1. Tree Plantings and Preservation
Trees and forests reduce stormwater runoff by capturing and storing rainfall in the canopy and releasing water into the atmosphere through evapotranspiration. Tree roots and leaf litter also create soil conditions that promote the infiltration of rainwater into the soil. In addition, trees and forests reduce pollutants by taking up nutrients and other pollutants from soils and water through their root systems. A development site can reduce runoff volume by planting new trees or by preserving trees which existed on the site prior to development. The volume reduction calculations either determine the cubic feet to be directed to the area under the tree canopy for infiltration or determine a volume reduction credit which can be used to reduce the size of any one of the planned structural BMPs on the site.
Tree Considerations:
Existing trees must have at least a four inch trunk caliper or larger.
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Existing tree canopy must be within 100 feet of impervious surfaces.
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A tree canopy is classified as the continuous cover of branches and foliage formed by a single tree or collectively by the crowns of adjacent trees.
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New tree plantings must be at least six feet in height and have a two inch trunk caliper.
∙ All existing and newly planted trees must be native to Pennsylvania. See http://www.dcnr.state.pa.us/forestry/commontr/commontrees.pdf for a guide book titled Common Trees of Pennsylvania for a native tree list.
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When using trees as volume control BMPs, runoff from impervious areas should be directed to drain under the tree canopy.
Determining the required number of planted trees to reduce the runoff volume:
1. Determine contributing impervious surface area:
Garage roof (right) | 6 ft. x 24 ft. | = | 144 ft. |
2. Calculate the required control volume:
(144 sq. ft. x 2 inches of runoff) / 12 inches = 24 cu. ft.
3. Determine the number of tree plantings:
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A newly planted deciduous tree can reduce runoff volume by 6 cu. ft.
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A newly planted evergreen tree can reduce runoff volume by 10 cu. ft.
24 cu. ft./ 6 cu. ft. = 4 Deciduous Trees
Determining the volume reduction for preserving existing trees:
1. Calculate approximate area of the existing tree canopy:
~22 sq. ft. x ~23 sq. ft. = 500 sq. ft.
2. Measure distance from impervious surface to tree canopy: 35 ft.
3. Calculate the volume reduction credit by preserving existing trees:
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For Trees within 20 feet of impervious cover:
Volume Reduction cu. ft. = (Existing Tree Canopy sq. ft. x 1 inch) / 12
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For Trees beyond 20 feet but not farther than 100 feet from impervious cover:
Volume Reduction cu. ft. = (Existing Tree Canopy sq. ft. x 0.5 inch) / 12
(500 sq. ft. x 0.5 inches) / 12 = 21 cu. ft.
This volume credit can be utilized in reducing the size of any one of the structural BMPs planned on the site. For example, the 21 cu. ft. could be subtracted from the required infiltration volume when sizing the infiltration trench;
510 cu. ft. - 21 cu. ft. = 489 cu. ft.
489 cu. ft. / 3 ft. (Depth) = 163 / 6 ft. (Width) = 27.1 ft. (Length)
Using the existing trees for a volume credit would decrease the length of the infiltration trench to 27.1 ft. instead of 28.3 ft.
2. Minimize Soil Compaction and Replant with Lawn or Meadow
When soil is overly compacted during construction it can cause a drastic reduction in the permeability of the soil and rarely is the soil profile completely restored. Runoff from vegetative areas with highly compacted soils similarly resembles runoff from an impervious surface. Minimizing soil compaction and re-planting with a vegetative cover like meadow or lawn, not only increases the infiltration on the site, but also creates a friendly habitat for a variety of wildlife species.
Design Considerations:
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Area shall not be stripped of topsoil.
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Vehicle movement, storage, or equipment/material lay down shall not be permitted in areas preserved for minimum soil compaction.
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The use of soil amendments and additional topsoil is permitted.
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Meadow should be planted with native grasses. Refer to Meadows and Prairies: WildlifeFriendly Alternatives to Lawn at http://pubs.cas.psu.edu/FreePubs/pdfs/UH128.pdf for reference on how to properly plant the meadow and for a list of native species.
