Information: This page will be periodically updated as we get questions about the Calculator. See the latest revision date at the bottom of this page.

Many people have expressed interest in knowing what to look for when reviewing MIDS calculator files, including submissions from projects where the calculator was used. This page provides a summary of some things to look for. The page will continue to evolve since we get questions about the calculator and will incorporate those communications into this page. The page includes information for entire sites and individual BMPs.

General considerations

  • Ensure that acreages for the site are accounted for
  • Except for the stormwater disconnection BMP, the area of a BMP should be included in the impervious acres since rain falls directly on the BMP and is therefore part of the performance goal for that BMP.
  • For the disconnection BMP, the BMP is the pervious area receiving runoff from an impervious area and therefore must be included in the pervious area for the BMP and the site.

Permeable pavement

  • The calculator limits the ratio of impervious acres to permeable pavement acres to 5:1, but the manual recommends a 2:1 ratio. Since this is a very effective infiltration BMP, it can be used to meet a performance goal by routing large impermeable acreages to the BMP. BMP ratios greater than 2:1 or 3:1 should be viewed with caution.
  • Permeable pavement is often used in combination with other BMPs and can therefore easily result in double counting. Water infiltrated through permeable pavement is considered to infiltrate into underlying soil. impermeable acres for this type of practice cannot be applied to other BMPs. For example, impervious acres routed to permeable pavement that infiltrates into a tree trench system should be applied to the tree trench system and not the permeable pavement.
  • If the permeable pavement BMP is not meeting or barely meeting the performance goal, see if the subsoil is compacted. If the answer to this question is "No", determine if this is correct, since many if not most permeable pavement practices will require compaction.

Bioretention with underdrain

  • Ensure the answers to a raised underdrain and impermeable liner are consistent with the intent of the practice
  • Compare the values for field capacity, wilting point, and porosity to tabled values for the soil(s) at the site


Questions and issues that we've encountered

This page was last edited on 12 May 2021, at 12:25.