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

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Symbol for biofiltration (Bioretention basin (with underdrain)) in MIDS calculator.

Screen shot of the watershed tab for bioretention with an underdrain.

Screen shot from MIDS calculator showing user inputs needed for a biofiltration basin

For a biofiltration BMP with an underdrain at the bottom of the engineered media, 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. Volume retention also occurs by evapotranspiration through the vegetation in the biofiltration BMP. For biofiltration systems with an elevated underdrain, additional volume retention is achieved through infiltration of water stored in the pore spaces of engineered media between the underdrain invert and the native soils. In a bioretention BMP with an underdrain, all pollutants in infiltrated water are credited as being removed, while a portion of the pollutant loads in the stormwater that flows through the underdrain are removed through filtration.

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 amount of water between media porosity and field capacity stored 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 question. 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. Once a soil type is selected, the corresponding infiltration rates 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(DDT_calc), which is given by

\(DDT_{calc}=D_U / (I_R / 12)\)

Where

D_U is the depth below the underdrain (ft); and

I_R is the infiltration rate of the native soils (inches/hr).

If DDT_calc is greater than the user defined required drawdown time then the user will be prompted to enter a new depth below the underdrain or infiltration rate of the native soils.

• Infiltration rates of the underlying soils are restricted to a maximum of 1.63 inches/hour
• Surface areas must be equal to or less than all surface areas at higher elevations
• The Depth below the underdrain cannot be greater than the Total media depth
• If the user enters a value for field capacity minus wilting point or porosity minus field capacity outside the recommended range a warning will appear. The user will not be required to enter a new value.

Methodology

Required Treatment Volume

Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP plus any water routed to the BMP from an upstream BMP. This stormwater is delivered to the BMP instantaneously following the Kerplunk method.

Volume Reduction

The volume reduction achieved by a BMP compares the capacity of the BMP to the required treatment volume. The Volume reduction capacity of BMP [V] is calculated using BMP inputs provided by the user. For this BMP, the volume reduction credit methodology is determined by the location of the underdrain.

Underdrain located at BMP bottom

If the underdrain is located at the bottom of the BMP, then the Volume reduction capacity of BMP [V] is determined based on infiltration into the bottom of the BMP (V_{inf_b}), infiltration into the side slopes of the BMP (V_{inf_s}), and evapotranspiration in the planting media above the underdrain (V_ET).

Even with an underdrain present, under saturated media conditions some water will infiltrate through the bottom soils as water in the basin draws down. The volume of water lost through the bottom (V_{inf_b}) of the BMP is given by

\(V_{Inf_B} = I_R * (DDT) * A_B / (12in/ft) = 0.06 * (DDT) * A_B / (12in/ft)\)

Where

I_R is an infiltration rate into the native soils of 0.06 inches per hour;

A_B is the surface area at the bottom of the BMP in ft²; and

DDT is the drawdown time in hours.

The default infiltration rate is set at 0.06 inches per hour to represent a D soil. This rate was selected because it is assumed most of the stormwater will pass through the underdrain before it can infiltrate through the bottom of the BMP. This may be a conservative assumption if underdrains are small, spaced far apart, and the underlying soil has an infiltration rate greater than 0.06 inches per hour. Conversely, more closely spaced or larger underdrains may allow the basin to drain in less than the required drawdown time, resulting in a slight overestimation of infiltration loss through the basin bottom. If the user specifies that an impermeable liner is present at the bottom of the BMP, then no credit is given for infiltration into the bottom soils.

Under saturated conditions within the filter media, water will infiltrate through the sides of the basin as the stormwater draws down through the underdrain. Stormwater lost from a sloped sidewall (V_{inf_s}) is considered to infiltrate vertically into the surrounding soil. The volume of water infiltrated through the sidewalls is given by

\(V_{InfS} = I_R * (DDT / 2) * (A_O - A_U ) / (12in/ft) = 0.06 * (DDT / 2) * (A_O - A_U ) / (12in/ft)\)

Where:

A_O is the surface area at overflow in ft²; and

A_U is the surface area at the underdrain in ft²

The drawdown time is reduced by a factor of 2 to account for the drop in water level within the BMP over the drawdown period. The drop in water level is therefore considered to be linear over the drawdown time. A conservative default infiltration rate of 0.06 inches per hour is used because it is assumed that most of the stormwater will pass through the underdrain before it can infiltrate through the side walls of the BMP. If the user specifies that an impermeable liner is present on the sides of the BMP, then no credit is given for infiltration into the side soils.

