Posted by Christina Greene on Tue, Apr 29, 2014 @ 7:01 PM
Controlling Stormwater Runoff with Trees
by Rick Magill
“A city’s green infrastructure (trees, plants, parks) is as important as roads, sewers and water systems. When we take care of trees, they can help us live better for a long, long time.” (healthytreeshealthycities.org, 2014)
- Trees draw moisture from the soil ground surface through evapotranspiration, thereby increasing soil water storage potential.
- Leaves, branches surfaces, and trunk bark intercept and store rainfall thereby reducing runoff volumes and delaying onset of peak flows.
- Root growth and decomposition increase capacity and rate of soil infiltration by rainfall and reduce overland flow.
- Tree canopies reduce soil erosion by diminishing the impact of raindrops on barren surfaces.
In Oakland, California, the continuous tree canopy is estimated to intercept 4 inches of rain over one acre in a typical year or approximately 108,000 gallons. (USDA Forest Service, 2002)
Trees in green infrastructure offer comprehensive solutions to rainfall interception, stormwater management, pollutant control, and water use issues, among others. Green infrastructure systems such as: bio-swales, enhanced tree pits, bio-retention basins, permeable pavements, structural soil cells, green curbs, backyard cisterns, and others, are enhanced by trees in integrated urban stormwater control systems. Strategies to enhance the urban forest and improve stormwater runoff control:
- Plant larger trees, where possible.
“Large species trees, whether as street trees or as part of an urban forest or green infrastructure all provide an opportunity to reduce the rate and volume of surface water runoff. Large species trees are more effective at intercepting rainfall due to their larger and more extensive canopies and networks of branches.” (CIRIA, 2011)
- Plant species with higher growth rates, where appropriate.
- Match trees to local rainfall patterns ((broad-leaved evergreens and conifers are particularly effective when winter rainfall patterns prevail).
“One study found that “…a typical medium-sized tree can intercept as much as 2,380 gallons of rainfall per year.” (USDA Forest Service, 2002)
- Plant low-water requiring trees where appropriate, although the species should be able to withstand short-term saturated soils conditions.
- Plant broad-leaved evergreens, where appropriate.
- Plant trees in stands and groves, when possible.
- Use tree species that have proven to be tolerant of urban pollutants.
- Use native plants that can withstand dry seasons (once plants are established), thus reducing the need for supplemental irrigation.
- Improve the maintenance of existing trees.
- Plant species with architectural features (wide spreading and dense canopies) that maximize rainfall interceptions.
“…the (tree) crown does provide a positive and mitigating influence towards urban runoff control. “ (Xiao, 2000)
In far too many cases, the effectiveness of urban stormwater runoff management is compromised by the selection and installation of trees that are not appropriate for intended functions and uses. However, if the preceding strategies are employed in the planning, design, implementation, and maintenance of urban green infrastructure (including street trees), the result will be: effective, integrated, and sustainable urban stormwater management systems.
https://healthytreeshealthycities.org/ Homepage (2014)
Center for Urban Forest Research, Pacific Southwest Research Station, USDA Forest Service, Davis, California, July 2002.
CIRIA, The benefits of large species trees in urban landscapes: a costing design and management guide. Technical Report, Draft 6, September 2011
Winter rainfall interception by two mature open-grown trees in Davis, California (2000) Qingfu Xiao, G. McPherson, S.L. Ustin, M.E. Grismer, and J.R. Simpson