Line 40: | Line 40: | ||
*The bottom surface area must be equal to or smaller than the media surface area. | *The bottom surface area must be equal to or smaller than the media surface area. | ||
*The number of trees must be 1 or more. | *The number of trees must be 1 or more. | ||
+ | |||
+ | ==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 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 | ||
+ | |||
+ | <math>V = [(A_M + A_B)/2) * (n - FC) * D_M]</math> | ||
+ | |||
+ | where | ||
+ | :A<sub>M</sub> = the media surface area, square feet; | ||
+ | :A<sub>B</sub> = the surface area at the bottom of the basin, square feet; | ||
+ | :''n - FC'' = media porosity minus field capacity, cubic feet/ cubic feet; and | ||
+ | :D<sub>M</sub> is the media depth, feet. | ||
+ | |||
+ | 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 (I<sub>c</sub>), as presented by Breuer et al. (2003) for deciduous and coniferous tree species. The volume of water lost through interception (V<sub>I</sub>) in cubic feet is given by | ||
+ | |||
+ | <math>V_I = I_C/12 * CP * N</math> | ||
+ | |||
+ | where | ||
+ | :I<sub>C</sub> = the interception capacity for an individual tree, inches; | ||
+ | :CP is the canopy projection area for an individual tree, ft<sup>2</sup>; and | ||
+ | :N is the total number of trees planted in the tree trench. | ||
+ | |||
+ | The interception capacity (I<sub>C</sub>) 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 (V<sub>ET</sub>) is assumed to be the smaller of two calculated values of potential ET and measured ET. | ||
+ | *Potential ET (ET<sub>pot</sub>) is equal to the amount of water stored in the media between field capacity and the wilting point. ET<sub>pot</sub> is given by | ||
+ | |||
+ | <math>ET_{pot} = [D_M * (A_M + A_B)/2 * (FC - WP)]</math> | ||
+ | |||
+ | where | ||
+ | :D<sub>M</sub> is the total media depth; | ||
+ | :A<sub>M</sub> is the surface area of the media; and | ||
+ | :(FC – WP) is the difference between field capacity and wilting point. | ||
+ | |||
+ | *Measured ET (ET<sub>mea</sub>) is the amount of water lost to ET as measured using available data. Measured ET is given by | ||
+ | |||
+ | <math>ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)</math> | ||
+ | |||
+ | where | ||
+ | :N = the number of trees in the BMP; | ||
+ | :CP = is the canopy projection area, square feet; | ||
+ | :LAI is the leaf area index. The LAI is stratified by tree type and tree size. For coniferous trees the LAI = 5.47. For deciduous trees LAI = 3.5 for small trees, 4.1 for medium sized trees, and 4.7 for large trees. These values are based on collected research for global leaf area from 1932-2000 (Scurlock, Asner and Gower, 2002).; | ||
+ | :E<sub>rate</sub> = the pan evaporation rate for a given area. This value is set to 0.02 ft/day which is based on evaporation data collected at the Southwest Research and Outreach Center in Lamberton, Minnesota. | ||
+ | :E<sub>ratio</sub> accounts for the efficiency of the leaves to transpire the available soil water or, alternately, the stomatal resistance of the canopy to transpiration and water movement, relative to evaporation from a pan surface. This is set at 0.20, or 20 percent based on research by Lindsey and Bassuk (1991). This means that a 1 square centimeter leaf transpires only about 1/5 as much as 1 square centimeter of pan surface. | ||
+ | :Three days is a typical time period between precipitation events in Minnesota based on analysis of rainfall data. | ||
+ | :The adjustment accounts for soil volume that is less than the recommended volume. Since the recommended soil volume equals 2 times the canopy project area (CP), the adjustment term is given by Adjustment=(S<sub>v)</sub>/(2*CP) where Sv is the actual soil volume available for each individual tree, in cubic feet. S<sub>V</sub> is given by (((A<sub>m</sub>+A<sub>b</sub>)/2 * D<sub>m</sub>)/N) where N is the number of trees planted in the tree trench/box. | ||
+ | |||
+ | 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 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. | ||
+ | |||
+ | 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. | ||
+ | |||
+ | ==Routing== | ||
+ | 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. | ||
+ | |||
+ | ==Assumptions for tree trench system/box== | ||
+ | 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. | ||
+ | *The tree trench system has been properly designed, constructed and will be properly maintained. | ||
+ | *Stormwater runoff entering the tree trench/box 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. | ||
+ | *Evapotranspiration is independent of plant type, plant density and weather conditions. |
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.