Green Infrastructure Implications

for Tennessee

Curt Jawdy, PE

865-310-4727

New MS4 Permit Language

Green Infrastructure is…

• “An approach to wet weather management that is cost-effective,

sustainable, and environmentally friendly. Green Infrastructure

management approaches and technologies infiltrate,

evapotranspire, capture and reuse stormwater to maintain or

restore natural hydrologies."

• “The interconnected network of open spaces and natural areas,

such as greenways, wetlands, parks, forest preserves and native

plant vegetation, that provide wildlife habitat, natural drainage,

recreational opportunities and help to sustain our Nation’s cities..."

• “A cost effective and environmentally friendly approach to

mitigating sewer overflows and works by diverting stormwater from

the sewer system and directing it to areas where it can be

infiltrated, evapotranspired or reused.

I will be focusing on…

Potential Urban Ecosystem Benefits

•Increased infiltration contributes to reliable base flows.

•GI treatments decrease channel erosion potential.

•Infiltration allows filtration of suspended pollutants, resulting in better water quality.

•GI is able to cool urban runoff by allowing the ground media to moderate temperatures.

• Infiltrate– Bioretention/rain gardens

– Pervious pavement

– Vegetated swales

• Evapotranspire– Bioretention/rain gardens

– Tree planters

– Vegetated swales

• Harvest and reuse – Cisterns

– Rain barrels

Primary GI Pathways

Hydrograph

Pre-

developed

Urbanized

Urbanized

w/detention

Channel

Eroding

Flow

Flow Duration CurveStorm Flows Mid Range Low FlowsMoist Conditions Dry Conditions

Pre-

developed

Decreased

Baseflow

Urbanized

Urbanized w/detention

Increased Erosive VolumeChannel

Eroding

Flow

Question 1:

• Do statewide hydrologic differences require

custom GI policies?

• a la Delaware

Median = 1.5%

Median = 7.0%

Median = 8.5%

Slopes

Hydrologic Soil GroupsMemphis HSGs

A

B

C

D

Nashville HSGs

A

B

C

D

Knoxville HSGs

A

B

C

D

Legend

MemphisHSGs

HydrolGrp

A

B

C

D

Memphis HSGs

A

B

C

D

Nashville HSGs

A

B

C

D

Knoxville HSGs

A

B

C

D

Legend

MemphisHSGs

HydrolGrp

A

B

C

D

Memphis HSGs

A

B

C

D

Nashville HSGs

A

B

C

D

Knoxville HSGs

A

B

C

D

Surface Soils

Knoxville

Nashville

Memphis

Soils at 36” Depth

Knoxville

Nashville

Memphis

24-hr Rainfall Depth Exceedance

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90 100

Exceedance Frequency (%)

Rain

fall

Dep

th (

in)

Memphis

Nashville

Knoxville

Rainfall Patterns

Surface Ponding Runoff

Surface Evaporation

Pre-Development SWMM Model

Infiltration per Green & Ampt

Unsaturated Soil

Saturated Soil

Water Table per moisture relations

Deep Percolation

Evapotranspiration • Goal

– Determine the water

balance prior to

development

• Parameters

– Soil physical

properties from

texture

– Slopes from GIS

– 3’ deep soil matrix

• Forcings

– Rainfall and PET for

1971-2006

Rainfall

Pre-developed Water BalanceKnoxville Water Balance

Evaporation

Direct

Runoff

GW

Recharge

Evapo-

transpiration

Memphis Water Balance

Evaporation

Direct

Runoff

GW

Recharge

Evapo-

transpiration

Nashville Water Balance

Evaporation

Direct

Runoff

GW

Recharge

Evapo-

transpiration

• Small differences for “average” conditions

• Some portion of GW recharge eventually becomes

quickflow and baseflow

State Hydrology Conclusions

• The variability of GI factors is greater within

counties than across the state, therefore

– Statewide policies make sense

– Significant flexibility must be available for

individual sites

Question 2:

• What impact will the 1” runoff reduction

requirement have on land use and runoff

volume?

Define “inch”

• Capture runoff from a 1” storm

• What intensity of storm?

– Assume 1” in 1 hr

Runoff Bed Depth Bed Area Bed Area Bed Area (in) (ft) (sf) (% of total) (% of pervious)

Knoxville Exurbs (2 acre) 0.112 3 3388 0.8% 0.9%

Knoxville Suburbs (1 acre) 0.187 3 5656.75 1.3% 1.6%

Knoxville Residential (1/4 acre) 0.355 3 10738.75 2.5% 4.0%

Knoxville Townhomes (1/8 acre) 0.61 3 18452.5 4.2% 12.1%Knoxville Commercial 0.794 3 24018.5 5.5% 36.8%

Knoxville Exurbs (2 acre) 1 3 30250 6.9% 7.9%

Knoxville Suburbs (1 acre) 1 3 30250 6.9% 8.7%

Knoxville Residential (1/4 acre) 1 3 30250 6.9% 11.2%

Knoxville Townhomes (1/8 acre) 1 3 30250 6.9% 19.8%

Knoxville Commercial 1 3 30250 6.9% 46.3%

ScenarioCriteria

1"

ov

er

an

ho

ur

1"

pu

re

run

off

Location

Urbanization Effects on Land

Landuse

%

Impervious

Flow

Length (ft)

Ponding

Depth (in)

