A SITE EVALUATION TOOL FOR ASSESSING WATER QUALITY IMPACTS FROM CONVENTIONAL AND LOW IMPACT DEVELOPMENT

Local governments are faced with the increasing challenge of managing stormwater runoff and meeting water quality regulations. Development increases nutrient export, sedimentation, and stream channel erosion, often resulting in violations of water quality standards and other negative water quality impacts. Since development is regulated at the local level, local governments can benefit from having a tool that can assess the impacts of development on water quality, evaluate whether a development site plan meets site performance standards (numeric targets for pollutant loading and/or hydrologic control), and perform a cost-benefit analysis of selected management measures to help evaluate the most sustainable practices.The Site Evaluation Tool (SET) was developed for the assessment of development impacts to water quality at the site level. The SET requires relatively little time and resources to operate, is founded upon sound scientific principles and models, and is capable of evaluating the impact of development on downstream water quality and the influence of Best Management Practices (BMPs) on hydrology and pollutant loads. It can also easily be configured to evaluate the performance of new and existing development against standards and targets for water quality protection. The SET is particularly useful for assessing various LID techniques for stormwater management.The SET is a Microsoft Excel spreadsheet with Visual Basic automation designed for a relatively quick assessment of site-scale annual pollutant loading, storm event peak flow and runoff volume, and costs associated with stormwater management. Three scenarios are simultaneously tested – pre-developed conditions, post-developed conditions with no BMPs, and post-developed conditions with BMPs. It uses an enhanced version of the Simple Method for relating annual runoff to annual pollutant loads, TR-55 for storm event peak flow and runoff volume, an NRCS method for site-scale storm event hydrographs, a simplified approach for estimating the effects of selected BMPs on the hydrograph, and BMP/stormwater management cost equations based largely on research conducted in North Carolina.The SET is easily tailored to model output for water quality regulations and performance standards specific to a jurisdiction, and has been enhanced to meet the goals of several organizations, including the City of Huntersville/Mecklenburg County (NC), the Upper Neuse River Basin Association (NC), Orange Water and Sewer Authority (Orange County, NC), Central Arkansas Water (Little Rock, AR), the Spa Creek Conservancy (Annapolis, MD), and the Clinton River Basin Intercounty Drainage Board (Oakland and Macomb Counties, MI). A case study is presented showing how LID techniques can be assessed within the SET for meeting stormwater management goals, and how these techniques compare to conventional stormwater treatment.By focusing on core goals – a scoping-level tool that is easy to modify and tailor to meet the needs of an organization, based on sound modeling and principles – the SET has continued to evolve as a useful means of estimating site-scale impacts of development, assessing a site's design against performance standards, and evaluating impacts within the larger context of the receiving watershed. Such tools are critical to aiding local municipalities, the development community, and others in planning and implementing sustainable practices.