Nashville’s Green Infrastructure
Master Plan
Andy Reese
AMEC Earth & Environmental
Study Objectives:
1. Estimate impact of GI on volume
reduction in CSS area
2. Identify potential projects
3. Explore design criteria for infiltrationbased controls
4. Investigate incentive and alternate
funding opportunities
CSS Area
• 12.31 square miles
• 1.36 B gal/yr of
runoff
• Impervious area
even split between
rooftops, roads and
parking
• 45.6% average
impervious cover
(30% to 80%)
“retain” means…
Evapotranspiration (“up”)
Alternate Use
(“out”)
Infiltration (“down”)
We looked at:
• Harvesting and Reuse
– Cisterns and rain barrels
• Green Roofs
• Treatment Structures
– Bioinfiltration, tree boxes, infiltration ditches,
porous surfaces
• Tree Cover
Harvesting and Reuse
• Daily rainfall 1948-2009
simulation tool
developed
• Average 66 gpd runoff
per 1000 sf roof
• Continuous modeling
for cisterns and rain
barrels for design
criteria development
and removal estimates
Green Roofs
• 2,132 flat roof buildings
in the CSS area
• 20.6 million SF of
potential Green Roof
area
• Typical Green Roof
removes 55% of all
rainfall
• 343,000,000 gallons
annual removal
Structural Controls
• Bioinfiltration, porous
surfaces, tree planter
boxes were simulated
• Hourly continuous
simulation 1971-2006
• Volume removal
through infiltration
and/or evapotranspiration
• 80% removal of volume
is attainable
Urban Trees
• Existing 19.5% canopy
cover
• 51,800 acceptable new
planting sites identified
• A tree intercepts 7.3%
and transpires 54.8%
of rain that falls within
its canopy.
• 660,000,000 gallons
can be removed
annually by new trees
Practice Results Summary
•
•
•
•
•
Cistern - 600 gal average
demand equals runoff
Rain Barrel – one barrel 48
hour drawdown
Green Roof – 4” media
depth 75% of rooftop
Tree – 30’ canopy diameter
Structures – 10:1 impsurface area ratio, 0.3 in/hr
infiltration, 2’, 2.5’ and 3’
media depth respectively
Potential Pilot Projects
• Interviews with staff
• Criteria matrix for pilot
projects
• Problem area
identification and
assessment
• Preliminary and final
pilot project selection
• Preliminary design
concept
West Eastland Avenue
Est. Volume Reduction
3.8 million g/yr
Features
Bioretention
3,800 sf
Porous Pavement
(on-street parking)
6,000 sf
Pervious Pavers in Alley
6,975 sf
Hume Fogg School
Est. Volume Reduction
347,000 g/yr
Features
Water Harvesting Tanks
13,200 gallons
Extensive Green Roof Tray
System
12,650 sf
Farmer’s Market
Est. Volume Reduction
4.0 M g/yr
Potential LID Features:
1. water harvesting/ irrigation
• irrigation - capture roof runoff for use
on-site (existing nurseries)
2.
water harvesting/ flushing
• capture roof runoff for flushing
3.
permeable pavement
• heavy duty permeable pavement in
drive areas
Features
4.
permeable pavement
• reduce impervious pavement in parking
Water Harvesting
135,000 gal
Pervious Pavers
16,370
Pervious Concrete
15,350
5.
tree planting
• tree planting along Rosa Parks Blvd.
Metro Parks Administrative
Facility
Est. Volume Reduction
6.1 M g/yr
Features
Water Harvesting Tank
54,800 gal
Bioswale
16,500 sf
Green Roof
1,550 sf
Pervious Concrete Parking
6,200 sf
Pedestrian Pervious
Pavers
1,500 sf
How to develop an alternate
design criteria?
Volume reduction is mandated in
the next round of MS4 permits
Nashville Airport
Daily Values 1/1/1948 - 8/31/2009
8
7
Rainfall Depth (in)
6
Total Rainfall
Continuous Rainfall
“Everything should be made as
simple as possible but not simpler”
4
5
Albert Einstein
3
2
1
0
50%
60%
70%
80%
% Less Than
90%
100%
When volume is a target then…
• The following become
important:
– Inter-storm-event dry
periods
– The many small
storms
– Vegetation cover and
green space
– Annual pollutant
removal values
• The following lose
relevance:
– Older “event based”
design criteria
– Big storms
• Continuous
simulation with
simple criteria will be
key
Green Infrastructure Signatures
1.
Infiltration-based BMPs
–
–
–
2.
Bioretention
Porous Surfaces
Amended soils
Rainwater Harvesting and
Use
–
–
3.
Cisterns
Rain Barrels
Evapotranspiration
–
–
Trees
Green roofs
Initial
“Kerplunk”
Storage
Volume
Volume Reduction Rate
Alternative
Criterion:
Mimic
Annual
50% Tree
Canopy Annual
Volume
CaptureCapture
Requirement
D
C
B
A
Example Results - Bioinfiltration
Cistern Sizing
2 barrels per
residence
with 48 hour
drain time
yields 23%
residential
rooftop runoff
removal
Do Rain Barrels Work?
No
tB
ein
gD
r ai
ne
d
!!
No
No
87% capture
40% Impervious DCIA
87% capture
vs. 58% capture
Nature + Man
Big Shift
Steps to “retain” water?
1. The very best way is to develop in
such a way that little water runs off –
“Better Site Design”
58% capture
87%
vs. 75%
58% capture
66% capture
vs. 96% capture
Steps to “retain” water?
1.
2.
3.
The very best way is to develop in such a way
that little water runs off – “Better Site Design”
After that has been done (or if it is too late)
then look to other kinds of Green
Infrastructure controls…
If not possible then look for (1) flow through
treatment, (2) off site mitigation, or (3)
payment in lieu of controls
It can change our local design criteria…
Nashville, TN
Bioinfiltration
Current Tree Canopy
Potential Tree Canopy
Trees remove 60% of annual rainfall
87% capture
vs. 87%
58% capture
Steps to “Mimic Nature”
0.42” vs. 1.2” Green Infrastructure capture
This means you get
better mimicry at
35% of the G.I.
volume
Next Steps
•
•
•
•
•
•
•
10 rain garden demonstration projects
6 other demonstration projects
EPA scorecard process with staff
Stakeholder input process
New ordinance, checklists, processes
Volume-based design criteria
Staff and public education
Howdy from Tennessee!!
I brought you the
internet in 1985 and now
I got me a new surprise
for all my SESWA
friends… climate change
induced flooding
Actually right now Nashville isn’t
focused on Green Infrastructure
Questions?
[email protected]