MADISON, WI, NOVEMBER 17, 2008– Global
land use patterns and increasing pressures on water resources demand
creative urban stormwater management. Traditional stormwater management
focuses on regulating the flow of runoff to waterways, but generally
does little to restore the hydrologic cycle disrupted by extensive
pavement and compacted urban soils with low permeability. The lack of
infiltration opportunities affects groundwater recharge and has
negative repercussions on water quality downstream. Researchers know
that urban forests, like rural forest land, can play a pivotal role in
stormwater mitigation, but developing approaches that exploit the
ability of trees to handle stormwater is difficult in highly built city
cores or in urban sprawl where asphalt can be the dominant cover
feature.
A group of researchers from Virginia Tech, Cornell, and
University of California at Davis have been investigating innovative
ways to maximize the potential of trees to address stormwater in a
series of studies supported by the U.S. Forest Service’s Urban and
Community Forestry Grants Program. The results of the studies were
published in the November-December issue of the Journal of Environmental Quality.
Virginia Tech scientists used two container experiments to
establish that urban tree roots have the potential to penetrate
compacted subsoils and increase infiltration rates in reservoirs being
used to store stormwater. In one study, roots of both black oak and red
maple trees penetrated clay loam soil compacted to 1.6 g cm-3, increasing infiltration rates by an average of 153%.
In another experiment, researchers created a small-scale
version of the stormwater best management practice (BMP) under study by
the three universities. This BMP includes a below-pavement stormwater
detention reservoir constructed of structural soil. Structural soils
are engineered mixes designed to both support pavement loads and
simultaneously provide rooting space for trees. In this study, green
ash trees increased the average infiltration rate by 27 fold compared
with unplanted controls. In the experiment, a structural soil reservoir
(CUSoil, Amereq Corp., New York) was separated from compacted clay loam
subsoil (1.6 g cm‑3) by a woven geotextile in 102-liter
containers. The roots of ash trees planted in the structural soil
penetrated both the geotextile and the subsoil within two years.
“Although we observed many roots penetrating the geotextile,
roots really proliferated where there was a slight tear in the fabric,”
said Susan Day, the project’s lead investigator. “Manipulating root
penetration through these separation geotextiles could potentially play
a large role in bioretention system function and design, especially
since the potentially saturated soils beneath detention reservoirs may
have reduced soil strength, increasing opportunities for root growth by
some species.”
Structural soil reservoirs may thus provide new opportunities
for meeting engineering, environmental, and greenspace management needs
in urban areas. Further research is needed on the effects of tree roots
and detention time on water quality in structural soils. Monitoring
continues at four demonstration sites around the country and updated
information is posted as it becomes available at www.cnr.vt.edu/urbanforestry/stormwater.
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