WATERSHED QUANTITY AND QUALITY APPROACHES FOR A BIG CREEK

The Big Creek watershed is located in the upper Chattahoochee River Basin, north of Atlanta, Georgia. Jurisdictions within the 100-square mile watershed include Fulton, Forsyth and Cherokee Counties, and the Cities of Cumming, Roswell and Alpharetta. Rapid growth is occurring in this area, particularly in Forsyth County, one of the top ten fastest-growing counties in the United States.Concerns about water quality led to two concurrent water quality modeling studies of the watershed. A study for the Atlanta Regional Commission (ARC) was initiated to assess growth impacts and benefits of alternative management strategies, with particular interest in water quality near the watershed outlet, where the City of Roswell drinking water intake is located. The Fulton County Big Creek Water Reclamation Facility (WRF) Water Resources ManagementPlan (WRMP) evaluated flooding, erosion, and water quality within the Fulton County boundaries (Roswell, Alpharetta, and unincorporated Fulton County), as part of a larger project that also included chemical and biological monitoring and hydrologic/hydraulic modeling to assess stream aquatic integrity and problems such as excessive instream velocities and flooding.The two water quality studies were conducted using different, yet coordinated, models. The ARC study used the U.S. Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) for water quality and water quantity. In contrast, the Fulton County WRMP used the EPA SWMM for water quantity and the EPA BASINS (Better Assessment Science Integrating Point and Nonpoint Sources) graphical user interface (GUI) with the Hydrologic Simulation Program – FORTRAN (HSPF). Both models were calibrated to United States Geological Survey (USGS) streamflow monitoring data (hydrology/hydraulics) and local National Pollution Discharge Elimination System (NPDES) stormwater runoff monitoring data (water quality). The calibrated models were then applied to assess water quality under existing conditions, future conditions (no controls), and future conditions with alternative management strategies. Modeled parameters included the 12 standard NPDES parameters, plus fecal coliform bacteria. Three stream segments in the Big Creek watershed, and two more in nearby watersheds, are on the State of Georgia 303(d) list for fecal coliform bacteria. Therefore, parameters of greatest concern in the study area include bacteria, along with sediment from hydrologic changes and erosion in the study area, and total phosphorus discharges to the Chattahoochee River, which may contribute to eutrophication impacts in a downstream reservoir.This paper compares and contrasts the application of the two public domain water quality models (SWMM and BASINS) to the same study area. Points of comparison include the ease of model calibration, model capabilities and limitations, and model results (e.g., annual loads, and concentration-frequency relationships). The annual load results will be compared to those generated by the Watershed Management Model (WMM), which is a public domain Windowsbased evaluation tool designed to calculate annual or seasonal loads based on annual or seasonal flows and concentrations from various sources (e.g., runoff, baseflow, point sources). The authors will discuss the differences between the less complex model WMM and the continuous simulation models SWMM and BASINS, and draw conclusions regarding the benefits of both approaches.