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**Surface area at underdrain (A<sub>U</sub>): 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. | **Surface area at underdrain (A<sub>U</sub>): 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. | ||
**Bottom surface area (A<sub>B</sub>): 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. | **Bottom surface area (A<sub>B</sub>): 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<sub>U</sub>): This is the depth below the underdrain to the native soils. If no underdrain is present, this is the thickness of the engineered | + | **Depth below underdrain (D<sub>U</sub>): This is the depth below the underdrain to the native soils. If no underdrain is present, this is the thickness of the engineered media. |
**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. Values for porosity based on soil type can be found here. | **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. Values for porosity based on soil type can be found here. | ||
**Will subsoil require compaction?: This 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. | **Will subsoil require compaction?: This 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. |
For permeable pavement, stormwater runoff captured by the BMP and stored below the underdrain (if underdrain is present) is infiltrated into the underlying soil between rain events. All pollutants in the infiltrated stormwater are credited as being reduced. Pollutants in stormwater captured by the BMP but entering the underdrain are treated as they pass through the filter media and out the underdrain.
For permeable pavement systems, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.
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.
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) given by
\(DDT_{calc} = D_U / (I_R / 12)\)
Where
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.
The volume reduction achieved by a BMP compares the capacity of the BMP to the required treatment volume. The Volume reduction capacity of BMP is calculated using BMP inputs provided by the user. For this BMP the volume reduction credit is equal to the amount of water that can be instantaneously captured by the BMP in the media below the underdrain. The capture volume (V) is therefore given by
\(V= (A_U + A_B)) / 2 * n * D_U \)
Where:
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 lesser of the Volume reduction capacity of BMP calculated using the above method or the Required treatment volume. This check makes sure that the BMP is not getting more credit than necessary to meet the performance goal. For example, if the BMP is oversized the user will only receive credit for the Required treatment volume routed to the BMP, which corresponds with meeting the performance goal for the site .
Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting the 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 (add link to addendum) developed from multiple modeling scenarios. The performance curves use the Volume reduction capacity of BMP, 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 the 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 since these pollutants are either attenuated within the media or pass into the underlying soil with infiltrating water. Stormwater that is not infiltrated is assumed to flow through the filter media and out the underdrain. A 74 percent TSS, 82 percent particulate phosphorus, and 0 percent dissolved phosphorus removals are applied to the filtered stormwater. A schematic of the removal rates can be seen in the sidebar.
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.
A permeable pavement BMP 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 infiltration basin already in the stormwater runoff treatment sequence. All BMPs can be routed to the permeable pavement, except for a swale side slope.
The following general assumptions apply in calculating the credit for a permeable pavement system. If these assumptions are not followed, the volume and pollutant reduction credits cannot be applied.
Half of an existing 1.4 acre parking lot is going to be converted to permeable pavement. The entire parking lot (1.4 acres) plus 0.4 acres of pervious area (Turf Area) surrounding the parking lot will drain into the permeable pavement. The soils across the area have a unified soils classification of SM (HSG type B soil). An underdrain will be installed under the permeable pavement 0.5 feet above the native soils. Following the MPCA Construction Stormwater General Permit requirement, the water below the underdrain needs to drawdown in a 48 hour time period. The media below the underdrain has a porosity of 0.4 cubic feet per cubic foot. 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 1.8 acre site with 1.4 acres of impervious area and 0.4 acres of pervious turf area in type B soils. The impervious area includes the area of parking lot that has permeable pavement.
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. 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 Permeable Pavement icon into the Schematic Window
Step 4: Open the BMP properties for the permeable pavement by right clicking on the “Permeable pavement” icon and selecting Edit BMP properties, or by double clicking on the Permeable pavement icon. Click on the Watershed tab.
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 Permeable pavement BMP.
Step 6: Determine the watershed characteristics for the permeable pavement. For this example the entire site is draining to the permeable pavement. The watershed parameters therefore include a 1.8 acre site with 1.4 acres of impervious area and 0.4 acres of pervious turf area in 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).
Step 7: Enter in the BMP design parameters into the BMP parameters tab. Permeable pavement requires the following entries:
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.