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[[File:Tree trench no drain schematic for example.jpg|300 px|thumb|alt=schematic showing dimensions for calculator example|<font size=3>Schematic showing the dimensions of the tree trench system. The surface area of the system is 5600 square feet and the bottom surface area is 3230 square feet. The media depth is 4 feet.</font size>]] | [[File:Tree trench no drain schematic for example.jpg|300 px|thumb|alt=schematic showing dimensions for calculator example|<font size=3>Schematic showing the dimensions of the tree trench system. The surface area of the system is 5600 square feet and the bottom surface area is 3230 square feet. The media depth is 4 feet.</font size>]] | ||
− | A tree trench system 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 tree trench BMP area). All of the runoff from the watershed will be treated by the tree trench system. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface area of the tree trench basin 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 4 feet. Following the MPCA Construction Stormwater General Permit requirement, the water in the media of the tree trench needs to drawdown in a 48 hour time period. The media will be Media Mix D, which is a | + | A tree trench system 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 tree trench BMP area). All of the runoff from the watershed will be treated by the tree trench system. The soils across the area have a unified soils classification of SM ([[Glossary#H|HSG]] type B soil). The surface area of the tree trench basin 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 4 feet. Following the [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html MPCA Construction Stormwater General Permit] requirement, the water in the media of the tree trench needs to drawdown in a 48 hour time period. The media will be [[Design criteria for bioretention#Materials specifications - filter media|Media Mix D]], which is a loamy sand composition resulting in a difference between the [[Soil water storage properties|media wilting point and field capacity]] of 0.05 cubic feet per cubic foot and a difference between the [[Soil water storage properties|media porosity and field capacity]] of 0.35 cubic feet per cubic foot. The tree trench will be planted with 10 [[Tree species list - morphology|medium sized deciduous trees]]. 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 tree trench basin. | '''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 tree trench basin. |
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/tree 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.
A tree trench system 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 tree trench BMP area). All of the runoff from the watershed will be treated by the tree trench system. The soils across the area have a unified soils classification of SM (HSG type B soil). The surface area of the tree trench basin 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 4 feet. Following the MPCA Construction Stormwater General Permit requirement, the water in the media of the tree trench needs to drawdown in a 48 hour time period. The media will be Media Mix D, which is a loamy sand composition resulting in a difference between the media wilting point and field capacity of 0.05 cubic feet per cubic foot and a difference between the media porosity and field capacity of 0.35 cubic feet per cubic foot. The tree trench will be planted with 10 medium sized deciduous trees. 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 tree trench 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 tree trench 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 “Tree trench system/Box (w/o underdrain)” icon into the “Schematic Window”
Step 4: Open the BMP properties for the tree trench by right clicking on the “Tree trench system/Box (w/o underdrain)” icon and selecting “Edit BMP properties”, or by double clicking on the “Tree trench system/Box (w/o underdrain)” icon.
Step 5: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Tree trench system/Box (w/o underdrain)” BMP.
Step 6: Determine the watershed characteristics for the tree trench. For this example the entire site is draining to the tree trench. 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).
Step 7: Enter in the BMP design parameters into the “BMP parameters” tab. Tree trench systems 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.