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{{alert|The anticipated construction period for this page is through July, 2014|alert-under-construction}} | {{alert|The anticipated construction period for this page is through July, 2014|alert-under-construction}} | ||
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+ | [[File:Symbol for tree trench.png|300px|thumb|alt=symbol for tree trench system-treebox|<font size=3>Symbol used in the MIDS calculator to represent the tree trench system-tree box BMP. Note the symbol shows multiple trees since tree trench systems contain multiple trees.</font size>]] | ||
For a tree trench system/box without an underdrain, stormwater runoff captured by the BMP in the media is infiltrated into the underlying soil between rain events or lost through evapotranspiration. A small portion of precipitation is also intercepted by trees in the BMP. All pollutants in the captured and intercepted water are credited as being reduced. Pollutants in the stormwater that bypasses the BMP are not reduced. | For a tree trench system/box without an underdrain, stormwater runoff captured by the BMP in the media is infiltrated into the underlying soil between rain events or lost through evapotranspiration. A small portion of precipitation is also intercepted by trees in the BMP. All pollutants in the captured and intercepted water are credited as being reduced. Pollutants in the stormwater that bypasses the BMP are not reduced. | ||
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==MIDS calculator user inputs for Tree trench system/box== | ==MIDS calculator user inputs for Tree trench system/box== | ||
+ | [[File:watershed tab tree trench no underdrain.png|300px|thumb|alt=schematic of watershed tab for tree trench in MIDS calculator|<font size=3>Schematic of the Watershed tab for tree trench system-tree box BMP in the MIDS calculator. In this example, 1 acre of Forest/open space, 1 acre of managed Turf, and 1 acre of Impervious Cover drain to the BMP. The BMP is being routed to a constructed stormwater pond.</font size>]] | ||
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+ | [[File:BMP Parameters tab tree trench no underdrain|300px|thumb|alt=schematic of BMP Parameters tab for tree trench in MIDS calculator|<font size=3>Schematic of the BMP Parameters tab for tree trench system-tree box BMP in the MIDS calculator.</font size>]] | ||
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For Tree trench system/box without an underdrain BMPs, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP. | For Tree trench system/box without an underdrain BMPs, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP. | ||
*Watershed tab | *Watershed tab | ||
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===Pollutant Reduction=== | ===Pollutant Reduction=== | ||
+ | [[File:Schematic of pollutant reductions tree trench no underdrain.jpg|300px|thumb|alt=schematic of pollutant reductions from tree trench BMP|<font size=3>Schematic illustrating how pollutant reductions (TSS, dissolved and particulate P) are calculated for the tree trench system-tree box BMP in the MIDS calculator. Note the BMP</font size>]] | ||
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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. | 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. | ||
For a tree trench system/box without an underdrain, stormwater runoff captured by the BMP in the media is infiltrated into the underlying soil between rain events or lost through evapotranspiration. A small portion of precipitation is also intercepted by trees in the BMP. All pollutants in the captured and intercepted water are credited as being reduced. Pollutants in the stormwater that bypasses the BMP are not reduced. The user should be aware of the difference between a tree trench system and a tree box.
For Tree trench system/box without 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.
\(DDT_{calc} = (D_m/(n - FC)) / (I_R/12)\)
where
If DDTcalc is greater than the user defined required drawdown time then the user will be prompted to enter a new overflow depth or infiltration rate.
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 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 capacity” is equal to the sum of the capture volume in the media, the amount of water stored in the media that is lost through evapotranspiration (VET), and the volume of water lost through interception by the tree canopy (VI).
Stormwater runoff will flow into the media of the tree trench and fill the pores of the soil, eventually reaching water saturation. Water will then drain from the soils through infiltration into the underlying soils until the water content in the media reaches field capacity. The volume of water stored in the media between saturation and field capacity is the capture volume of the BMP. The capture volume (V) is given by
\(V = [(A_M + A_B)/2) * (n - FC) * D_M]\)
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 BMP media.
The second 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.
\(ET_{pot} = [D_M * (A_M + A_B)/2 * (FC - WP)]\)
where
\(ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)\)
where
Measured ET and potential ET are compared and the volume lost to ET is the smaller of the two values.
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.
All pollutants associated with the reduced volume of water are captured for a 100 percent removal. Water that bypasses the BMP through the overflow is not treated for a 0 percent removal. A schematic of the removal rates can be seen below.
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.