Determining the volume reduction by minimizing soil compaction and planting a meadow:
1. Calculate approximate area of preserved meadow:
~22 sq. ft. x ~23 sq. ft. = 500 sq. ft.
2. Calculate the volume reduction credit by minimizing the soil compaction and planting a lawn/meadow:
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For Meadow Areas: Volume Reduction (cu. ft.) = (Area of Min. Soil Compaction (sq. ft.) x 1/3 inch of runoff) / 12
(500 sq. ft. x 1/3 inch of runoff) / 12 = 13.8 cu. ft.
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For Lawn Areas: Volume Reduction (cu. ft.) = (Area of Min. Soil Compaction (sq. ft.) x 1/4 inch of runoff) / 12
(500 sq. ft. x 1/4 inch of runoff) / 12 = 10.4 cu. ft.
This volume credit can be used to reduce the size of any one of the structural BMPs on the site. See explanation under the volume credit for preserving existing trees for details.
Alternative BMP to Capture and Reuse Stormwater
Rain Barrels
Rain barrels are large containers that collect drainage from roof leaders and temporarily store water to be released to lawns, gardens and other landscaped areas after the rainfall has ended. Rain barrels are typically between 50 and 200 gallons in size. It is not recommended for rain barrels to be used as a volume control BMP because infiltration is not guaranteed after each storm event. For this reason, a rain barrel is not utilized in the site plan example. However, the information is included to provide an alternative for a homeowner to utilize when considering capture and reuse stormwater methods.
Design Considerations:
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Rain barrels should be directly connected to the roof gutter/spout.
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There must be a means to release the water stored between storm events to provide the necessary storage volume for the next storm.
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When calculating rain barrel size, rain barrels are typically assumed to be 25% full because they are not always emptied before the next storm.
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Use screens to filter debris and cover lids to prevent mosquitoes.
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An overflow outlet should be placed a few inches below the top with an overflow pipe to divert flow away from structures.
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It is possible to use a number of rain barrels jointly for an area.
Sizing Example for a Rain Barrel
1. Determine contributing impervious surface area:
Garage Roof (right) | 6 ft. x 24 ft. | = | 144 sq. ft. |
2. Determine the amount of rainfall to be captured by the Rain Barrel. A smaller storm, no more than two inches, is recommended to calculate the runoff to be captured. This example chose the one-inch storm event.
3. Calculate the volume to be captured and reused:
(144 sq. ft. x 1 inch of runoff ) / 12 inches = 12 cu. ft.
4. Size the rain barrel:
12 cu. ft. x 7.48 = 90 gallons
90 gallons x (0.25*) = 22.5 gallons (*assuming that the rain barrel is always at least 25% full)
90 gallons + 22.5 gallons = 112 gallons
The rain barrel or barrels should be large enough hold at least 112 gallons of water.
REFERENCES:
Center for Watershed Protection and US Forest Service Watershed Forestry Resource Guide (2008). Retrieved on May 26, 2010 from http://www.forestsforwatersheds.org/reduce-stormwater/.
Department of Environmental Protection. Pennsylvania Stormwater Best Management Practices Manual (2006).
Wissahickon Watershed Partnership. Pennsylvania Rain Garden Guide. Retrieved on May 4, 2010 from http://pa.audubon.org/habitat/PDFs/RGBrochure_complete.pdf.
Building a Backyard Rain Garden. North Carolina Cooperative Extension. Retrieved on May 4, 2010 from http://www.bae.ncsu.edu/topic/raingarden/Building.htm.
Delaware County Planning Commission. Draft Crum Creek Watershed Act 167 Stormwater Management Plan. Ordinance Appendix B. Simplified Approach to Stormwater Management for Small Projects (2010).
Solebury Township. Solebury Township Stormwater Management Ordinance (2008). “Appendix J Simplified Stormwater Management Procedures for Existing Single Family Dwelling Lots”
(Ord. 239, passed 10-14-2020)