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This page provides a list of considerations when using models to meet MS4 (Municipal Separate Storm Sewer System) total maximum daily load permit requirements. For detailed guidance or additional information, see the links section for each model.

MIDS calculator quick guide

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schematic showing routing of BMPs
MIDS calculator screenshot
image of Minimal Impact Design Standards logo

The Minimal Impact Design Standards calculator is a moderately simple tool for estimating pollutant load reductions associated with best management practices (BMPs).

Applicability

  • The MIDS Calculator is capable of predicting annual runoff, total suspended solid (TSS) and total phosphorus (TP) loading from three land cover categories (impervious area, forest/open space, and managed turf), and annual runoff and pollutant removal for a variety of BMP types.
  • The MIDS Calculator reports on an annualized-basis, and is therefore not suitable for evaluating WLA compliance for non-annualized WLAs.
  • The MIDS Calculator is an empirical model which is limited in its ability to model pollutant removal through BMPs in series (e.g. treatment train) and bypass from undersized BMPs. For this reason, the MIDS Calculator should only be used to demonstrate wasteload allocation WLA compliance for simplified study area where the impact of pollutant removal through BMPs in series and bypass from undersized, non-infiltration BMPs is considered minimal.

Model inputs

The MIDS calculator has a Site informationtab for site information and a Schematics'' tab for inputting information for individual BMPs.

Site information tab

  • Ensure you have the correct zip code for the site
  • Choose the most appropriate event mean concentration (emc) for your site or input a site-specific emc if data is available. If you have widely differing emcs across your site, run the model separately for areas with different emcs and compile results from different calculator simulations.
  • Select "No" for the question regarding compliance with the Construction Stormwater General Permit.
  • Ensure all pervious and impervious acreages are accounted for and pervious acres are assigned to the correct soil type (see Design infiltration rates.

Schematic tab

  • Ensure that BMPs are properly sized
  • Ensure that BMPs are properly routed
  • Ensure that all acreages are accounted for - check the site summary on the left side of the calculator

Results tab

The Results tab displays summary information for the site and for each BMP.

  • Examine the BMP Summary table. Ensure that BMPs are not over- or under-sized and that inflow and outflow, including pollutant mass, makes sense.
  • Examine the Summary Information. This will be the pounds of TSS or TP reduced for this site. If you have multiple sites, compile the results across all sites.

The following table summarizes a QAQC list for interpreting results.

General MIDS Calculator result QAQC list
Link to this table

QAQC / Review Item Related MIDS Calculator Output
Review device routing Review routing in "Schematic" window
Confirm area routed to individual BMPs is correct BMP Properties --> Watershed
Confirm total watershed area is correct Review "Site Information" window
Confirm annual precipitation total is correct Review zip code in "Site Information" window
Review watershed pollutant concentrations Compare watershed areal loading to literature values
Review TSS/TP removal rates Compare pollutant removal values to literature values
Confirm no routing between non-volume reduction BMPs Removal estimates for non-volume reduction BMPs in series are incorrect. For this reason, routing between non-volume reduction BMPs should be disconnected
Review BMP sizing recommendations and review contributing watershed area Bypass from undersized BMPs is not calculated for non-volume reduction BMPs. For this reason, only the area for which the BMP is correctly-sized should be modeled to the BMP (the remainder should be modeled as untreated)


Links


MPCA Simple Estimator quick guide

The MPCA Simple Estimator is a spreadsheet-based tool used to predict watershed-based loading for total phosphorus (TP) and total suspended solids (TSS), and estimate pollutant removal at water quality best management practices (BMPs).