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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

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

screen shot of simple estimator
Screen shot of the Simple Estimator

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).

Applicability

  • The Simple Estimator is capable of predicting annual total suspended solid (TSS) and total phosphorus (TP) loading from several land use categories (e.g. commercial, industrial, residential, etc.), several BMP types (e.g. biofiltration, infiltration, constructed ponds, etc.), and up to 10 sub-watersheds.
  • The Simple Estimator is easy to use but has limitations, including the following.
    • The MPCA Simple Estimator reports on an annualized-basis, and is therefore not suitable for evaluating wasteload allocation WLA compliance for non-annualized WLAs.
    • The MPCA Simple Estimator is an empirical model which has limited ability to model pollutant removal through BMPs in series and bypass from undersized BMPs. For this reason, the MPCA Simple Estimator should only be used to demonstrate WLA compliance for simplified study areas where the impact of pollutant removal through BMPs in series (i.e. treatment train) and bypass from undersized BMPs in considered minimal.
    • The MPCA Simple Estimator is not capable of identifying and/or distinguishing unique removal rates for individual BMPs that may be undersized or would not otherwise meet BMP design criteria.

Model inputs

screen shot of summary sheet
Screen shot of the summary tab from the Estimator. The sheet shows total initial and adjusted loads, pounds and percent load reduced, and final loading rates. Click on image and then click on image again to enlarge.
  • Identify the sub-watersheds you will model. Consider modeling undersized BMPs or BMPs in series as separate sub-watersheds.
  • Within each sub-watershed, determine land uses and acreages within each land use.
  • Within each land use, determine acreages treated by each BMP type.
  • Evaluate the default values for event mean concentration and runoff coefficient for each land use and adjust if necessary.

Model output

The spreadsheet consists of 10 worksheets, one for each sub-watershed. Data from these worksheets is compiled on an eleventh worksheet called "Summary sheet". This summary sheet shows results for each sub-watershed and cumulative for the project/study.

Quality assurance/quality control procedures are summarized in the following table.

General MPCA Simple Estimator result QAQC list
Link to this table

QAQC / Review item Related MIDS Calculator Output
Confirm total watershed area is correct Review land use-based area assigned in the "total load" table
Confirm annual precipitation totals are correct Review annual rainfall values in column G and confirm all are the same value
Review watershed pollutant concentrations Compare watershed areal loading values in the "total load" table to literature values in Table 9
Review watershed routing to BMPs Review land use-based area assigned to each BMP type
Confirm only the directly-contributing watershed area is applied for each BMP The area assigned to a given BMP should be only the directly-contributing watershed area and should NOT include area tributary to other BMPs upstream
Review TSS/TP removal rates Compare pollutant removal values to literature values. Note: percent load reduction shown for each BMP type is calculated based on the total watershed loading
Review BMP sizing recommendations and review contributing watershed area Bypass from undersized BMPs is not calculated. 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

P8 quick guide

P8 icon

P8 is a continuous water quality model capable of summarizing runoff and associated total suspended solids (TSS) and total phosphorus (TP) generation, removal, and outflow loading from individual catchments, individual BMPs and pipes (referred to as “devices”), or as a model wide summary.

Applicability

P8 is capable of accurately predicting pollutant removal through BMPs in series as well as predicting runoff and pollutant bypass from undersized BMPs. Due to this flexibility, P8 is capable of providing accurate pollutant removal estimates regardless of BMP network and subwatershed configuration, and is capable of demonstrating compliance to mass-based wasteload allocations (WLAs) (e.g. pounds of TSS per year), concentration-based WLAs (e.g., mg/L of TSS), and areal-loading based WLAs (e.g., pounds of TSS per acre per year) for both TSS and TP.

WinSLAMM quick guide