A PLANNING TOOL FOR ASSESSING SITE DEVELOPMENT IMPACTS AND IMPLEMENTING SITE PERFORMANCE STANDARDS

Local governments are faced with the increasing challenge of managing stormwater runoff. Federal, state, and local regulations designed to protect water quality in urban areas cannot be met without addressing nonpoint source runoff. New development is often the source of increases in nutrients, sedimentation, and stream channel erosion, 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 at their disposal for assessing the impacts of development on water quality and evaluating whether a new development site plan meets site performance standards (numeric targets for pollutant loading and/or hydrologic control).The Site Evaluation Tool (SET) was developed for Mecklenburg County to assess how new development impacts 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 development against standards and targets for water quality protection.The SET is a Microsoft Excel spreadsheet designed for a relatively quick assessment of site-scale annual pollutant loading and storm event runoff. Its modeling approach is based on the Simple Method for estimating annual runoff, and relates runoff to annual pollutant loads using event mean concentrations. Loads from infiltrated shallow groundwater and from septic systems are also evaluated. Storm event runoff is calculated using the NRCS TR-55 Curve Number approach. The SET is easy to use because there are relatively few model inputs required (chiefly a division of pervious and impervious land uses by site drainage areas). Pollutant loads and storm event runoff are calculated for the site with existing land use, with the proposed site plan, and the proposed site plan with selected BMPs. In addition, pollutant load and storm runoff targets can be easily incorporated in the SET based on a local government's needs. The approach employed by the SET allows users to easily test various BMP configurations to determine the best option for achieving pollutant load targets. Two case studies (a residential site and an institutional site) illustrate how conventional stormwater treatment versus Low Impact Design techniques can be compared and evaluated by the SET.The SET provides a very practical means for implementing new development site performance standards. Since the model calculates pre- and post-development loads and hydrologic measures, it is easy to compare results to a diverse set of targets. Examples include fixed nutrient targets, a comparison of pre- and post-development storm event runoff, and a target of an 85 percent reduction in post-development sediment loading. Another beneficial feature of the SET is that it can be distributed to the development community, allowing developers to test various BMP configurations and more easily produce site plans that meet performance standards.