A performance goal specifies what level of stormwater treatment must be achieved. The MIDS performance goals were developed to satisfy the legislation by determing how much precipitation must be retained on a particular site.
An important objective of the Work Group was to determine a performance goal for new development sites that do not have restrictions. Restrictions refer to site factors that prevent attainment of the performance goal. Examples of site restrictions include shallow depth to bedrock, presence of contaminated soils, and lack of access due to presence of utilities.
In deciding a performance goal for new development without restrictions, the Work Group began by identifying and discussing the pros and cons of three common approaches to consider as the MIDS performance goal approach. Those approaches involved retainage of a runoff volume
To help the Work Group select a performance goal that would mimic a site's natural hydrology, the approaches were assessed in comparison with runoff rate and volume from native soil and vegetation conditions. The goal of the assessment was the answer the question, "How well do the performance goal alternatives mimic natural hydrology?" Details of the methods, results, and conclusions of the analyses are included in a report, Assessment of MIDS Performance Goal Alternatives: Runoff Volumes, Runoff Rates, and Pollutant Removal Efficiencies (Barr Engineering Co., 2011: ' In April 2011, MIDS work group members agreed on a performance goal for new development: For new, nonlinear developments that create more than one acre of new impervious surface on sites without restrictions, stormwater runoff volumes will be controlled and the post-construction runoff volume shall be retained on site for 1.1 inches of runoff from impervious surfaces statewide.
In June, 2013, the MIDS work group members agreed on a performance goal for redevelopment and linear. Nonlinear redevelopment projects on site without restrictions that create one or more acres of new and/or fully reconstructed impervious surfaces shall capture and retain on site 1.1 inches of runoff from the new and/or fully reconstructed impervious surfaces. Linear projects on sites without restrictions that create one acre or greater of new and/or fully reconstructed impervious surfaces, shall capture and retain the larger of the following:
Mill and overlay and other resurfacing activities are not considered fully reconstructed. All projects shall first attempt to meet the volume reduction Performance Goal on site. However, if an applicant is unable to achieve the full Performance Goal due to site restrictions as attested by the local authority and documented by the applicant, the Flexible Treatment Options Approach shall be followed.