Using Trees To Reduce Stormwater Runoff

Center for Watershed Protection USDA Forest Service, Northeastern Area, State and Private Forestry Center for Watershed Protection USDA Forest Service, Northeastern Area, State and Private Forestry

using trees to reduce stormwater runoff

stormwater runoff is the number one factor in the decline of

urban streams and

decreasing urban water

quality !

old news

The first models for predicting runoff appeared in the 1800s and used runoff coefficients to account for different land use and land covers.

•The Rational Method (Mulvaney 1851)

•Peak Discharge = Runoff Coefficient x Rainfall x Basin Area

•Runoff Coefficients:

•Forested ground is typically assigned a value of near 0.

•Pavement is given values approaching 100 percent

But why emphasize trees in particular versus other

stormwater practices?

save energy improve air quality provide habitat better quality of life neighborhood stability aesthetic values increase property value reduce noise good for business

trees are the original “multi-taskers”

??so if trees and forests provide so many benefits beyond reducing stormwater …shouldn’t they be more of a priority consideration

in site design.??

… yes, but only if trees are given credit for their work…and to give credit, we must be

able to provide numbers!

(How else can engineers calculate credits?)

don’t trees get some credit already?

not in most municipalities not enough to matter to most developers. not enough to equal their true value

Source: CWP

trees & stormwater: some conclusions

• Watershed health is linked to the amount of forest in the watershed and its distribution.

• Increases in tree cover and tree size will result in reduced total runoff and peak runoff rates (10% = 2-5%)

• Tree canopy has a greater effect on small storm events than on large storm events (2 year storm frequency)

• Effects on runoff are greatest when urban trees are large and well-established (site quality).

• Trees and stormwater management practices can coexist if planned and designed from the start.

Approaches to use trees for stormwater management

•Incorporate trees into stormwater practices•Pursue site opportunities for stormwater forestry•Make stormwater practices an amenity

incorporating trees into stormwater practices

•Traditionally not a celebrated union

•Some engineers don’t want trees in STPs

•Foresters want to know where are all the trees?

•Most of our practices end up losing trees due to poor maintenance

challenges…

Urban Foresters

Engineers/Planners

Now remember, you have to work

together Leftover disturbed areas are not the only place for trees

We can retain valuable trees that improve watershed functions

We can use trees in SW practices if we design it differently

We need wider streets and curbs

Keep the trees in the park, we need the tax revenue

Don’t worry, we’ll come back and plant trees later.

potential engineering conflicts

•Tree litter may clog pipes/outlets•Maintenance may be more difficult•Tree roots may puncture underdrains/filter fabric•Trees can reduce storage capacity•Limited maintenance access•Trees may compromise embankment stability

Source: Tim Schueler

conditions that limit tree growth in

stormwater/urban practices

• Compacted soils/limited soil volume

• High temperatures/drought• High winds• Physical damage from

mowers/browsers• Too much/too frequent

inundation• Urban pollutants (metals,

chloride)• Ice damage/scour

most stormwater projects are…

• built on the least valuable land

• constructed at the lowest cost

• designed only to manage quantity

• not intended for people

• UGLY!!!

Historic Approach to Stormwater Management: move water off the site as quickly as possible

typical stormwater pond with no trees

More recent approach is to temporarily detain runoff to control peak flows – this does not reduce

the volume of runoff or mimic pre-development hydrology

What opportunities are we missing?

Why integrate trees and stormwater?

• Potential benefits of trees in stormwater treatment practices: infiltration, pollutant removal, stabilization, habitat, reduced mowing costs, landscaping value

• Implementation of stormwater forestry practices has been limited - lack of collaboration among foresters and stormwater engineers

• Use of trees (non-structural practices) is more cost-effective than use of structural practices

Stormwater projects can reduce water quantity, improve water quality…and be beautiful.

