Stormwater Runoff Landscaping With Urban Canopy Cover and Groundcover

Summary: Researchers look at stormwater runoff landscaping with urban grassy, shrubby groundcover and tree canopy cover during summer and winter storms in Oregon.

Stormwater runoff landscaping incorporates ground and tree canopy covers to control stormwater runoff through ground infiltration; ground cover in rain garden designed to capture stormwater runoff in 7-SIGMA's parking lot, South Minneapolis, Minnesota; Oct. 15, 2007: BrianAsh at English Wikipedia, Public Domain, via Wikimedia Commons

Groundcover acts against acceleration and accumulation of surface runoff in urban watersheds more during summer than winter storms, according to an article for the September 2016 issue of Arboriculture & Urban Forestry.

The article Statistical Analysis of Vegetation and Stormwater Runoff in an Urban Watershed During Summer and Winter Storms in Portland, Oregon, U.S., broaches stormwater runoff landscaping. Corresponding author Geoffrey Donovan and co-authors David Butry and Megan Mao consider random-effects regression model consequences of vegetation for summer and winter peak and total runoff. Previous research in the forms of small-scale experiments and of wildland studies does not describe the interactions of built, impervious-surfaced components with vegetation in urban watersheds.

The study examines grassy, shrubby groundcover, green infrastructure and tree canopy cover, at 34 sewer-monitoring sites June 15 and 16 and Dec. 18 and 19, 2010.

The same combined-sewer system pipes fill with sanitary flow from 70 percent of Portland's homes and storm runoff from the city's mean annual 42.91-inch (109-centimeter) precipitation.

The Columbia Slough gets untreated flows four times during winter and once every third summer and the Willamette River once every fifth winter and tenth summer. Untreated flows no longer happen 50-plus times a year because of the Northwest Environmental Advocates' suit against the City in 1991 for Clean Water Act violations. Portland's experience with back-up sewage in residences and untreated flows from insufficient, undersized pipes is possible for 40 million combined-sewer system users in 772 U.S. communities.

Portland juggles gray infrastructure through three big-pipe project storage tunnels and green infrastructure through stormwater runoff landscaping of bioswales, grassy, shrubby groundcover and tree canopy cover.

Canopies and groundcovers kindle stormwater runoff landscaping by infiltration from roots into soil pores, interception by shoots for evaporation and transpiration from tissues for water vapor. Stormwater runoff peaks, rates and totals link to canopy and groundcover density, storm duration, intensity and seasonality, tree age and type and wild and urban interfaces.

Previous research in wildland settings matches more stormwater runoff with forest structures of meager over denser canopies and with trees 200-plus years old over 25-year-old Douglas-firs. Previous research in urban rain interception notes less stormwater runoff with low-intensity short-duration over high-intensity long-duration storms and with open-growth coniferous over open-growth deciduous tree species.

Previous research offers for the study the conclusions that stormwater runoff vegetation obstructs surface runoff rates and totals in urban and wildland settings during summer storms.

The study presents low-lying grassy and shrubby ground cover and tree canopy cover as respectively significant and insignificant in change-in-flow models for June stormwater runoff measurements. Grasses, shrubs and trees queue into insignificance, perhaps from leaf loss, in change-in-flow and flow models of peak and total runoff for December stormwater runoff measurements. The results remain indefinitive in terms of relationships between stormwater runoff management and tree canopy cover even though they reveal peak flow reductions through low-lying vegetation. They suggest the continued significance of grassy, shrubby groundcover and of low-lying vegetation in the green infrastructure of bioswales and the need for further tree-related research.

The co-authors talk of research beyond the study's two seasonal storms since light-, moisture-, noise-, pollution-mitigating trees never take space from impervious and vegetation rain-interactive surfaces.

The 2010 study of stormwater runoff landscaping in Portland, Oregon, considers ground and tree canopy cover for effectively controlling urban runoff as impervious surfaces of urban built environments allow for excessive runoff; diagram illustrates relationships of ground cover, impervious surfaces and surface runoff, February 2003: U.S. Environmental Protection Agency, Public Domain, via Wikimedia Commons

Acknowledgment

My special thanks to:
talented artists and photographers/concerned organizations who make their fine images available on the internet;
University of Illinois at Urbana-Champaign for superior on-campus and on-line resources.

Image credits:

Stormwater runoff landscaping incorporates ground and tree canopy covers to control stormwater runoff; ground cover in rain garden designed to capture stormwater runoff in 7-SIGMA's parking lot, South Minneapolis, Minnesota; Oct. 15, 2007: BrianAsh at English Wikipedia, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:7sigma_RainGarden_66.JPG

The 2010 study of stormwater runoff landscaping in Portland, Oregon, considers ground and tree canopy cover for effectively controlling urban runoff as impervious surfaces of urban built environments allow for excessive runoff; diagram illustrates relationships of ground cover, impervious surfaces and surface runoff, February 2003: U.S. Environmental Protection Agency, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Natural_%26_impervious_cover_diagrams_EPA.jpg

For further information:

Donovan, Geoffrey H.; Butry, David T.; and Mao, Megan Y. September 2016. "Statistical Analysis of Vegetation and Stormwater Runoff in an Urban Watershed During Summer and Winter Storms in Portland, Oregon, U.S." Arboriculture & Urban Forestry 42(5): 318-328.

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