The volume of water lost through evapotranspiration (VET) is the smaller of two calculated values, potential ET and measured ET.

• Potential ET (ET_pot) is equal to the amount of water stored between field capacity and the wilting point in the media above the underdrain. ET_pot is given by

\(ET_{pot} = (D_M - D_U ) * (A_M + A_U) / 2 * (FC - WP)\)

Where

D_M is the total media depth in feet;

D_U is the depth under the underdrain in feet;

A_M is the surface area of the media in square feet;

AU is the surface area at the underdrain in square feet; and

(FC – WP) is the difference between field capacity and wilting point.

• Measured ET (ET_mea) is the amount of water lost to ET as measured using available data. Pan evaporation (PE) measurements collected at the University of Minnesota Southwest Experiment Station at Lamberton, Minnesota were used to estimate an average daily PE (Source: Climate of Minnesota Part XII- The Hydrologic Cycle and Soil Water, 1979). A rate of 0.2 inches per day was used, which is an intermediate value between the summertime maximum rate and the lowest rates in October. PE is converted to ET by multiplying by a correction factor of 0.5. Analysis of rainfall patterns indicates that a typical time period between precipitation events is 72 hours in Minnesota. Therefore, a volume loss from ET is calculated over a 3 day period to measure conformance to the MIDS performance goal. Therefore, the measured ET volume equals the media surface area (A_M) in square feet times the daily ET rate in inches per day times 3 days.

\(ET_{mea} = A_M * 0.2 in/day * 0.5 * 3 days / 12 in/ft = 0.025 A_M\)

If trees are planted in the bioretention basin then ET_mea is multiplied by a factor of 3.

Measured ET and potential ET are compared and the volume lost to ET is the smaller of the two values.

Elevated Underdrain

If the underdrain is elevated above the bottom of the BMP, then the volume reduction credit is determined based on the storage capacity in the media between the underdrain and the native soils, infiltration through the sides of the BMP (V_{inf_s}), and evapotranspiration in the planting media above the underdrain (V_ET).

When the underdrain is elevated, storage capacity becomes available in the media between the underdrain and the native soils. The storage capacity credit replaces the credit given for infiltration into the bottom of the BMP below the underdrain (V_{Inf_B}). The volume of water captured below the underdrain is given by

\(V= (A_U + A_B) / 2 * (n - FC) * D_U \)

Where:

A_U is the surface area at the underdrain in ft²;

A_B is the surface area at the bottom of the basin in ft²;

(n - FC) is the media porosity minus field capacity of the soils; and

D_U is the depth of the media below the underdrain in ft

The stored water must drain within the specified drawdown time. The underlying soil controls the infiltration rate. The user must input the soil with the most restrictive hydraulic conductivity in the 3 feet directly below the basin.

In addition to the credit given for the storage capacity below the underdrain, a biofiltration system with an elevated underdrain also receives volume reduction credit for infiltration into the sloped sidewall as well as evapotranspiration. Credit is given following the same methods described when the underdrain is located at the bottom of the BMP (see discussion above). A biofiltration system with an elevated underdrain thus behaves as a dual system, with the portion above the drain acting like a biofiltration system with an underdrain at the bottom and the portion below the underdrain acting like a bioinfiltration system.

The Volume of retention provided by BMP is the amount of volume credit the BMP provides toward the performance goal. This value is equal to the Volume reduction capacity of BMP [V], calculated using the above method, as long as the volume reduction capacity is less than or equal to the Required treatment volume. If Volume reduction capacity of BMP [V] is greater than Required treatment volume, then the BMP volume credit is equal to Required treatment volume. This check makes sure the BMP is not getting more credit than the amount of water it receives. For example, if the BMP is oversized the user will only receive credit for Required treatment volume routed to the BMP.

