MIDS Calculator terminology

The following terms are used in the MIDS Calculator.

• Bottom surface area [[A_B]]: This is the surface area at the bottom of the engineered media. It represents the area where the engineered media changes to native soils. The user inputs this value in square feet.
• Depth below underdrain (D_U): This is the depth below the underdrain to the native soils. If no underdrain is present, this is the thickness of the engineered media.
• Infiltration rate of underlying soils: This is the infiltration rate corresponding with the Hydrologic Soil Group chosen by the user. This value is calculated and cannot be changed. however, if the user selects a User defined infiltration rate, this cell becomes blank.
• Media porosity: This is the ratio of pore space in the engineered media to the total volume of the engineered media. Units are volume/volume (e.g., cubic centimeters per cubic centimeter). If various types of media are used in the BMP, this value should be an average of the media installed between the underdrain and the native soils. Click here to see values for porosity based on soil type.
• Media surface area (A_M): This is the surface area at the bottom of the ponded water within the BMP. This is therefore the area at the surface of the engineered media. Units are in square feet.
• Overflow depth (D_O): This is the maximum depth of ponded water within the BMP (i.e., vertical distance from the overflow elevation to the top of the soil or media). Units are in feet. The maximum allowable depth is either 1.5 feet or the depth defined by the requirement to drain ponded water within 48 hours, whichever is less.
• Required drawdown time (hrs): This is the time in which the stormwater captured by and ponded within the BMP must drain into the underlying soil/media. The user must select from predefined values of 48 or 24 hours. The MPCA Construction Stormwater General Permit requires drawdown within 48 hours, but 24 hours is Highly Recommended when discharges are to a trout stream. The calculator uses the Infiltration rate of underlying soils and the Overflow depth (D_O) to check if the BMP is meeting the drawdown time requirement. The user will encounter an error and be required to enter a new Overflow depth (D_O) if the stormwater stored in the BMP cannot drawdown in the required time.
• Required treatment volume: This is the volume that must be treated to meet the performance goal. The value is equal to the impervious surface area contributing to the BMP times the performance goal, plus any excess runoff routed to the BMP from another BMP. The default performance goal is 1.1 inches, which results in a required treatment volume of 3993 cubic feet per acre of impervious acre contributing to the BMP.
• Surface area at overflow (A_O): This is the surface area at the lowest outlet point from the BMP. Units are in square feet.
• Surface area at underdrain (A_U): This is the surface area of the BMP at the invert of the underdrain. If an underdrain is not present, it is the surface area of the permeable pavement. The user inputs this value in square feet. The calculator will display the surface area in acres for comparison with the watershed impervious cover acres entered for the BMP.
• Underlying soil - Hydrologic Soil Group: The user selects the most restrictive soil (lowest hydraulic conductivity) within the 3 feet below the media/native soil interface of the bioinfiltration basin. There are 14 soil options that fall into 4 different Hydrologic Soil Groups (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. Once a soil type is selected, the corresponding infiltration rate will populate in the Infiltration rate of underlying soils field. The user may also select User Defined. This selection will activate the User Defined Infiltration Rate cell allowing the user to enter a different value from those in the predefined selection list. The maximum allowable infiltration rate is 1.63 inches per hour.
• User defined infiltration rate: If the user enters a site specific infiltration rate rather than selecting a soil from the Hydrologic Soil Group, they enter the infiltration rate in this cell. The value is in inches per hour. The maximum infiltration rate is 1.63 inches per hour.
• Volume reduction capacity of BMP [V]: The volume that is captured by the BMP and available for infiltration into the underlying soil. It is equal to the area of the BMP times the depth of the BMP. If the volume is captured within media (e.g. tree trench, permeable pavement), the volume is also a function of the porosity of the media.
• Volume of retention provided by BMP: This is the volume credited as being infiltrated through the BMP. This will equal the smaller of Volume reduction capacity of BMP [V] or Required treatment volume. This term corrects for BMPs that are oversized, thus preventing credit beyond the performance goal.
• Will subsoil require compaction?: This term applies only to permeable pavement and is a YES/NO question. Select YES if compaction of the soil subgrade is needed to support vehicular loads. This selection does not affect the calculation of volume or pollutant reduction credits, but will likely reduce the infiltration rates of the underlying soils and the associated volume and pollutant credits. The USER should consider selecting a lower infiltration rate if the subsoil is compacted.