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**Is a soil amendment used to attenuate phosphorus?: This is a YES/NO question. Answer YES if the filter media contains [[Soil amendments to enhance phosphorus sorption|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 a soil amendment used to attenuate phosphorus?: This is a YES/NO question. Answer YES if the filter media contains [[Soil amendments to enhance phosphorus sorption|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 tree trench/box. There are 14 soil options that fall into 4 different [[Glossary#H|Hydrologic Soil Groups]] (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. These correspond with [[Design infiltration rates|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. | **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 tree trench/box. There are 14 soil options that fall into 4 different [[Glossary#H|Hydrologic Soil Groups]] (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. These correspond with [[Design infiltration rates|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 (hrs): This is the time in which the stormwater captured by the BMP must drain into the underlying soil/media. The user may select from predefined values of 48 or 24 hours. The MPCA [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html 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 | + | **Required drawdown time (hrs): This is the time in which the stormwater captured by the BMP must drain into the underlying soil/media. The user may select from predefined values of 48 or 24 hours. The MPCA [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html 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. | *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. | ||
For a tree trench system/box 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 (ET) and interception also occur from the trees planted in the system. For a tree trench/box system with an elevated underdrain, in addition to volume losses through the sidewalls and through evapotranspiration and interception, a portion of the water stored in the media between the underdrain and the native soils is infiltrated. In a tree trench/box 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. All pollutants in water lost to ET and interception are removed.
The user should be aware of the difference between a tree trench system and a tree box.
For Tree trench system/tree box with an underdrain BMPs, 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 meet an error message will inform the user to change the input to meet the requirement.
\(DDT_{calc} = (D_M/(n - FC)) / (I_R/12)\)
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 volume 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 (Vinf_b), infiltration into the side slopes of the BMP (Vinf_s), evapotranspiration in the planting media above the underdrain (VET), and interception from the tree canopy (VI).
Even with an underdrain present, under saturated media conditions some water will infiltrate through the native soils as water in the basin draws down. The volume of water lost through the bottom (Vinf_b) of the BMP equals the following
\(V_{Inf_B} = I_R * (DDT) *A_B /(12in/ft)\)
where
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 (Vinf_s) is considered to infiltrate vertically into the surrounding soil. The volume of water infiltrated through the sidewalls equals the following
\(V_{Inf_S} = I_R * (DDT/2) * (A_M - A_U ) / (12in/ft) \)
where
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 third mechanism contributing to the Volume reduction capacity of BMP is interception. Water intercepted by a tree canopy may evaporate or be slowly released such that it does not contribute to stormwater runoff. An interception credit is given by a simplified value of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous and coniferous tree species. The volume of water lost through interception (VI) in cubic feet is given by
\(V_I = I_C/12 * CP * N\)
where
The interception capacity (IC) is determined based on data presented by Breuer et al. (2003) for deciduous and coniferous tree species (IC = 0.087 inched for coniferous trees and 0.043 inches for deciduous trees).
The canopy projection area (CP) is the perceived tree canopy diameter at maturity and varies by tree species. Canopy projection is determined based on the size of the tree (CP = 315 square feet for a small tree, 490 square feet for a medium sized tree, and 707 square feet for a large tree)
The final mechanism contributing to the Volume reduction capacity of BMP is evapotranspiration (ET). The water stored in the media between field capacity and wilting point is available for evapotranspiration. The volume of water lost through evapotranspiration (VET) is assumed to be the smaller of two calculated values of potential ET and measured ET.
\(ET_{pot} = [D_M * (A_M + A_B)/2 * (FC - WP)]\)
\(ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)\)
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 above the underdrain (Vinf_s), evapotranspiration in the planting media (VET), and interception of rainfall from the tree canopy (VI).
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 (VInf_B). The volume of water captured below the underdrain equals the following
\(V = [(A_U + A_B)/2 * (n - FC) * D_U]\)
where
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 tree trench system with an elevated underdrain also receives volume reduction credit for infiltration into the sloped sidewall as well as evapotranspiration and interception. Credit is given following the same methods described when the underdrain is located at the bottom of the BMP (see discussion above).
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 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 (add link to addendum) 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 tree trench/box system but not infiltrated or consumed through ET/interception is assumed to flow through the filter media and out the underdrain. A constant 68 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: 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.
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.
\(credit = 20 * (D_M - D_U) / (2 ft)\)
where
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.
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:
The removal rates of the filtered stormwater for annual particulate phosphorus and dissolved phosphorus is summarized in the following table. 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 tree trench/tree box BMP can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause water to be rerouted back to the tree trench/tree box BMP. All BMPs can be routed to a tree trench/tree box BMP except for a swale side slope BMP.
The following general assumptions apply in calculating the credit for a tree trench/box. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.