Pollutant Reduction

Schematic showing how pollutant load reductions are calculated for bioretention with an underdrain. All TSS and phosphorus in infiltrated water is reduced. TSS loads are reduced by 60 percent for the portion captured by the underdrain. Phosphorus reductions are a function of the media and media thickness.

Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting Volume reduction capacity of BMP, which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves developed from multiple modeling scenarios. The performance curves use Volume reduction capacity of BMP [V], the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above Required treatment volume will not provide additional credit towards the performance goal volume, it may provide additional pollutant reduction.

A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater. Stormwater captured by the bioretention system but not infiltrated or consumed through ET is assumed to flow through the filter media and out the underdrain. A constant 60 percent removal rate is applied to the filtered stormwater for TSS reduction. The removal rates of the filtered stormwater for annual particulate phosphorus and dissolved phosphorus depend on the answers given to the three user inputs: Bioretention planting media mix, Is the P content of the media less than 30 mg/kg? and Is a soil amendment used to attenuate phosphorus?

Particulate Phosphorus: The particulate phosphorus credit given is either 0 percent or 45 percent depending on the media mix used and the P content of the media.

• If Media Mix C or D is used, the annual particulate phosphorus reduction credit is 45 percent of the filtered water volume.
• If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is 30 milligrams per kilogram or less, the annual particulate phosphorus reduction credit is 45 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is greater than 30 milligrams per kilogram, the annual pollutant phosphorus reduction credit is 0 percent of the filtered water volume.
  • If a media mix other than C or D is used and the soil phosphorus has not been determined, the annual particulate phosphorus credit is 0 percent of the filtered water volume.

Dissolved Phosphorus: The dissolved phosphorus credit given is between 0 percent and 60 percent depending on the media mix, the media P content, and if the media was amended to attenuate phosphorus.

• If Media Mix C or D is used, or if a media mix other than C or D is used and soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is 30 milligrams per kilogram or less, the annual dissolved phosphorus credit applied to the filtered water volume is calculated by

\(credit = 20 percent (D_M- D_U) / 2 ft\)

Where

D_M - D_U represents the media depth above the underdrain.

The credit is calculated as a percent reduction with a maximum value of 20 percent for media depths above the underdrain greater than 2 feet. If the media depth above the underdrain is less than 2 feet the credit is reduced equivalently.

• If a media mix other than C or D is used and the soil phosphorus as measured using the Mehlich 3 test or a suitable alternative test is greater than 30 milligrams per kilogram, the annual dissolved phosphorus credit is 0 percent of the filtered water volume.
• If a media mix other than C or D is used and the soil phosphorus has not been determined, the annual dissolved phosphorus credit is 0 percent of the filtered water volume.
• An additional annual dissolved phosphorus credit of 40 percent of the filtered water volume may be received if phosphorus-sorbing amendments are used. Acceptable amendments include the following:
  • 5 percent by volume elemental iron filings above the internal water storage (IWS) layer or elevated underdrain;
  • minimum 5 percent by volume sorptive media above IWS layer or elevated underdrain;
  • minimum 5 percent by weight water treatment residuals (WTR) to a depth of at least 3.9 inches (10 centimeters);
• An additional annual dissolved phosphorus credit commensurate with the research results can be applied if other phosphorus-sorptive amendments are proposed that have supporting third party research results showing dissolved phosphorus reduction for at least a 20-year lifespan.

Phosphorus credits for bioretention systems with an underdrain. This includes tree trenches and dry swales.
Link to this table

Particulate phosphorus (PP)	Dissolved phosphorus (DP)
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1? If yes, particulate credit = 100% of the particulate fraction (assumed to be 55% of total P) If no or unknown, particulate credit = 0% The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here. Example PP removal credit Particulate fraction (55% of TP) * removal rate for that fraction (80% for bioretention and tree trench and 68% for swale main channel) = 0.55 * 0.80 = 0.44 or 44% for bioretention and tree trench (with underdrain) and 0.55 * 0.68 or 36% for swale main channel (with underdrain) If particulate is 75% of TP, removal = 0.75 * 0.80 or 60% for bioretention and tree trench (with underdrain) and 0.75 * 0.68 or 51% for dry swale	1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1? If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2) If no or unknown, credit = 0% 2. Does the system include approved P-sorbing soil amendments2? If yes, additional 40% credit Example DP removal credit DP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent Adjust DP removal if depth is less than 2 feet. Example depth = 1 foot then DP removal = 0.45 * 10% * 1/2 = 0.045 Adjust DP removal if dissolved credit is higher due to use of P-sorbing soil amendments Adjust if the fraction (percentage) of DP is different than 45% The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.
TP removal = PP removal + DP removal

