calibration was completed, the validation period, 1/1/2015 to 3/26/2015, showed a satisfactory relationship for the upstream site but not for the downstream site. Limited available rain gauge data (due to equipment failure-may have restricted more adequate calibration and validation results). Through continuous recording and analyzing the levels of these nutrients and water quality indicators in the LCC, this study hopes to provide a better understanding of the impact of highway runoff on receiving water bodies in the context of post-construction stormwater management of highways, as well as expand the SWMM knowledge base to include more detailed studies on the impacts of roadways on small stream waters or headwaters.