Using Trees To Reduce Stormwater Runoff

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Information about Using Trees To Reduce Stormwater Runoff

Published on October 9, 2008

Author: watershedprotection

Source: slideshare.net

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?

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”

save energy

improve air quality

provide habitat

better quality of life

neighborhood stability

aesthetic values

increase property value

reduce noise

good for business

?? 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?)

… 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

not in most municipalities

not enough to matter to most developers.

not enough to equal their true value

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.

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

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

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

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

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

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!!!

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

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

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

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

Wooded Wetland Third Generation The Forested Wetland—water quality + tree benefits

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 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

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)

schools

parks

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

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

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

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

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

Stephen Epler Hall, Portland State

 

“ beckoning cistern” by buster simpson Growing Vine, Seattle Washington

 

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

Reinforced or thicker concrete slabs

Pervious concrete

Decomposed granite and compacted gravel

Permeable pavers

Recycled rubber

Other ideas?

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?

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….?

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

What if….?

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