Measuring and modeling stormwater runoff from an interstate in a rural/forested watershed
Type of DegreePhD Dissertation
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In hydrological research, roadways are often defined simply as an increased impervious portion of a watershed or a flow blocking structure. The research presented in this dissertation examines the effects of highway crossings on stream water quality, groundwater-surface water interactions, and stormwater runoff. Management of stormwater runoff from highways has been identified as an important aspect of highway development or redevelopment in recent years. Research to date has shown that stormwater runoff, including that from roadways, may have constituents that cause adverse impacts to aquatic ecosystems. Such investigations have focused either on characterizing the runoff directly generated on roads or on the effectiveness of various pollutant removal techniques. Unlike these works, the research presented herein focuses on impacts created by stormwater runoff from roadways measured in small headwater streams. This dissertation presents the results of an investigation on the impact of stormwater runoff from Interstate 59 (I-59) measured at the Little Cahaba Creek (LCC), a small headwater stream in Trussville, AL. Water samples were collected at selected points and hydrological and water quality parameters were continuously monitored in selected stations. In addition to field measurements of streamflow, groundwater levels, and water quality parameters within the LCC, two hydrologic models (SWMM and GSSHA) were developed to explore the relationship between the interstate features and the watershed’s response to rain events, including groundwater fluctuations. After model calibration, the relationship between observed streamflow and groundwater table fluctuations was determined: bank storage was a dominant process in the near-stream subsurface. Comparisons of modeled data to measured streamflow and groundwater table elevation data are made for multiple major storm events. Both programs performed well in calculating the groundwater table elevation change due to stream bank storage. Water quality upstream and downstream of the roadway was very good, with low concentrations of nutrients, suspended sediment, and other parameters and high biodiversity as measured in this study. Of particular importance, these parameters generally do not increase as the stream receives stormwater runoff from I-59. Additionally, the complex response of streamflow to rain events across the roadway could not be well defined through field measurement alone. Modeled data indicates that a substantial amount of groundwater table elevation change could be represented as bank storage at the downstream site, but the median drainage area captures and retains more runoff than could be modeled in this work. Since groundwater-surface water interactions are often neglected in hydrological modeling, an accurate understanding of the location of the groundwater table is unlikely when this is not considered. With this in mind, the ability to properly design and install infiltration based stormwater volume reduction approaches will be severely reduced and unreliable. Future research should include the consideration of road-crossing hydrology. In particular, grass-lined medians are common in roadway development, yet it is unclear how much stormwater runoff could be captured in the subsurface of these drainage systems.