MIDS Calculator terminology

The following terms are used in the MIDS Calculator.

• Are the sides of the basin lined with an impermeable liner?: This is a YES/NO question. Answering YES means the sides of the basin are lined, preventing water from infiltrating into the native soils. Answering NO means the sides are not lined and infiltration is allowed through the side of the basin into the native soils.
• 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.
• Channel length (LC): This is an input for using swale in the MIDS calculator. This is the length of the swale channel from the furthest upstream point to the furthest downstream point. Units are in feet.
• Channel slope (S): This is an input for using swale in the MIDS calculator. This is the slope of the channel. Units are in percent. The slope is calculated by taking the difference between vertical elevations at the upstream and downstream points of the swale and dividing by the horizontal distance between the two locations. The slope therefore represents an average slope over the length of the swale. If the slope varies over the length of a swale, the user should enter the average slope over the entire length rather than break the swale into separate smaller lengths. Based on design restrictions, the channel slope cannot be less than 0.5 percent or greater than 4 percent.
• Depth at check dam (DCD): This is an input for using swale in the MIDS calculator. This parameter represents the elevation change between the overflow point of the check dam (top of check dam) and the soil surface of the swale. Units are in 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.
• Does the swale have a bioretention base?: This is an input for using swale in the MIDS calculator. This is a YES/NO question. Answer YES if the swale main channel has engineered soils similar to a bioretention basin. Answer NO if the swale main channel has native soils rather than engineered media. If YES is selected, then the design parameters associated with the bioretention base will be activated.
• Does the swale have a check dam?: This is an input for using swale in the MIDS calculator. This is a YES/NO question. The user selects YES if one or more check dams have been installed in the swale main channel. By selecting YES, the “Is check dam permeable?” input box will become active.
• 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.
• Is the underdrain elevated above native soils?: This is a YES/NO question. Answering YES means the underdrain is elevated within the media. This creates storage capacity between the underdrain and the native soils. Answering NO means that the underdrain is not elevated within the media and is directly above the native soils with no storage capacity below the underdrain.
• Is the bottom of the basin lined with an impermeable liner?: This is a YES/NO question. Answering YES means the bottom of the basin is lined, preventing water from infiltrating into the native soils. Answering NO means the bottom is not lined and infiltration is allowed through the bottom of the basin into the native soils.
• Is a soil amendment used to attenuate phosphorus?: This is a YES/NO question. Answer YES if the filter media contains soils amendments to enhance phosphorus sorption and NO if amendments are not used. This value is used to determine the annual phosphorus load reduction credit.
• Is check dam permeable?: This is an input for using swale in the MIDS calculator. This is a YES/NO questions. The user selects YES if the check dams are permeable, allowing stormwater to filter through the media and NO if they are impermeable, acting as a dam and providing extended detention in the swale main channel. If NO is selected, the design parameters associated with the check dam will become active. If YES is selected, the design parameters will remain inactive as no credit toward the performance goal is obtained for permeable check dams.
• Is the P content of the media less than 30 mg/kg?: This is a YES/NO question. The P content of the planting media should be tested using the Mehlich 3 test or an acceptable alternative method. Select YES if the P content of the planting media is less than 30 milligrams per kilogram and NO if it is greater. P content testing is not needed for planting media C or D; therefore, this item will automatically populate to YES if one of those two media types are selected. This value is used to determine the annual phosphorus load reduction credit.
• Manning’s n (Vegetation): This is an input for using swale in the MIDS calculator. The user selects a type of vegetation cover on the swale main channel, which populates a corresponding Manning’s n. The vegetation types are mowed turf or native grasses. Mowed turf would be selected if the main channel is mowed on a consistent basis. Native grasses would be selected if the main channel is left to grow or is mowed infrequently. Once a vegetation cover is selected, the corresponding Manning’s n will populate in the “Manning’s n” field. The user may also select “User Defined.” This selection will activate the “User Defined Manning’s n” cell, allowing the user to enter a value different from the values in the predefined selection list.
• Media depth (D_M): The depth of the media, equal to the distance from the bottom of the media (i.e., top of underlying drainage layer) to the top of the media. The user inputs this value in inches.
• Media field capacity minus wilting point (FC - WP) - This is the amount of water between field capacity and the permanent wilting point stored in the media. This is water often considered to be available for uptake by plants. If multiple types of media are used in the BMP, this value should be an average of the media installed above the underdrain. Values for field capacity and wilting point based on soil type can be found here. The user inputs this value in cubic feet of water per cubic feet of media.
• Media holding capacity: Moisture content at the maximum media density. This term applies to green roofs and represents the amount of water that the soils are able to hold onto instead of percolating into the drainage layer. The value is in cubic feet per cubic feet and is often expressed as percent volume This value is restricted to 0.33 cubic feet per cubic feet (33 percent).
• 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 porosity minus filed capacity (n - FC) - This is the amount of water stored in the media between media porosity (soil saturation) and field capacity. This is the amount of water that is stored in the media and infiltrated into the underlying soils. If multiple types of media are used in the BMP, this value should be an average of the media installed. Values for porosity and field capacity based on soil type can be found here. The user inputs this value in cubic feet of pore space per cubic feet of media.
• 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.
• Number of check dams: This is an input for using swale in the MIDS calculator. This parameter is the number of check dams built throughout the length of the swale main channel. If multiple check dams are constructed, enter the average value of all dams for top width and depth.
• Number of trees: This is an input for tree trench/tree box in the MIDS calculator. The user enters the total number of trees planted in the tree trench/tree box system.
• 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.
• Planting media mix: This is an input for vegetated BMPs, such as bioretention and tree trench/tree box. The user selects the type of media mix installed for planting from a predefined list of Media mixes: Media mix A (water quality blend), Media mix B (enhanced filtration blend), Media mix C (North Carolina State University water quality blend), Media mix D (tree mix), or Other. This value is used to determine the annual phosphorus load reduction credit.
• 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.
• Swale bottom width (WB): This is an input for using swale in the MIDS calculator. This is the average bottom width of the swale main channel. Units are in feet. Based on design restrictions, the bottom width cannot be less than 2 feet.
• Top surface area (A_M): This term applies to green roofs and is surface area of the green roof. The user inputs this value in square feet. The calculator will display the surface area in acres for comparison with the watershed areas entered.
• Top width at check dam (WT): This is an input for using swale in the MIDS calculator. This is the width of the check dam at the overflow elevation (top of check dam) and is used to calculate the storage volume behind the check dam. Units are in feet.
• Tree Type: This is an input for tree trench/tree box in the MIDS calculator. The user selects the type of tree planted in the tree trench/box from a drop down menu. The user can select a tree type of deciduous or coniferous. If both deciduous and coniferous trees are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
• Tree Size: This is an input for tree trench/tree box in the MIDS calculator. The user selects the size of tree planted in the tree trench/box system. The user can select small, medium or large. Tree size for different tree types are listed in the tree species list. If multiple tree sizes are planted at the site, they should be treated as separate Tree trench system/tree box BMPs.
• 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.