¹Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
²Acceptable P sorption amendments include

• 5% by volume elemental iron filings above IWS or elevated underdrain
• minimum 5% by volume sorptive media above IWS or elevated underdrain
• minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
• other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results

NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.

Routing

A biofiltration basin can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause stormwater to be rerouted back to the biofiltration basin already in the sequence. All BMPs can be routed to a biofiltration, except for a swale side slope BMP.

Assumptions for biofiltration basin with underdrain

The following general assumptions apply in calculating the credit for a biofiltration basin. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.

• The biofiltration basin has been properly designed, constructed and will be properly maintained.
• Stormwater runoff entering the biofiltration basin has undergone pretreatment.
• Stormwater captured by the BMP enters the BMP media instantaneously. This will slightly underestimate actual infiltration since some water will infiltrate through the basin bottom and sidewalls during a rain event, thus creating more volume for storage in the BMP.
• Infiltration rates used to calculate the infiltration credit through the bottom and sidewalls of the basin are 0.06 inches per hour.
• Evapotranspiration is independent of plant type, plant density and weather conditions.

Biofiltration Basin with an elevated underdrain example (Version 2)

Schematic for the MIDS calculator example for bioretention with an underdrain. Impervious surface area is 1.4 acres. Pervious surface area, which includes a turf area and the bioretention basin, is 0.8 acres. See Step 1.

Screen shot of site information tab for bioretention with elevated underdrain example. The User must input impervious acres and zip code. Other fields are optional. See Step 2.

Schematic used for MIDS calculator example for bioretention with elevated underdrain. See Step 1.

Screen shot of the MIDS calculator Schematic tab for the bioretention with elevated underdrain example. See Step 3.

An unlined biofiltration basin with an elevated underdrain is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious area (turf area and the bioretention BMP area). All of the runoff from the watershed will be treated by the biofiltration basin. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface overflow is located 1 ft above the media surface. The surface area of the biofiltration basin at the overflow point will be 6534 square feet. The area is 5600 square feet at the media surface. The surface area at the invert of the underdrain will be 3948 square feet. The area at the media-soil interface is 3320 square feet. The total media depth will be 3 feet with 1 foot of media between the underdrain and the native soils. Following the MPCA Construction Stormwater General Permit requirement, the water below the underdrain in the biofiltration basin needs to drawdown in a 48 hour time period. The media will be Media Mix C, which is mostly sand resulting in a difference between the media wilting point and field capacity of 0.11 cubic feet per cubic foot and a difference between the media porosity and field capacity of 0.26 cubic feet per cubic foot. The P content of the media is less than 30 mg/kg (milligrams per kilogram) and no soil amendments will be used to attenuate phosphorus. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious area in type B soils. The pervious area includes the turf area and the area of the biofiltration basin.

Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. The Managed turf area includes the turf area and the area of the bioretention basin. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the Bioretention basin (with underdrain) icon into the Schematic Window

Step 4: Open the BMP properties for the bioretention basin by right clicking on the Bioretention basin (with underdrain) icon and selecting Edit BMP properties, or by double clicking on the Bioretention basin (with underdrain) icon.

Step 5: If help is needed click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculation specific to the Bioretention basin (with underdrain) BMP.

Step 6: Determine the watershed characteristics for the Bioretention basin. For this example the entire site is draining to the bioretention basin. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed turf in B soils).

• MIDS calculator screen shots for inputs for bioretention with an elevated underdrain. Click on an image for enlarged view.
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  Screen shot showing watershed tab for bioretention with an elevated underdrain. See Step 6.

