- Tree trench/tree box with a raised underdrain was not receiving annual volume credits, resulting in no pollutant removal for the portion of captured water that infiltrates below the underdrain.
- Multiple stormwater disconnection BMPs, beyond the first disconnection BMP, were not receiving credit for volume and pollutants.
- Volume stored beneath an underdrain for permeable pavement was not being calculated correctly when multiple permeable pavement BMPs were used (i.e. the first permeable pavement calculation was correct but not for additional permeable pavement BMPs)
We corrected the formulas and these three issues should be resolved. However, when you create a new file within Version 3 or use an existing Version 3 file, these errors will not be resolved. To access a file with these issues resolved you must download the following file and continue to use this file or saved versions of this file to avoid the previous errors.
If you are interested in seeing an example with the first two items corrected, you can access the following file. File:Corrected April 10 example.xls
It has also been noted the TSS removal for bioretention with an underdrain is 60%, while the manual provides a removal rate of 85%. The 60% value was derived from the Virginia calculator and stormwater manual. We are conducting a literature review to determine the appropriate value. For now, we are recommending that people use the numbers from the Minnesota stormwater manual (85%) instead of the calculator.Download Version 3
View changes made from the previous version.
- Volume and pollutant reductions given in the MIDS calculator are based on the assumption that the BMP is properly designed, constructed and maintained. Guidance and recommendations for design, construction and maintenance are contained within the individual BMP sections of this Manual. You can find lists of relevant web pages at the following links.
- Design criteria (NOTE the page Design criteria for filtration includes design criteria for swales.)
- Construction specifications
- Operation and maintenance
- BMPs must be designed and constructed in accordance with the Construction Stormwater General Permit (CSW permit). Requirements of the CSW permit are highlighted in red throughout the various sections of this Manual. It is important to note that volume reduction BMPs need at least three feet of separation to the seasonally saturated soils to be eligible under the CSW permit to receive credit for volume reduction. The pollutant removal credit is not valid for undersized or oversized practices.
- The MIDS calculator does not require sizing criteria for BMPs that do not achieve volume reductions (ponds, wetlands, sand filters). The pollutant removal credit in the calculator assumes these BMPs are properly sized per recommendations in this Manual.
We welcome comments and suggestions on the calculator. Please send comments to Anne Gelbmann. We will compile comments and, if resources allow, incorporate suggestions into future versions of the calculator.