For a tree trench system/box without an underdrain, stormwater runoff captured by the BMP 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 (w/o underdrain) BMPs, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.
If the following requirements for inputs into the MIDS calculator are not met, then 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.
The Volume reduction capacity of BMP [V] is calculated using BMP inputs provided by the user. For this BMP, the Volume reduction capacity [V] is equal to the sum of the Volume reduction stored in soil media (infiltration), the Volume reduction of BMP from ET (VET), and the Volume reduction of BMP from interception (VI).
The total instantaneous Volume reduction capacity of BMP [V] due to these three mechanisms is calculated using BMP design inputs provided by the user. This Volume reduction capacity of BMP [V] is then compared to the Required treatment volume in order to determine the Volume of retention provided by BMP, which is the instantaneous volume credit that can be claimed for that BMP.
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 5 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 estimated through a simplification of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous (Ic = 0.043 in) and coniferous (Ic = 0.086 in) tree species. To determine the interception volume of an individual tree, the Ic is multiplied by the canopy projection area (CP), which is the perceived tree canopy diameter at maturity. Although CP varies by tree species and size, for the MIDS calculator CP is determined solely 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 volume of water lost through interception (VI) in cubic feet is given by
\(V_I = I_C/12 * CP * N\)
where
The final mechanism contributing to the Volume reduction capacity of BMP is evapotranspiration (ET) of a portion of the water stored in the media between field capacity and wilting point. The volume of water lost through evapotranspiration (VET) is assumed to be the smaller of two calculated values, potential ET and measured ET.
Potential ET (ETpot) is equal to the amount of water stored in the media between field capacity and the wilting point, and is given by
\(ET_{pot} = D_M * (A_M + A_B)/2 * (FC - WP)\)
Measured ET (ETmea) is the amount of water lost to ET as measured using available data. For this BMP in the MIDS Calculator, ETmea is calculated based on the leaf area index (LAI); canopy projection (CP); an average pan evaporation rate; an adjustment to the pan evaporation rate that accounts for leaf and stomata effects; the average time between rain events in MN; the soil volume available for tree rooting (Sv); and the number of trees in the BMP. Measured ET is given by
\(ET_{mea} = N * CP * LAI * E_{rate} * E_{ratio} * 3 days * (adjustment)\)
The "adjustment" factor is a multiplier that accounts for BMP conditions where the soil volume that is less than the recommended volume. Since the recommended soil volume equals 2 times the canopy projection area (CP), the adjustment term is the lesser between 1 and the following term
\((adjustment) = S_v / (2*CP)\)
\(S_v = (0.5*(A_M + A_B) * D_M) / N\)
The LAI is stratified by tree type and tree size. For coniferous trees, the LAI = 5.47. For deciduous trees, the 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). The pan evaporation rate is set at 0.02 ft/day, which is based on evaporation data collected at the University of Minnesota Southwest Experiment Station at Lamberton, Minnesota. The Eratio term accounts for the reduced 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 a 1 square centimeter pan surface.
The MIDS calculator compares the Volume reduction capacity of BMP [V] with the Required treatment volume, and the lesser of the two values is used to populate the Volume of retention provided by BMP. This comparison between potential and actual treatment volumes ensures that the BMP does not claim more credit than is due based on the actual amount of water routed to it. The Volume of retention provided by BMP is the actual volume credit the BMP receives toward the instantaneous performance goal. For example, if the BMP is oversized the user will only receive volume credit for the Required treatment volume routed to the BMP.
Annual volume retention is assessed by converting the instantaneous Volume reduction capacity of BMP [V] to an annual volume reduction percentage. This is accomplished through the use of performance curves developed from a range of modeling scenarios. These performance curves use the Volume reduction capacity of BMP [V], the infiltration rate of the underlying soils, the percent imperviousness of the contributing watershed area, and the size of the contributing watershed to calculate the Percent annual runoff volume retained and annual Retention volume provided by BMP.
Recommended values for porosity, field capacity and wilting point for different soils.1
Link to this table.
Soil | Hydrologic soil group | Porosity 2 (volume/volume) | Field capacity (volume/volume) | Wilting point (volume/volume) | Porosity minus field capacity (volume/volume)3 | Field capacity minus wilting point (volume/volume)4 |
---|---|---|---|---|---|---|
Sand | A (GM, SW, or SP) | 0.43 | 0.17 | 0.025 to 0.09 | 0.26 | 0.11 |
Loamy sand | A (GM, SW, or SP) | 0.44 | 0.09 | 0.04 | 0.35 | 0.05 |
Sandy loam | A (GM, SW, or SP) | 0.45 | 0.14 | 0.05 | 0.31 | 0.09 |
Loam | B (ML or OL) | 0.47 | 0.25 to 0.32 | 0.09 to 0.15 | 0.19 | 0.16 |
Silt loam | B (ML or OL) | 0.50 | 0.28 | 0.11 | 0.22 | 0.17 |
Sandy clay loam | C | 0.4 | 0.07 | |||
Clay loam | D | 0.46 | 0.32 | 0.15 | 0.14 | 0.17 |
Silty clay loam | D | 0.47 to 0.51 | 0.30 to 0.37 | 0.17 to 0.22 | 0.16 | 0.14 |
Sandy clay | D | 0.43 | 0.11 | |||
Silty clay | D | 0.47 | 0.05 | |||
Clay | D | 0.47 | 0.32 | 0.20 | 0.15 | 0.12 |
1Sources of information include Saxton and Rawls (2006), Cornell University, USDA-NIFA, Minnesota Stormwater Manual. (See References for trees)
2Soil saturation is assumed to be equal to the porosity.
3This value may be used to represent the volume of water that will drain from a bioretention media.
4This value may be used to estimate the amount of water available for evapotranspiration
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 to the right.
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.
The following pages address incorporation of trees into stormwater management under paved surfaces