Summary of changes in Version 4 Minimal Impact Design Standards (MIDS) Calculator

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.

Constructed ponds with an iron bench

Pollutant removal percent for different pond conditions and design levels
Design level	Particulate phosphorus		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

Screen shot of the constructed pond BMPs parameter tab in the MIDS Calculator.

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:

"Have amendments to attenuate phosphorus been incorporated?"

Although the question does not specify iron as the amendment, iron is the most likely amendment, based on current knowledge. Answering "YES" to the question about an amendment results in dissolved phosphorus reduction of 40%. Removal of particulate phosphorus and total suspended solids are unchanged. Design level requirements are the same with or without an amendment. It is assumed that the amendment is incorporated following recommended design, construction, and maintenance guidance in this manual.

For information on using the constructed pond BMP in the Calculator, go to this link.

This change was made in the Calculator because there are numerous ponds constructed with iron benches. Performance results have been mixed, but data suggests that properly designed, constructed, and maintained iron benches retain dissolved phosphorus.

Green roofs - intensive roofs

Screen shot of the Green roof parameter tab showing an 8 inch media depth and a warning stating the User has created an intensive green roof.

Users can now incorporate intensive green roofs into a green roof design. This change was created by allowing the user to select a media depth greater than 4 inches. When the user specifies a depth greater than 4 inches the following warning appears:

"you have specified an intensive green roof design. See the green roof design section in the Stormwater Manual."

Other inputs and restrictions on green roof design are unchanged. For information on using green roof BMPs in the Calculator, go to this link.

To view the green roof section in this manual, link here.

This change was incorporated because there are intensive green roofs and increased media depth has a significant effect on the volume of water that can be retained.

Updated pollutant removal values

Pollutant removal vales were updated for some BMPs. These changes were based on published research and to improve consistency between the Calculator and values posted in this Manual. An important source of information used in evaluating these data was the International BMP Database (IBMPD).

• Reduce the wet swale removal rate for TSS from 68% to 35%. Published research, including the IBMPD, show removal rates between 20 and 40 percent. Higher values are reported in the literature, but often these values are associated with swale systems that include pretreatment or other specific design features.
• Adjust the particulate phosphorus (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%). Version 3 included PP removal values greater than TSS removal, which is not physically possible unless the BMP was selectively removing sediment with a greater PP concentration. Removal values in Version 3 were 82, 73, and 62 percent for permeable pavement, swale, and Level 1 pond, respectively.
• 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%. This was an error in Version 3 and has been changed to align particulate and dissolved phosphorus removal in BMPs amended to remove phosphorus.

Bioretention media mix options

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).

This change was made because a variety of media mixes are being used in stormwater applications, including site-specific mixes. Phosphorus removal and phosphorus leaching from media is a function of the phosphorus concentration and form of phosphorus in the media. Research suggests that media with a phosphorus concentration greater than about 30 mg/kg will leach phosphorus. This is an area of active research. For more information on engineered media and phosphorus, link here. For more information on compost, which is typically the source of phosphorus leached from engineered media, link here.

Tree interception

The value for tree interception was updated. When tree interception was initially incorporated into the Minnesota Stormwater Manual and the MIDS Calculator, research was limited. Research conducted over the past 5-10 years indicated the initial values for tree interception were low. MPCA executed a work order to conduct a literature review to determine if the tree interception value in the Calculator should be modified. The Contractor submitted a technical memo to the MPCA, which included the following recommendations.

• Setting the annual rainfall interception performance of deciduous and coniferous trees at 30% and 57%, respectively. These values are based on the average of the performances documented in all reviewed studies for trees and climates that are applicable to Minnesota.
• Setting the rainfall interception depth capacity of deciduous and coniferous trees at 0.14 inches and 0.40 inches, respectively. This interception depth translates into an annual rainfall interception performance of 30% and 57% respectively, thereby aligning both tree metrics.

These changes were incorporated into Version 4 of the Calculator.

The technical memo reference above File:TreePerformanceMemo.docx can be found here.

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

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. 10. 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. 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. 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.