Using the Minnesota Stormwater Manual and MIDS Calculator to help meet industrial stormwater permit requirements

At permitted industrial stormwater facilities, failure to meet permit benchmark values requires implementation of management practices that reduce the concentration of pollutants in runoff. the Minnesota Stormwater Manual and Minimal Impacts Design Standards (MIDS) calculator can be used to help select, design, construct, and maintain appropriate practices and quantify pollutant reductions associated with those practices.

Using the Minnesota Stormwater Manual

The Stormwater Manual contains considerable additional information useful for selecting, designing, constructing and maintaining stormwater BMPs. The following web pages may be useful in managing stormwater at your site.

• Process for selecting Best Management Practices: This page provides a general overview of BMP options and how to select the most appropriate BMPs.
• Using the treatment train approach to BMP selection: this page provides information on combining multiple treatment practices.
• BMP design: This page contains links to the design sections for several BMPs. For example, clicking on the link for Design criteria for permeable pavement takes you to a page that describes how to properly design a permeable pavement system.
• BMP construction: This page contains links to the guidance on how to construct several BMPs. For example, clicking on the link Construction specifications for permeable pavement takes you to a page that describes how to properly construct a permeable pavement system.
• BMP operation and maintenance: This page contains links to the operation and maintenance sections for several BMPs. For example, clicking on the link for Operation and maintenance of permeable pavement takes you to a page that describes how to properly operate and maintain a permeable pavement system.

Using the MIDS calculator

The MIDS calculator estimates reductions in stormwater volume, total suspended solids, and phosphorus associated with implementation of stormwater Best Management Practices. The screen shot of the calculator below shows a site where several BMPs are used together to achieve a reduction in runoff and pollutant loads. The user must know the soil type, the facility’s zip code, and the acres of impervious and pervious surfaces being treated. A user-friendly interface allows for easy BMP selection. Inputs for each BMP vary, but the calculator can be used to properly determine the size of the BMP(s). The calculator displays results in terms of pounds of pollutant removed or percentage of pollutant removed.

Although the calculator is fairly intuitive and easy to use, the Minnesota Stormwater Manual contains detailed descriptions for the calculator, including several sample exercises. Check out these links to help you use the manual:

• Links to Manual pages that address the MIDS calculator
• MIDS calculator sample exercises
• Download the MIDS calculator (click on the zip file that starts with: "MIDSCalculator")

Example applications of the MIDS calculator and Minnesota Stormwater Manual

Below are two relatively simple example applications for the MIDS calculator.

MIDS calculator example 1

A 10 acre facility contains 5 acres of turf and 5 acres of impervious surfaces or compacted soils. Results from several rounds of sampling show that concentrations of total suspended solids (TSS) are 200 mg/L, which exceeds the benchmark of 100 mg/L. Soils at my facility consist mostly of clay (Soil Group D). What BMP(s) can I implement to meet the benchmark concentration (i.e. reduce my load by 50 percent or more)?

Because my soils do not infiltrate well, I need to select BMPs that filter or settle the runoff. Examples include the following:

   Filtering BMPs: Sand filter, swale, bioretention (rain garden) with an underdrain.
   Settling BMPs: Constructed pond or wetland

I decide to use a bioretention system (rain garden). I can use the MIDS calculator to determine the size of my rain garden necessary to meet the reduction requirement of 50 percent. If I route all the runoff on site to a rain garden that is 1,000 feet wide, 1,000 long, and 1.5 feet deep, I will reduce my TSS load by 61 percent, which meets the reduction required to meet the benchmark concentration.

Within the MIDS Calculator, you’ll note a “Help” button in the MIDS calculator. Clicking on that button will take users to the Minnesota Stormwater Manual page that describes the specific BMP, in this case the bioretention basin with underdrain. The manual page includes a description of the BMP in the calculator, examples from the calculator, and links to information on how to design, construct and maintain a bioretention basin.