Requirements, recommendations and information for using bioretention with an underdrain BMPs in the MIDS calculator

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Biofiltration (Bioretention with underdrain)

For a biofiltration BMP with an underdrain at the bottom, most of the stormwater captured by the BMP is lost to the underdrain. However, some stormwater infiltrates through the basin bottom and sidewalls if these do not have an impermeable liner. Evapotranspiration also occurs from vegetation in the biofiltration BMP. For a biofiltration system with an elevated underdrain, in addition to volume losses through the sidewalls and through evapotranspiration, the water stored between the underdrain and the native soils is captured and infiltrated. In a bioretention BMP with an underdrain, all pollutants in infiltrated water are removed, while pollutants are removed through filtration for the water that flows through an underdrain.

MIDS calculator user inputs for biofiltration

For biofiltration systems, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.

• Watershed tab
  • BMP Name: this cell is auto-filled but can be changed by the user.
  • Routing/downstream BMP: if this BMP is part of a treatment train and water is being routed from this BMP to another BMP, the user selects the name of the BMP from the dropdown box to which water is being routed. All water must be routed to a single downstream BMP. Note that the user must include the BMP receiving the routed water in the Schematic or the BMP will not appear in the dropdown box.
  • BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP. Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for three Land Cover types (Forest/Open Space, Managed Turf and impervious). The surface area of the BMP should be included as a managed turf land cover under the hydrologic soils group of the native soils located under the BMP. Units are in acres.

• BMP Parameters tab
  • Is the underdrain elevated above native soils?: This is a YES/NO question. Answering YES means the underdrain is elevated within the media (i.e., Biofiltration with a raised underdrain). 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 (i.e., Biofiltration with underdrain at the bottom).
  • 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.
  • 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 infiltration 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.
  • Surface area at overflow (A_O): This is the surface area of the BMP at the lowest outlet point of the surface overflow from the ponding area of the BMP. The user inputs this value in square feet.
  • Surface area at media surface (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. The user inputs this value in square feet.
  • Overflow depth (D_O): This is the maximum depth of ponded water within the BMP (i.e., distance from the overflow elevation to the top of the soil or media). The user inputs this value in feet. The maximum value allowed for this depth is 1.5 feet.
  • Surface area at underdrain (A_U): This is the surface area of the BMP at the invert elevation of the underdrain. If the response to “Is the underdrain elevated above native soils?” is set to NO, then this cell will become inactive and populated with the “Bottom surface area” value. The user inputs this value in square feet.
  • 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.
  • Total media depth (D_M): This is the depth of the engineered media between the media surface and the native soils. The user inputs this value in feet.
  • Depth below underdrain (D_U): This is the depth of the media between the underdrain invert and the native soils. If the response to “Is the underdrain elevated above native soils?” is set to NO, then this cell will become inactive and populated with a 0. The user inputs this value in feet.
  • 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 above the underdrain. 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. The recommended range for this value is 0.05 to 0.17.
  • Media porosity minus filed capacity ('n' - FC) - This is the ratio of media pore space to the total media volume between the underdrain invert and the bottom of the media (top of native soil). If multiple 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. 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. The recommended range for this value is 0.15 to 0.35.
  • Is a tree(s) planted in the BMP?: This is a YES/NO questions. If trees are planted within the bioretention basin then additional volume loss associated with evapotranspiration will be applied.
  • Bioretention planting media mix: 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, or Other. This value is used to determine the annual phosphorus load reduction credit.
  • 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.
  • Is a soil amendment used to attenuate phosphorus?: This is a YES/NO question. Answer YES if the bioretention 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.
  • Underlying soil - Hydrologic Soil Group: The user selects the most restrictive soil (lowest hydraulic conductivity) within 3 feet of the soil/media interface in the biofiltration 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. These correspond with soils and infiltration rates contained in this Manual. 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 the values in the predefined selection list. The maximum allowable infiltration rate is 1.63 inches per hour.
  • Required drawdown time: This is the time in which the stormwater captured by and ponded within the BMP must drain into the underlying soil/media. The user may 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 underlying soil infiltration rate and the “Depth below underdrain” to check if the BMP is meeting the drawdown time requirement. The user will encounter an error and be required to enter a new “Depth below underdrain” if the stormwater stored in the BMP cannot drawdown in the required time.
  • BMP Summary Tab: The BMP Summary tab summarizes the volume and pollutant reductions provided by the specific BMP. It details the performance goal volume reductions and annual average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary are the total volume and pollutant loads received by the BMP from its direct watershed, from upstream BMPs and a combined value of the two. Also included in the summary, are the volume and pollutant load reductions provided by the BMP, in addition to the volume and pollutant loads that exit the BMP through the outflow. This outflow load and volume is what is routed to the downstream BMP if one is defined in the Watershed tab. Finally, percent reductions are provided for the percent of the performance goal achieved, percent annual runoff volume retained, total percent annual particulate phosphorus reduction, total percent annual dissolved phosphorus reduction, total percent annual TP reduction, and total percent annual TSS reduction.

Model input requirements and recommendations

The following are requirements or recommendations for inputs into the MIDS calculator. If the following are not met, an error message will inform the user to change the input to meet the requirement.

• Overflow depth cannot be greater than 1.5 feet.
• The water underneath the underdrain must meet the drawdown time requirement specified. The drawdown time requirement is checked by comparing the user defined drawdown time with the calculated drawdown time(DDTcalc) calculated using the following: