Version 3 of the Minimal Impact Design Standards (MIDS) Calculator (Calculator) was updated to Version 4 in Fall, 2020. This page provides a summary of updates from Version 3 to Version 4 of the Calculator.

rowspan=2 Design level !! colspan=2 Particulate phosphorus !! colspan=2 Dissolved phosphorus !! TSS
No amendment With amendment No amendment With amendment No amendment With amendment
1 60 60 0 40 60 60
2 84 84 8 40 84 84
3 90 90 23 40 90 90

Constructed ponds

Users can now specify a constructed stormwater pond with an iron-enhanced filtration bench to allow dissolved phosphorus removal. To achieve this, the Calculator incorporates the same question and same dissolved phosphorus removal percent as for sand filters in Version 3.


5. Allow intensive green roofs (i.e. greater than 4 inches of media depth), but add the following warning when the media depth exceeds 4 inches – you have specified an intensive green roof design. See the green roof design section in the Stormwater Manual and create a link to https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_green_roofs 6. Incorporate swale side slope into swale main channel BMPs (swale main channel and swale main channel with an underdrain), to be named swale with no underdrain and swale with underdrain. In the new BMP interface: a. Questions and calculations are described in Appendix A b. For each side slope, utilize the same inputs and calculations as the Version 3 side slope BMP c. Modify the Results tab to reflect results for the side slope(s) and main channel 7. Reduce the wet swale removal rate for TSS to 35% 8. For bioretention and tree trench with underdrain, change the question Is a soil amendment used to attenuate phosphorus? to Do you have a properly designed phosphorus treatment layer or area in your system (e.g. iron enhanced layer)? and provide a link to https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_bioretention#Layered_systems 9. For bioretention basin with underdrain, tree trench with underdrain, and swale with underdrain, when a user includes an amendment for phosphorus retention, change the particulate phosphorus retention from 0% to 40% 10. For bioretention basin with underdrain and tree trench with underdrain, for the question Bioretention planting media mix, add Other. In addition to Mixes A and B, if the user selects Other for media mix, display the cell for the question Is the P content of the media less than 30 mg/kg? (i.e. this cell is greyed out when the user selects Mix C or Mix D). 11. For sand filter, create three treatment levels similar to those currently used for constructed stormwater ponds. MPCA will communicate the three levels to Barr after meeting with stakeholders. 12. For infiltration BMPs (BMPs without an underdrain), if the user answered yes to the question Are you using the calculator to determine compliance with a Construction Stormwater permit? on the Site Information tab, do not allow the user to select D soils. If the user selects a user-defined infiltration rate, ensure this value is >= 0.2 inches per hour. 13. Adjust the PP removal numbers down to match TSS removal for Permeable Pavement (74%), Swale Main Channel w/o underdrain (68%) and Level 1 Stormwater Pond (60%) 14. For non-volume reduction BMPs, request the user to manually input a “bypass percentage” (e.g., 28% bypass) within the BMP properties dialog box. 15. For non-volume reduction BMPs, track treated vs untreated pollutant load and assign a fractional adjustment of the percent reduction (PR%) applied to the treated load 16. For practices with an underdrain, if the practice is undersized, calculate water that bypasses the practice and provide no treatment for the bypass water 17. On the Results screen, under Summary Information, create a line in the P section for Total P removed by BMPs (add particulate and dissolved removed). Units for this are lbs. 18. On the Results screen, in the BMP Summary section, add a section called Total Phosphorus Summary, which contains information similar to the other P sections. 19. Update the value for tree interception. This value will be provided by the MPCA PM. 20. For tree trench system/Box, after the input asking for total media depth, add this input: Total media and soil depth available for tree rooting. When computing the number of trees allowed at a site, use this value instead of the media depth. 21. When the above changes are completed, allow MPCA to review the Calculator to ensure functionality. Incorporate any changes submitted by MPCA.