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  Screen shot showing BMP Parameters tab for bioretention with an elevated underdrain. See Step 7.

• BMP Parameters tab bioretention with elevated underdrain example 2.png

  Screen shot showing BMP Parameters tab for bioretention with an elevated underdrain. See Step 7.

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  Screen shot of the BMP Summary tab for bioretention with an elevated underdrain. See Step 8.

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  Screen shot for the example of a bioretention system with an elevated underdrain. See Step 10.

Step 7: Enter in the BMP design parameters into the BMP parameters tab. Bioretention basin with an underdrain requires the following entries.

• Is the underdrain elevated above native soils – Yes;
• Are the sides of the basin lined with an impermeable liner – No;
• Is the bottom of the basin lined with an impermeable liner – No;
• Surface area of overflow which is 6534 square feet;
• Surface area at media surface which is 5600 square feet;
• Surface area at underdrain which is 3948 square feet;
• Bottom surface area (area at media-soil interface) which is 3230 square feet;
• Overflow depth which is 1 foot;
• Total media depth which is 3 feet;
• Depth below underdrain which is 1 foot;
• Media field capacity minus wilting point which is 0.11 cubic feet per cubic foot;
• Media porosity minus field capacity which is 0.26 cubic feet per cubic foot;
• Is a tree(s) planted in the BMP – No;
• Bioretention planting media mix which is Media Mix C;
• Is the P content of the media less than 30 mg/kg which autofills to “Yes” for Media Mix C;
• Is a soil amendment used – No;
• Underlying soil – Hydrologic Soil Group which is SM (HSG B; 0.45 in/hr) from the dropdown box; and
• Required drawdown time (hrs) which is 48 from the dropdown box.

Step 8: Click on BMP Summary tab to view results for this BMP.

Step 9: Click on the OK button to exit the BMP properties screen.

Step 10: Click on Results tab to see overall results for the site.

Biofiltration Basin with an underdrain at bottom example (Version 2)

Schematic used for MIDS calculator example for bioretention with an underdrain at the bottom. In this example there is 1.4 acres of impervious parking draining to the bioretention basin. Pervious area is 0.8 acres and includes the turf area and the bioretention BMP. See Step 1.

Screen shot of the Site information tab for the MIDS example for bioretention with an underdrain at the bottom. The User must input impervious acres and zip code. Other fields are optional. See Step 2.

Schematic used for the MIDS calculator example for bioretention with an underdrain at the bottom. See Step 1.

Screen shot of the Schematic tab for the MIDS calculator example for bioretention with an underdrain at the bottom. See Step 3.

An unlined biofiltration basin with an underdrain at the bottom is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious area (turf area and the bioretention BMP area). All of the runoff from the watershed will be treated by the biofiltration basin. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface overflow is located 1 ft above the media surface. The surface area of the biofiltration basin at the overflow point will be 6534 square feet. The area is 5600 square feet at the media surface. The area at the media-soil interface is 3320 square feet. The total media depth will be 3 feet. The media will be Media Mix C, which is mostly sand resulting in a difference between the media wilting point and field capacity of 0.11 cubic feet per cubic foot and a difference between the media porosity and field capacity of 0.26 cubic feet per cubic foot. The P content of the media is less than 30 mg/kg (milligrams per kilogram) and no soil amendments will be used to attenuate phosphorus. The following steps detail how this system would be set up in the MIDS calculator.

Step 1: Determine the watershed characteristics of your entire site. For this example we have a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious area in type B soils. The pervious area includes the turf area and the area of the biofiltration basin.

Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. The Managed turf area includes the turf area and the area of the bioretention basin. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.

Step 3: Go to the Schematic tab and drag and drop the Bioretention basin (with underdrain) icon into the Schematic Window

Step 4: Open the BMP properties for the bioretention basin by right clicking on the “Bioretention basin (with underdrain)” icon and selecting Edit BMP properties, or by double clicking on the Bioretention basin (with underdrain) icon.

Step 5: If help is needed click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculation specific to the “Bioretention basin (with underdrain)” BMP.

Step 6: Determine the watershed characteristics for the Bioretention basin. For this example the entire site is draining to the bioretention basin. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed turf in B soils).

