Green Stormwater Infrastructure for Flood Risk Reduction

June 2016

Presented by:

Kari Mackenbach, CFM

ms consultants

Lynn Mayo, PE, CFM

AECOM

Green Stormwater

Infrastructure for

Flood Risk Reduction

Topics

1. Proposed ASFPM Policy Paper

2. Background

– Why is the issue important

3. Policy Paper

– Proposed terminology

– Use of green infrastructure for flood mitigation

– Calculating benefits of green infrastructure for flood mitigation

4. Next steps and comments on ASFPM Policy Paper

Green Stormwater Infrastructure for Flood Risk Reduction 2

Proposed ASFPM

Policy Paper

Consistent use of terms

Effectiveness of Green

Infrastructure for flood risk

reduction


Background


Natural water cycle: Pre-development


Natural water cycle: Post-development

• Increased

imperviousness

increases flows

• Decreased water

quality

• Increased erosion

and sediment in

streams


DistributedStormwater Features

Traditional RegionalTechnique

Integrated Stormwater Controls

Distributed stormwater controls close to runoff sources


Is this green infrastructure?


Is this green infrastructure?


Green Infrastructure Technology Types

Why are

consistent terms

important?


Definitions of “green infrastructure”

American Planning Association (first used term in 1994)*:

– Interconnected system of natural areas and open space that:

• conserves ecosystem values,

• helps sustain clean air and water, and

• provides benefits to people and wildlife

EPA Definition (first used term in 2007)*:

– Preserving and recreating natural landscape features

– Minimizing effective imperviousness to create functional and

appealing site drainage

– Treating stormwater as a resource rather than a waste product

• from Karen Firehock, 2010


Definitions of “green infrastructure”

US Army Corps of Engineers:

– Integration of natural systems and processes, or engineered

systems that mimic natural systems and processes, into investment

in resilient infrastructure

ASFPM:

– No Adverse Impact Toolkit… Green infrastructure is a proactive,

systematic, multifunctional model that views open space on a large

scale and better integrates open/green space planning with other

efforts to manage growth and development. It essentially uses

stormwater storage areas, water conveyance areas and other

natural flooded areas as part of the community infrastructure for

stormwater and flood damage reduction, as well as for parks, trails

and other recreation areas.


Can green stormwater

infrastructure really

reduce flooding?


How is green infrastructure and flooding connected?

Seeing the

significance of the

hydrologic impacts

of urbanization

over time


http://1558vs-d8ap17:81/deltekvision/Stock Photos/Water Resources/stromDrain_321496.jpg

Street

In a 1”

rainfall

Potential

Runoff:

7,040

gallons

2,500 s.f.

“impervious” x 1” rain =

1,559 gallons

of runoffStormdrain

1,500 s.f. house (& patio) x 1” rain =

935 gallons of runoff

+ “Green Concrete” Compacted Lawn

8,390 s.f. “impervious” x 1” rain (if infiltrates first ¼” of rain)

= 3,923 gallons of runoff

1,000 s.f.

driveway x 1”

rain = 624

gallons of

runoff

How is green infrastructure and flooding connected?

Green infrastructure flood risk reduction

potential benefits

Depends on type of GI: • Non-urban green infrastructure

• Urban green stormwater infrastructure

– New development

– Existing development


Non-urban green infrastructure

18Green Stormwater Infrastructure for Flood Risk Reduction

New development

Non-urban green infrastructure

• Land Preservation

• Wetland Preservation/Creation

• Floodplain and Stream Restoration (includes set-

back levees)


Non-urban green infrastructure success story

Charles River – USACE acquired 8,000 acres wetlands for

flood control for $9M

– Average Annual flood damage reduction

$17M


Urban green stormwater infrastructure


New development

Urban green infrastructure (examples)

• Green roof

• Pervious pavers or pavement


• Rain garden or bioswales

• Infiltration trench

Green stormwater infrastructure used

effectively for flood risk reduction

City of Columbus,

Blueprint Columbus

Green Infrastructure

Strategy



success story

Cuyahoga Falls

green infrastructure

mitigation park project

– FEMA grant to acquire 4 homes

– Space converted into green infrastructure

– Reduced flooding for remaining homes




University of

Louisville Belknap

Campus




Copenhagen,

Denmark Climate

Resiliency and Flood

Mitigation Strategy


On July 2, 2011, a cloud burst inundated Copenhagen

with 6 inches of rain in less than three hours, flooding

cellars, streets, and key roads. ($1.04 billion) in damage.

http://cphpost.dk/news/local/drenched

Proposed ASFPM

Policy Paper

Consistent use of terms

Effectiveness of Green

Infrastructure for flood risk

reduction


Green infrastructure flood risk reduction

potential benefits

Depends on type of GI: • Non-urban green infrastructure

• Urban green stormwater infrastructure

– New development or redevelopment

– Existing development




New development


To be effective flood risk

reduction, need to:

– Change land use to

impact curve number

(i.e., reduce runoff)

or

– Have available storage

at time of peak flow

Rainfall Hyetograph

Inch r

ain

fall



New

Development

GI can change land use to

impact curve number and

reduce flowa

GI can add storage so water

detained at time of peak flow a



New

Development

Existing

Development

GI can change land use to

impact curve number and

reduce flowa s

GI can add storage so water

detained at time of peak flow a s



New development or redevelopment


EPA study on GI for flood loss avoidance

(for new development or redevelopment)

• Assumed GI retained:

– Redevelopment: Approximately 0.7–1.3 inch rainfall

– New development: Approximately 0.9–1.5 inch rainfall

• Evaluated 20 HEC8 watersheds with and without GI retention out

to year 2040

• Benefits: Flood losses without GI / flood losses with GI

• Losses Avoided- HAZUS modeling results

– Present value 2020 to 2040 = $0.4 to $1 billion

– Average over 20 years = $30 to $65 million per year

Source: EPA, Flood Loss Avoidance Benefits of Green Infrastructure for Stormwater Management, December 2015



Existing development


Existing Development Green Stormwater

Infrastructure

Urban GSI Limitations:

– Limited impact for

downstream large

watershed

– Limited impact on large

flood events


Urban GSI Opportunities:

– Opportunities for localized

flooding

Typically limited land available to install green infrastructure

– Opportunities for small

flood events


May affectNot going to affect



Good News: Most flood reduction benefits come

from addressing smaller, frequent events.

May affectNot going to affect


Example estimated damages from different storm events in Concord, MA

Total expected dollar losses from flooded buildings and contents

10% (10 yr) 4% (25 yr) 2% (50 yr) 1% (100 yr) 0.2% (500 yr)

$593 M $749 M $905 M $1,199 M $1,459 M

Source: Concord, MA, Flood Risk Report




10% (10 yr) 4% (25 yr) 2% (50 yr) 1% (100 yr) 0.2% (500 yr)

$593 M $749 M $905 M $1,199 M $1,459 M

10% chance have $593M in damage any given year

$593M x 10% = $59M





10% (10 yr) 4% (25 yr) 2% (50 yr) 1% (100 yr) 0.2% (500 yr)

$593 M $749 M $905 M $1,199 M $1,459 M

10% chance have $593M in damage any given year

$593M x 10% = $59M

0.2% chance have $1,459M in damage any given year

$1,459M x 0.2% = $3M



Average Annualized Loss


Average Annualized Loss = estimated long-term weighted average value

of losses to a property in any single year




10% (10 yr) 4% (25 yr) 2% (50 yr) 1% (100 yr) 0.2% (500 yr)

$593 M $749 M $905 M $1,199 M $1,459 M

Total annualized losses from flooded buildings and contents

10% (10 yr) 4% (25 yr) 2% (50 yr) 1% (100 yr) 0.2% (500 yr)

TOTAL

Annualized

(& / yr)

$40 M $17 M $10 M $10 M $3 M $80 M


Commercial flooding- often most effective

Addressing “nuisance” flooding in commercial areas

– Typically more dense

development

– Loss of function, including

loss of access to area

– Customers stay away

– High content value

– Vehicle detour time and mileage

Potentially significant benefit of reducing

flood elevation by less than a foot


Non-urban green

infrastructure

– Potential flood

benefits for

large events

Urban green stormwater

infrastructure – existing

development

– Potential flood benefits

for small events

– Biggest long-term

benefits are from small

events

Urban green stormwater

infrastructure – new

development

– Potential flood

benefits for large

events

Summary


Next steps and comments on ASFPM Policy

Paper



Average Annualized Loss (AAL)

AAL = estimated long-term weighted average value of losses to a

property in any single year in a specified geographic area

AAL = (10% - 4%) * (Loss 10% + Loss 4%) / 2 +

(4% - 2%) * (Loss 4% + Loss 2%) / 2 +

(2% - 1%) * (Loss 2% + Loss 1%) / 2 +

(1% - 0.2%) * (Loss 1% + Loss 0.2%) / 2 +

0.2% * Loss 2%


Documents

Green Stormwater Infrastructure for Flood Risk Reduction