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)
Long-term continuous simulation XP-SWMM models, developed for three regions of Minnesota (Twin Cities, Southeast and North-Central), estimated the runoff from a 10-acre site with native soils (Hydrologic Soil Group A, B, C, and D) and native vegetation (100 percent deciduous woods and 100 percent meadow). Long-term continuous XP-SWMM models of the same three Minnesota regions also simulated the effectiveness of implementing different volume control performance goals on development scenarios of varying imperviousness and soil type. Between twenty-six and thirty-five years of measured precipitation data with a time increment of 15 minutes was used in the modeling effort. Precipitation in the form of rain and snow on frozen and unfrozen ground conditions was modeled to determine the effectiveness of common volume control performance goals on annual runoff (Barr Engineering Co., 2010).
Performance goals were assessed based on estimated total phosphorus (TP) and total suspended solids (TSS) removal efficiency on an average annual basis. The portion of average annual runoff volume captured onsite varies depending on the performance goal and resulting BMP volume. While strongly correlated with the amount of runoff captured and infiltrated, the overall pollutant removal efficiency is also dependent on other factors such as the varying concentration of pollutants in runoff (such as the “first flush effect”) and pollutant removal that occurs through sedimentation or other mechanisms. Six of the Twin Cities region development scenarios were modeled using P8 modeling software to evaluate the overall average annual phosphorus and total suspended solids removal efficiencies expected from the four performance goals (Barr Engineering Co., 2010). Those four performance goals included retainage of a runoff volume
Table 3-1 of the 2011 Barr Engineering report summarizes the various development scenarios and Minnesota regions modeled with XP-SWMM and P8.
The following conclusions are based on the XP-SWMM and P8 modeling results:
The Work Group discussed the modeling results and considered the level of simplicity, incentive to reduce impervious surfaces, and accounting for different regions of the state for each modeling approach. After a vote, the Work Group selected an approach that would retain a runoff volume equal to an amount times the proposed impervious surfaces. The Work Group decided that one value would be best rather than varying the value by soil type, impervious surface percentage, and location within the state. They selected 1.1 inches as the statewide average.
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.