A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges

8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges

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backgRouNd

Nashville, located on the Cumberland River in Tennessee,

covers 526 square miles and has a metropolitan area that

spans 13 counties. The Metro Nashville area still has 47

percent o its urban tree canopy; in the city center, the

gure dips to 13 percent.1 The city’s combined sewer system

(CSS) was built in the late 1880s. It carried both stormwater

and sewage to the Cumberland River without treatment

until the late 1950s, when the city constructed the Central

Wastewater Treatment Plant to treat wastewater prior to

release. Today Nashville has a CSS servicing 7,878 acres, or12.3 square miles, in the core o the city. Its land cover is 46.5

percent impervious and contains 19.5 percent o the urban

tree canopy.2 O the 2,500 miles o streams running through

Nashville and Davidson County, 350 miles are on Tennessee’s

ocial list o impaired waters.3

NashvI lle , TeNNessee

emeRald cITy cRITeRIa*

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portion o the existing impervious suraces?

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n3 Long-term Green Inrastructure (GI) Plan?

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NashvIlle’s sToRmwaTeR maNagemeNT

backgRouNd

In August 2007, the Metropolitan Government o Nashville

and Davidson County signed a consent decree with the

United States and the state o Tennessee that called or

a nine-year plan to reduce the estimated 765.2 million

gallons o combined sewer overfow (CSO) discharged to the

Cumberland River each year.4 In response, the Metropolitan

Department o Water and Sewerage Services o Nashville

and Davidson County (MWS) are currently leading eorts

to develop a CSO long-term control plan. In 2008, Metro

Nashville established a Stormwater Master Planning

District covering the entire CSS area and directed MWS

and other Metro Nashville departments to develop a green

inrastructure plan or the area; it was completed in 2009 andis explained below.5 The remainder o Nashville is serviced by

separate sewer systems that are regulated through a National

Pollutant Discharge Elimination System (NPDES) Phase I

MS4 permit.6 A new MS4 permit or Nashville is expected to

be issued in late 2011 or early 2012.

Responsibility or Metro Nashville’s stormwater program

also belongs to MWS,7 and in 2006 the agency updated Metro

Nashville’s Stormwater Management Manual (SWMM).

The manual provides the ramework or site development,

including erosion and sediment control during construction

and post-development water quantity and quality

requirements. The 2006 SWMM contains guidance or greeninrastructure practices including green roos, bioretention,

and use o pervious pavement.8 MWS is currently in the

process o developing a new volume o the SWMM designed

to encourage the use o green inrastructure, which

will establish an alternative compliance path based on

stormwater volume reduction and will provide incentives

or the use o bioretention, permeable pavements, trees,

green roos, cisterns, and other green inrastructure practices

that reduce stormwater volume. The approach will remain

voluntary until required under the city’s new MS4 permit.

Under Mayor Karl Dean’s guidance, the use o green

inrastructure to address stormwater and fooding concerns

has taken on increased signicance. In 2008 Metro Nashville joined a group o local governments promoting sustainability

through peer-to-peer advice on stormwater issues. That

same year, the mayor signed a green building permits and

green certicate o occupancy ordinance; appointed an

environmental sustainability manager; and created both a

Green Ribbon Committee and a Green Team Committee,

whose members, among other things, provided guidance on

the use o green inrastructure to address stormwater runo

and commissioned a downtown Tree Master Plan.

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In 2009, the Green Ribbon Committee released a ull

report that set orth 16 goals, including the establishment o

tree canopy and tree-planting objectives or various property

types to achieve; the greatest reduction o stormwater runo possible; the establishment o a dedicated source o unding

or stormwater management; and the removal o all Nashville

streams rom the state’s list o impaired waters by 2020. 9 In

May 2010, these eorts were diverted or a time to deal with

the atermath o a catastrophic food that caused the loss o

11 lives and more than $2 billion o private property damage.

