CREATIVE WATERSHED PERFORMANCE REQUIREMENTS FOR NEW DEVELOPMENT Dale Jones', James H. Scarbrough', David Chastant 2 , Ken C. Hall' and Farhan Shaikh' AUTHORS: 'Environmental Scientist, CH2M HILL, 115 Perimeter Center Place, Suite 700, Atlanta, GA 30346-1278; and 2 Gwinnett County Department of Public Utilities, 684 Winder Hwy., Lawrenceville, GA 30045. REFERENCE: Proceedings of the 2001 Georgia Water Resources Conference, held March 26-27, 2001, at the University of Georgia. Kathryn J. Hatcher, editor, Institute of Ecology, the University of Georgia, Athens, Georgia. Abstract. Gwinnett County's Watershed Protection Plan represents the culmination of a two-year watershed assessment and modeling project. One key component of the watershed protection plan is the development and implementation of requirements for new development in the watershed. In areas where water quality criteria provide little practical guidance for developing watershed protection plans, statistical relationships between biotic integrity (benthos, fish, and habitat scores) and pollutant loadings for key parameters in kg/ha/yr (lb/ac/yr) were used to develop watershed improvement guidelines. An automated spreadsheet analysis tool (WISE) was used to facilitate this analysis and allow interactive evaluation with the County and citizens' group. Performance based strategies were used to provide needed protection as well as maximum flexibility for the development community. A site specific improvement guideline for TSS was developed and a spreadsheet tool that assists with new development layout to meet the target was developed. Options are provided for implementing BMPs on the site and designating the tributary drainage area to each BMP. The form automatically graphs and compares the uncontrolled and controlled loading rates to the performance criterion. This tool can be used iteratively in the site design process. In addition to the new development tool which focuses mainly on water quality controls, Gwinnett's regulations were also revised to control water quantity from new developments including four key hydrologic design events, 1) major flooding (100 year events), 2) out of bank flooding (10 to 25-year events), 3) channel protection (1-year events) and 4) water quality protection (1.2 inch rainfall). These design events are managed to protect the environment and the public. In combination with the BMP form, these strategies provide needed protection to streams as well as maximum flexibility for the development community. INTRODUCTION Gwinnett County, like many suburban areas near Atlanta, is experiencing rapid population growth and corresponding changes in land use. These changes dramatically affect the character of many of the County's watersheds. The State of Georgia recently began requiring watershed assessments, and corresponding watershed protection plans, to help mitigate the secondary impacts of development on the health of its lakes and streams. In early 1998, the Gwinnett County Department of Public Utilities (DPU) began a comprehensive watershed assessment project covering all 437 square miles of the county. The project's major components are watershed characterization (including habitat, benthos, fish, and water quality monitoring), watershed modeling (using BASINS), watershed protection, and public involvement. Currently all tasks in the project are complete. METHODOLOGY The project included an integrated assessment of the habitat, the biological community, water quality, and pollutant loadings in the streams; the assessment was used to evaluate impacts on the streams and their uses. The integrated assessment of these impacts is the key to developing an effective, efficient watershed protection and improvement strategy. Project Methodology The Gwinnett County project built on earlier work in the region (such as Hall and Richards, 1998). In particular, it refined the modeling approaches (using BASINS) and the tools for evaluating relationships among habitat, pollutant loading rates, and biotic integrity. The integrated assessment was used to determine the impacts on the streams and their uses (CH2M HILL, 2000a). The assessment confirmed results from other studies that uncontrolled imperviousness in watersheds significantly degrades 296