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8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges
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Ni, Tnn 1 | Rt t Rir II
NashvIlle, TeNNessee A CAse study o How Green InrAstruCture 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*
Requirement to use GI to reduce some
portion o the existing impervious suraces?
Incentives or private-party actions?
Retention Standard?
n3 Long-term Green Inrastructure (GI) Plan?
n3
Guidance or other afrmative assistanceto accomplish GI within City?
n3 Dedicated unding source or GI?
Tt critri sr
3
o a i 6
8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges
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Ni, Tnn 2 | Rt t Rir II
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.
nahvi cm h f g i h sha, amig a maj a i a ia-i
ci icaig iak-v ii a, ii c m-, a aca mia, cc iak,
a aig 102 ha .
8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges
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Ni, Tnn 3 | Rt t Rir II
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
8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges
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Ni, Tnn 4 | Rt t Rir II
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
8/3/2019 A Case Study of How Green Infrastructure is Helping Manage Urban Stormwater Challenges
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Ni, Tnn 5 | Rt t Rir II
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
*emeRald cITy RaTINg sysTem
each h cii f i Rooftops to Rivers II i a a i g
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a iac. th cii vag ig i caiv a. th
vi iia a cmmcia a f
iva i a aiz h mi f vi g
iac. I h, h a chagig h cii k a ci.
nrdC’ ema Ci raig sm iif ix aci cii h
ak maximiz hi g iac ivm. o mic ic cma ci ah, ggahica,
ai, ga a h ic. Ia, i iif h
c ac cmm ac cc ha nrdC iv
a ia m a g iac cmmim.
o ci f, phiahia, i akig ach h aci
iif, ahgh ach ci i akig a a .
RefeReNces
1 Gaham, J., a I. Ha (2010). “M nahvi t Ca
Am pjc,” acc a h://.ahvi.g/imag/
ca//ca..
2 M wa svic (2009). “Mia Gvm
nahvi a davi C G Iac Mapa,” acc a .ahvi.gv/ma/c//
GIacr101120..
3 Gaham a Ha.
4 M nahvi (2009). “si oiac n. bl2008-345:
a iac amig ti 15 h Mia C ca a
ma ma aig iic,” acc a .ahvi.gv/mc/
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5 M wa svic. (2009). “Mia Gvm nahvi
a davi C G Iac Ma pa.”
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C Ms4 npdes pmi n. tns068047 Aa r: pmi
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Cc2_ya7_Aa_r..
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i_a..
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acc a .ahvi.gv/ma/g/ix.a.
9 G ri Cmmi evima saiaii (2009).
“tgh Makig nahvi G, a h Ha Ka da,
Ma,” acc a .ahvi.gv/aiaii/gc/ix.a.
10 tak m Ma Ka da’ K A a h i
naia G Iac Cc, h a h naia
Cvai taiig C, shh, w Vigiia,
a 24, 2011, acc a h://.agiigi.cm/
GIacrcnGIC2.hm.
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a davi C G Iac Ma pa.”
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o sac pa: A r nahvi: naa,” acc a h://
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13 M nahvi (A. 14, 2011). “Ma da, la t
t ra o sac pa” ( a), acc a h://
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14 Mia dam wa a sag svic
nahvi a davi C (2011). “rai Ga nahvi,”acc a .aigaahvi.cm/ix.hm.
15 Imac nahvi (2011). “V oi: rai Ga
paig,” acc a imacahvi./ii/114.
16 M pic wk (2009). “daick: t’ i ‘G
s’,” acc a .ahvi.gv//jc/aick.a.
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2009–2013.”
18 M wa svic (2010). “Mia nahvi a davi
C Ms4 npdes pmi n. tns068047 Aa r: pmi Cc
2, ya 7.
19 M nahvi, 2011. “Ca wa Iac pgam,”
acc a h://.ahvi.gv/a/ci/ma_.a.
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Maa,” acc a .ahvi.gv/a/ci/c/
swuCiMaa..
21 M nahvi, 2011. “G Cci,” acc a .
ahvi.gv//g_g.a.
22 M wa svic (2009). “Mia Gvm nahvi
a davi C G Iac Ma pa.”
23 A, emi. pa cmmicai, J 5, 2011.