As the city recovered rom the experience, however, a new

approach to open-space planning took shape. Since then,

Nashville has moved orward with a plan to buy and remove

more than 300 structures in the foodway to restore and

preserve the land as open space. The city is also addressing

stormwater by increasing the number o incentives and

requirements that encourage the use o green inrastructurepractices.10

NashvIlle’s gReeN INfRasTRucTuRe

masTeR plaN

Downtown Nashville’s 12.3-square-mile CSS was designated

a stormwater planning district in 2008 under an ordinance

that directed MWS, the Metropolitan Planning Department,

the Metropolitan Development and Housing Agency, and the

Department o Public Works to create a Green Inrastructure

Master Plan; the plan was nalized and approved in the all o

2009. In addition to identiying various green inrastructurepractices in the stormwater planning district, the plan

provides a detailed analysis o the impacts that our types o

practices have on the volume o stormwater runo: rainall

harvesting; green roos; urban trees; and three inltration

practices (bioinltration areas, permeable suraces, and

tree planters).

For rainall harvesting, the plan evaluated the eect that

capturing runo rom the 1,300 acres o rootops in the CSS

area would have on stormwater runo. On average, rootops

in Nashville were estimated to generate 65.5 gallons per day

per 1,000 square eet, or a total o 1.36 billion gallons o

runo per year. I all o the 708 buildings suitable or green

roos were converted, 112 million gallons o runo could beremoved rom the annual total. Similarly, the plan evaluated

the impact o additional tree plantings within the CSS area

and ound 51,800 acceptable new planting sites; these

would add 811 acres o urban trees and increase the canopy

coverage rom 19.5 percent to 30 percent. By doing so, Metro

Nashville could expect to reduce stormwater volume by 660

million gallons annually. Similar evaluations were prepared

or other green inrastructure practices. In addition, the

plan identied 50 potential green inrastructure projects

that MWS could implement and provided brie overviews

o six. Under the ordinance, the list o green inrastructureprojects must be updated annually, and MWS was authorized

to promulgate and enorce rules and regulations or the

implementation o green inrastructure techniques.11

NashvIlle’s opeN-space plaN:

Nashville: Naturally

In April 2011, Nashville released its rst open-space plan,

which aims to protect 22,000 acres over the next 25 years,

including 10,000 acres o foodplain. The plan “Nashville:

Naturally,” builds upon the lessons learned rom the food

o 2010 by ocusing protection eorts on land in each o the

nine bends o the Cumberland River. The network o open

spaces is intended to provide buers against foodwaters,

improve water quality, protect agricultural soils, and

oer recreational opportunities. Other goals include the

restoration o the endangered Nashville craysh population

and the removal o all area streams rom the impaired waters

list. The plan urther aims to double the 85-acre downtown

tree canopy within 10 years and to transition 110 acres, or 20

percent, o the suitable impervious suraces downtown to

pervious or natural plantings.

To help Nashville reach these goals, the plan makes

numerous policy recommendations to connect wildlie and

water networks, support urban and rural arming, connectpeople to green inrastructure, and preserve historic and

iconic resources. From a stormwater perspective, some o the

more important recommendations are to:

n integrate Metro department activities related to orest and

water resource protection

n create incentives that encourage green inrastructure

stormwater management on private properties;

n establish a stronger stream buer to protect and restore a

riparian buer system;

n institute a no-adverse-impact policy that restricts

development in food-prone areas and requires

development that alters fooding conditions to mitigate

the impact o such actions; and

n explore sustainable open space unding and incentive

programs that could be oset by the creation o green

spaces such as green roos in dense urban areas.12




The Metro Council and Mayor Dean have already set aside

$5 million rom Metro Nashville’s capital spending budget to

begin an acquisition und; they expect to build the und with

private contributions. Additionally, they’ve taken the rst steptoward meeting the 22,000-acre green space goal by agreeing

to purchase a 135-acre ormer private airport or $1.2 million.

Doing so will serve to connect two adjacent parks, create

a 936-acre swath o open space, and provide an additional

buer to surrounding neighborhoods to protect them rom

uture food events. To raise unds, Metro is partnering with

the Land Trust or Tennessee and the Friends o Shelby Park. 13

oTheR gReeN INfRasTRucTuRe

INITIaTIves

Metro Nashville and MWS have implemented several other

projects to better engage and inorm the general public on

the purpose and utility o green inrastructure practices.