Stormwater practices can be amenities for people…

• Good Stormwater Practices = Water Amenities

how do we get there?

stormwater forestry practice concept designs…what if?• Wooded wetland• Bioretention and bioinfiltration• Tree check dam• Alternating side slope plantings• Multi-zone filter strip• Forested filter strip• Linear stormwater tree pit• Stormwater dry pond

Third Generation The Forested Wetland—water quality + tree benefits

Wooded Wetland

Tree Clusters in Stormwater Ponds and Wetlands

Bioretention with trees

tree check dams

tree check dam section tree check dam axon

Alternating side slope plantings

Multi-zone filter strip

Forested filter strip

linear stormwater tree pits

stormwater drypond

Stormwater forestry opportunities in a watershed

• Schools• Parks• Highway rights-of-way • Vacant lots• Streams and shorelines• Utility corridors• Street medians and roadways• Parking lots• Home lawns (education and incentives are key)

Opportunities to build urban tree canopy that can enhance stormwater treatment

and improve watershed health

schools

forest cover in parks limits runoff

highway right of way

vacant lots are a good opportunity for reforestation and

neighborhood beautification

Combining Reforestation and Soil Compost Amendments to Restore a Vacant Urban Lot

Reforesting stream buffers provides stream shading, bank stabilization, pollutant removal and other benefits

planting under utilities

Plant Trees During Development/Redevelopment

More shade means more time between repaving. 20% shade on a street improves pavement condition by 11%, which is a 60% savings for resurfacing over 30 years.

Planting along local roads

local roads median planting

Trees planted in a median strip

A typical cul-de-sac is a large expanse of pavement with no vegetation

Trees can be incorporated into cul-de-sac islands

No infiltration of rainfall& limited soil volume

Parking Lots

Trees in parking lots perimeter

What opportunities

are we missing?

So what’s with turf cover? fastest growing land cover in the US!

Sources: MTC (1996), VASS (1998) and PTC (1989)

(% of total turf cover)

the truth about turf

• Turf is fastest growing urban land cover in United States

• High compaction of urban soils produces more urban runoff, particularly at edges

• Lawns have highest nitrogen and phosphorous concentrations of any urban source area

See Urban Watershed Forestry Manual Part 1

Trees planted on home lawns provide shade and other benefits at maturity

Use trees to: block winter winds,

shade summer sun,

accent important views, and

screen private areas

Residential landscapes can be functional and beautiful

Stormwater Design Amenities, Emerging Discoveries

• Visible water trail that intrigues and Engages• Character of Water Design that responds to context• Information systems that educate• Design for Maintenance that shows intention• Physical Accessibility• Multiple Use • Public Support• Perceived Value • Municipal Support

Source: Echols, Pennypacker 06

12th street green street, portland oregon

Stormwater runoff from 8000 square feet flows downhill along the existing curb unit. Stormwater planter system reduces intensity of

a 25 year storm event by 70% or more.

Infiltration = 4 inches per hours

Seattle SEA Streets

Seattle SEA (Street Edge Alternatives) Streets Project uses graded swales to reduce impervious cover by 11% over

traditional streets. Project has decreased stormwater volume by 97 percent.

A concept drawing of the $800,000 street edge alternative (SEA) project shows 14-foot-wide curved street, swales, additional trees, vegetation and diagonal

parking areas for cars.

SEA Streets

The Cascade Prototype project at N. 110th St. after most of the construction has been completed and

before planting.

• Stephen Epler Hall, Portland State

Visible water trail that intrigues and Engages

Growing Vine, Seattle Washington

“beckoning cistern” by buster simpson

alternative sidewalk design

Traditional sidewalk designs utilize individual tree pits (left) which confine roots; alternative designs cluster trees (right), which allows them to share rooting space. This reduces the

need for tree roots to grow under pavement.

alternative sidewalk materials

• Reinforced or thicker concrete slabs

• Pervious concrete

• Decomposed granite and compacted gravel

• Permeable pavers

• Recycled rubber

• Other ideas? *Goal is to allow tree roots to grow under sidewalks without causing damage

so what if we…

create watershed projects that target areas and actions needed to protect, expand, reforest and integrate tree cover with other development, stormwater, and landscaping activities?

create plans that facilitate goal setting and allows tracking of progress?

• What opportunities can you identify to integrate urban forestry, watershed forestry, engineering and design?

• What if….?

Using Trees To Reduce Stormwater Runoff

trees stormwater

stormwater runoff

stormwater engineers

urban trees

dont trees

stormwater projects

stormwater drypond

good stormwater practices

Technology

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