Creative Watershed Performance Requirements for New Development

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.