• MIDS calculator screen shots for inputs for bioretention with an underdrain at the bottom. Click on an image for enlarged view.
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  Screen showing watershed tab for bioretention with an underdrain at the bottom. See Step 6.

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  MIDS calculator screen shot showing BMP Parameters tab for bioretention with an underdrain at the bottom. See Step 7.

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  MIDS calculator screen shot showing BMP Parameters tab for bioretention with an underdrain at the bottom. See Step 7.

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  MIDS calculator screen shot of the BMP Summary tab for bioretention with an underdrain at the bottom. See Step 8.

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  MIDS calculator screen shot for the example of a bioretention system with an underdrain at the bottom. See Step 10.

Step 7: Enter in the BMP design parameters into the BMP parameters tab. Bioretention basin with an underdrain requires the following entries.

• Is the underdrain elevated above native soils – No;
• Are the sides of the basin lined with an impermeable liner – No;
• Is the bottom of the basin lined with an impermeable liner – No;
• Surface area of overflow which is 6534 square feet;
• Surface area at media surface which is 5600 square feet;
• Bottom surface area (area at media-soil interface) which is 3230 square feet;
• Overflow depth which is 1 foot;
• Total media depth which is 3 feet;
• Media field capacity minus wilting point which is 0.11 cubic feet per cubic foot;
• Media porosity minus field capacity which is 0.26 cubic feet per cubic foot;
• Is a tree(s) planted in the BMP – No;
• Bioretention planting media mix which is Media Mix C;
• Is the P content of the media less than 30 mg/kg which autofills to “Yes” for Media Mix C;
• Is a soil amendment used – No;
• Underlying soil – Hydrologic Soil Group which is SM (HSG B; 0.45 in/hr) from the dropdown box; and
• Required drawdown time (hrs) which is 48 from the dropdown box.

Step 8: Click on BMP Summary tab to view results for this BMP.

Step 9: Click on the OK button to exit the BMP properties screen.

Step 10: Click on Results tab to see overall results for the site.

Requirements

Measurement of depth from the bottom of the infiltration BMP (bioretention with elevated underdrain) to the seasonally high water table or bedrock. Note that there must be a minimum of 2 feet separation when soils beneath the BMP are ripped, with a total separation distance of 3 feet or more. Infiltration BMPs include any BMP that allows water to infiltrate into the underlying soil.

Warning: The following are requirements of the Minnesota Construction Stormwater General Permit

• 3 foot separation from the bottom of an infiltration system (bioretention with elevated underdrain) to the seasonal high water table
• Use the most restrictive infiltration rate within 3 feet of the bottom of the BMP
• For measured infiltration rates, apply a safety factor of 2
• Pretreatment for infiltration systems (bioretention with elevated underdrain)

Recommendations

Caution: The following are recommendations for inputs into the MIDS calculator

• Drawdown time of 24 hours when the discharge is to trout streams
• Field tested infiltration rates rather than table values

Information

Information: The following information may be useful in determining inputs for the MIDS calculator

• Guidance on determining infiltration rates
• Information on site constraints (shallow soil, karst, etc.)
• Guidance on pretreatment
• Construction specifications for bioretention BMPs
• Information on operation and maintenance of bioretention BMPs.

Links to MIDS pages

• Overview of Minimal Impact Design Standards (MIDS)
• Performance goals for new development, re-development and linear projects
• Design Sequence Flowchart-Flexible treatment options
• Community Assistance Package
• MIDS calculator
• Performance curves for MIDS calculator
• Training and workshop materials and modules
• Technical documents

Related pages

• Bioretention terminology (including types of bioretention)
• Overview for bioretention
• Design criteria for bioretention
• Construction specifications for bioretention
• Operation and maintenance of bioretention
• Cost-benefit considerations for bioretention
• Soil amendments to enhance phosphorus sorption
• Summary of permit requirements for bioretention
• Supporting material for bioretention
• External resources for bioretention
• References for bioretention
• Requirements, recommendations and information for using bioretention with no underdrain BMPs in the MIDS calculator
• Requirements, recommendations and information for using bioretention with an underdrain BMPs in the MIDS calculator