To encourage rain gardens, MWS has partnered with the

Nashville District o the Army Corps o Engineers and the

Cumberland River Compact, a nonprot organization that

engages businesses, individuals, community organizations,

and government in the restoration and protection o the

Cumberland River, to create a resource guide.14 In the spring

o 2011, the Cumberland River Compact, MWS, and Impact

Nashville built 50 rain gardens on residential properties with

the help o volunteers. The rain gardens were oered ree to

homeowners (or renters with owner permission) and were

concentrated within the watershed o Brown’s Creek, one o Nashville’s most polluted small streams.15 Plans are in place

or Nashville Metro to partner with the Cumberland River

Compact, the Nashville Tree Foundation, the Nashville Earth

Day Festival, and Sound Forest to plant shade trees with

the greatest benets or stormwater mitigation on selected

residential properties and in community spaces around

Davidson County. While individual websites exist or each

program, Nashville has developed a unique site, Impact

Nashville (impactnashville.net), aimed at engaging residents

in various citywide initiatives.

In 2009, Nashville completed a $4.5 million pilot

“green street” project along Deaderick Street, converting a

major downtown road into a pedestrian-riendly corridorby incorporating sidewalk-level bioretention planters,

bioretention curb bump-outs, a landscaped median, and

porous concrete sidewalks, and by planting 102 shade trees.

It is also one o the rst green streets constructed in the

Southeast.16

fINaNce sTRaTegy

In 2008, MWS prepared a stormwater business plan that

ound the stormwater program’s annual budget o $12 million

was below projected needs; an additional $85 million wasnecessary just to resolve the backlog o projects, and to ully

operate the stormwater program an annual operating budget

o $25.8 million was required. To ll the gap, the business

plan recommended that a dedicated user ee or stormwater

drainage be developed, with the rate structure based on

a property’s total impervious surace area. For customers

with existing MWS water accounts, the ee would be billed

monthly on the MWS utility bill. For customers without

water accounts, a quarterly “stormwater only” bill would

be issued.17 In 2009, a stormwater user ee came into eect

or Metro Nashville. Currently, monthly rates or residents

range rom $0 to $4.50, with an average residential bill o

$3.00.18 Nonresidential property rates range rom $0 to $400,

depending on the amount o impervious surace.19 Property

owners can receive a credit or mitigating stormwater runo

impacts through education or the implementation o source

controls or water quantity or quality (up to 20 percent or

each practice, capped at 50 percent).20 However, the monthly

stormwater ee does not appear to be signicant enough to

make much dierence in customer behavior. In other words,

the cost savings resulting rom stormwater improvements

seem unlikely to oset the cost o installing them.

In addition to the stormwater ee, Metro Nashville draws

rom its general und, internal service und, ederal unds,

and private unding to implement stormwater, open space,green building, and tree planting programs. And to encourage

green buildings, the Metro Codes Department established

a ast-track permitting process in 2009. To receive the green

stamp o approval, units must be third-party certied.21

However, no additional incentives other than ast-track

permitting are oered at this time, nor has Metro Nashville

included any stormwater management requirements that

encourage the use o green inrastructure practices in their

green building permitting process, such as requiring green

roos or the use o volume-based controls on-site.

Like many o the original case studies in 2006, Nashville’s

green inrastructure programs are still developing tools

and incentives used to encourage green inrastructurepractices are expected to increase over the years. The Green

Inrastructure Master Plan, which provides a summary o

various incentive practices that other cities use to encourage

green inrastructure, provides some hints as to incentives

Metro Nashville might implement to encourage participation

From these incentives, ve were selected or urther




consideration or Metro Nashville: stormwater ee discounts,

rebates and installation nancing, development incentives,

grants, and awards and recognition programs.22 In addition,

Metro Nashville is working to identiy incentives that will beincorporated in the upcoming stormwater management low-

impact development manual.23

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RefeReNces

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Am pjc,” acc a h://.ahvi.g/imag/

ca//ca..

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GIacr101120..

3 Gaham a Ha.

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ma ma aig iic,” acc a .ahvi.gv/mc/

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Cc 2, ya 7,” acc a .ahvi.gv/ma/c/h/

Cc2_ya7_Aa_r..

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2013,” acc a .ahvi.gv/a/ci/c/ma_

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Ma,” acc a .ahvi.gv/aiaii/gc/ix.a.

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naia G Iac Cc, h a h naia

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23 A, emi. pa cmmicai, J 5, 2011.

Documents

A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges