Urban Forestry & Stormwater Management

Using trees to enhance stormwater management efforts

High intensity land use patterns and increasing pressure on water resources demands creative stormwater management. Trees dissipate the energy of falling raindrops to help prevent erosion and buffer intense rainfalls. Urban tree roots have the potential to penetrate compacted soils and increase infiltration rates in open space areas, stormwater basins and subsurface stormwater storage (structured soil). Uptake of water from trees limits the volume of runoff discharged downstream, and their canopies offer interception of rainfall and shading (cooling) in an urban environment. Trees also absorb nutrients that could otherwise run off to local receiving waters.

This fact sheet provides an overview of the benefits of protecting existing trees and planting new trees in stormwater treatment practices of new development or redevelopment sites and includes activities that can be implemented by an MS4.

Benefits / Pollution Reduction

Urban forestry strategies can help satisfy many of the MS4 stormwater management requirements in a cost effective manner. Trees, forests, and other natural areas effectively manage water through interception, evapotranspiration, and infiltration. Together, these processes can significantly reduce peak stormwater rates and volumes, naturally filter runoff, enhance ground water recharge, stabilize base flows and reduce erosion in streams.

Trees also take up nutrients and various pollutants through their root systems. A study of the City of Fayetteville, Arkansas, estimated that increasing tree canopy from 27 percent to 40 percent would reduce stormwater runoff by 31 percent (American Forests, UEA of Benton and Washington Counties, Arkansas, 2002).

This image shows urban trees
Urban Trees Image Courtesy of Center for WatershedProtection

A study at University of California at Davis evaluated pollutant removals for structural soils, soils designed to meet requirements for pavement support while still allowing sufficient pore space to support tree roots. Three soil types averaged 73-77 percent removal of nitrate, 52-58 percent of phosphorus, 75-80 percent of zinc and 78-92 percent of chromium (see Managing Stormwater for Urban Sustainability Using Trees and Structural Soils in Additional Resources). The term “phytoremediation” has been used to describe the ability of certain trees to take up and alter contaminants that occur in soil and shallow ground water. This has become an effective and low cost remediation approach for brownfield restoration.

Program Development & Implementation

Preventing Tree Loss during Development and Redevelopment

Regulatory tools can be adopted, perhaps as part of a tree ordinance, to reduce forest clearing during development, as well as to prevent inadvertent injury to trees. Some of these techniques include:

  • Bonus or incentive zoning – provides the right to build more intensely on a portion of the property in exchange for conserving forested areas
  • Clearing and grading requirements – set limits on the amount of clearing that may occur onsite
  • Forest conservation and protection regulations – establish the criteria by which trees are identified for conservation, including buffer and fencing requirements
  • Open space design – a compact form of development that relaxes minimum lot sizes, setbacks, road widths and other ordinances to provide common open space (also see the Open Space Design fact sheet)
  • Overlay zoning – or the stacking of additional standards onto existing development criteria
  • Performance-based zoning – designed to ensure an acceptable level of performance is met with a development, in this case, for protection of specified percentage of forest land
  • Stormwater credits – may be granted for the conservation of forested areas (also see the Minnesota Stormwater Manual)
  • 8Stream buffer ordinances – specify protection of existing forest cover or may even specify reforestation along corridors lacking tree cover

Trees and Stormwater Management BMPs

Urban development and redevelopment sites provide many opportunities (e.g. during the installation of stormwater treatment practices) to plant new trees that provide water quality treatment and storage of stormwater runoff from impervious surfaces. Many stormwater treatment practices such as roadside rain gardens have not traditionally been considered appropriate locations for planting trees. Research on the benefits of trees, however, shows they have enormous potential to improve the efficiency of these practices through nutrient uptake and runoff reduction.

To encourage tree planting in stormwater treatment practices, detailed guidance has been developed by the Center for Watershed Protection (CWP) (see Additional Resources) for the selection of appropriate species, identification of areas suitable for planting, and modification of the design or planting environment. The CWP resource also provides conceptual designs for the following stormwater management features:

  • Wooded wetland
  • Bioretention and biofiltration facilities
  • Alternating side slope plantings (swale)
  • Tree check dams (swale)
  • Forested filter strip
  • Multi-zone filter strip
  • Linear stormwater tree pit

Examples of stormwater credits for urban trees
Link to this table

Municipality Credit Website
Pine Lake, GA Saved trees contribute to site runoff requirements: < 12 inches DBH = 10 gal/in; > 12 inches DBH = 20 gal/in http://pinelakega.com/environment-green-space/waterfirst-plan/
Sacramento, CA A portion of impervious cover underneath the canopy of new or existing interceptor trees from approved species list may be subtracted from the site’s overall impervious cover:

New deciduous = 100 ft2; New evergreen = 200 ft2

Existing = ½ existing canopy
2007 Stormwater Quality Design Manual: http://www.sacramentostormwater.org/SSQP/documents/DesignManual/SWQ_DesignManual_May07_062107.pdf
Portland, OR A portion of impervious cover underneath the canopy of new or existing trees from approved species list and within 25 feet of impervious surfaces may be subtracted from the site’s overall impervious cover. 2004 Stormwater Management Manual: http://www.portlandonline.com/BES/index.cfm?c=35122
Indianapolis, IN 100 ft2 impervious area reduction for each new tree within 20 feet of impervious surfaces and from approved species list. http://rrstormwater.com/city-indianapolismarion-county-stormwater-management-regulation-compliance

The Minnesota Stormwater Manual (Appendix E) contains recommendations for trees suitable for Minnesota conditions such as salt tolerance. Proprietary devices are available that consist of the structural framework into which the soil medium and tree is installed and above which the sidewalk is constructed. For examples, see Additional Resources.

Tree Credits for Stormwater Management

Incentives for implementing trees for stormwater management can include providing stormwater management credit in development or redevelopment rules. Some metro watershed districts are considering allowing credit for the interception of rainfall by trees. For example, the Capitol Region Watershed District is considering a volume reduction credit equal to 0.15 inches over the area of the tree canopy. Other credits have been established in municipalities across the country. Pine Lake, Georgia site runoff requirements are lessened by 10 gallons/inch for trees less than 12 inches diameter at breast height (DBH) and those greater than 12 inches DBH are credited 20 gallons/inch. The following table presents this and other stormwater credit approaches.

This image shows a typical cross-section of a tree box filter adjacent to a parking lot or roadway
Typical cross-section of a tree box filter adjacent to a parking lot or roadway

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