Pre-developed 0 300 0.35

Exurbs (2 acre lots) 12 200 0.3

Suburbs (1 acre lots) 20 150 0.25

Residential (1/4 acre lots) 38 100 0.2

Townhomes (1/8 acre lots) 65 75 0.15Commercial 85 50 0.15

• Pre-development models were altered to represent

conditions after settlement

• Impervious areas were routed directly out to

represent a piped conveyance system

Surface Evaporation

Infiltration per Green & Ampt

Evapotranspiration

Unsaturated Media

Saturated Media

Water Table per moisture relations

Deep Percolation

Surface PondingSurface Ponding Runoff

Surface Evaporation

Runon

3’ Deep Bioretention Model

Infiltration per Green & Ampt

Unsaturated Soil

Saturated Soil

Water Table per moisture relations

Deep Percolation

Evapotranspiration

Source Area GI Facility

Rainfall Rainfall

Memphis Bioretention

Memphis 1" Runoff Bioretention

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Bioretention

Pre-development target

Pre-Development

Level

Nashville Bioretention

Nashville 1" Bioretention

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Bioretention

Pre-development target

Pre-Development

Level

Knoxville Bioretention

Knoxville 1" Bioretention

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Bioretention

Pre-development target

Pre-Development

Level

Question 3:

• How much does soil infiltration rate affect GI

performance?

Infiltrating

flow w/o

runoffMedia has

saturated and

begun to

create a water

table

Runoff occurs

only when

media and

ponding depth

are both full

Causes of

Runoff

• Flow out of the cell is the limiting factor

• VERY few storms can overwhelm the inflow capacity

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Site

Ru

no

ff (

in/y

r)

Bioretention Percolation Out Rate (in/hr)

3' Deep Bioretention at Commercial Site in Knoxville

Importance of Testing

• Infiltration rates can vary

significantly over small

areas

• Tests must be performed

at the bottom of proposed

facilities

• Siting facilities well is

crucial

Question 4:

• Do I have any alternatives to putting in

constructed GI facilities at sites with tight

soils?

Impervious Disconnection

• Is the practice of directing

runoff from impervious

areas to flow over pervious

areas and thus allowing

infiltration

• Goal: check for possibility

for maintaining pre-

development hydrology

• Method: model various

levels of development with

disconnection

Memphis Disconnection

Memphis Disconnection

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Disconnection

Pre-development target

Pre-Development

Level

Nashville Disconnection

Nashville Disconnection

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Disconnection

Pre-development target

Pre-Development

Level

Knoxville Disconnection

Knoxville Disconnection

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 20 38 65 85

Imperviousness (%)

Ru

no

ff (

%)

Developed

Developed w/Disconnection

Pre-development target

Pre-Development

Level

Additional Options

• Reuse BMPs

– Reuse water for toilets

• AMEC work in Nashville showed reasonably sized cisterns can

provide 40% - 80% runoff reduction

– Reuse water for irrigation

– Reuse water for cooling towers

– Reuse water for car washing

– Reuse water for ….

• Evapotranspiration BMPs

– Green Roofs

• AMEC work in Nashville showed ~55% runoff reduction

– Lush, shallow and large bioretention

– Trees

Question 5:

• Can we plan GI well with event storm

techniques?

Benefits of Event Storm Design

• Good at designing

facilities to handle

flood flows

• Simple

• Well-known

Event Storm Shortcomings

• False: A hypothetical

storm shape

represents actual

rainfall events

• True: Actual storm

shapes vary widely

– some runoff is caused

by saturation over a

long storm

– some runoff is caused

by overwhelming

rainfall intensity

Event Storm Shortcomings• False: A 25-yr storm causes a 25-yr flood

• True: Existing soil moisture plays a large role

– especially for small storms (e.g. most storms)

Event Storm Shortcomings

• False: GI facilities

are always empty

at the beginning

of a storm

• True: GI facilities

may be partially

full

Event Storm Shortcomings

• False: Inter-storm periods aren’t important

• True: GI facilities mimic nature by evapotranspiring

and draining between storms

Nashville Water Balance

Evaporation

Direct

Runoff

GW

Recharge

Evapo-

transpiration

Single Continuous

Model Regulation

Regulate for outcomes:– Baseflows, WQ & Channel Stability

Flow duration curve

– Flooding

Flow peaks for X, Y and Z real storms

– Infrastructure

Safe elevations for X, Y and Z real storms

Being used by several U.S. cities

FDC StandardStorm Flows Mid Range Low FlowsMoist Conditions Dry Conditions

Disallow flow

increases here

I’m not sure we’re

ready for that…

Question 6:

• Is there any way to make this easier for my

local designers?

Are the current methods

really that simple?

Estimate BMP Area Required

Subcatchment Area 10 ha Bioretention Pervious Paving Veg. Swale Green Roof Cistern

% Impervious 50 % Media Depth (m) Media Depth (m) Media Depth (m) Media Depth (m) Storage Depth (m)

Capture Depth 25 mm 1 0.65 0 0.15 1

Porosity Porosity Porosity Porosity Porosity

0.4 0.4 0 0.58 1

Volumed Needed 1250 m3 Wilting Point Wilting Point Wilting Point Wilting Point Wilting Point

0.07 0.04 0 0.04 0

Effective Storage (m3/m2) Effective Storage (m3/m2) Effective Storage (m3/m2) Effective Storage (m3/m2) Effective Storage (m3/m2)

0.33 0.234 0.15 0.081 1

Area (m2) Area (m2) Area (m2) Area (m2) Area (m2)

2000 2000 500 500 100

Total Storage (m3) Total Storage (m3) Total Storage (m3) Total Storage (m3) Total Storage (m3)

660 468 75 40.5 100

Volume Supplied

1343.5 m3

• Should occur early in the planning process

and be easy enough for anyone to use

• Example above from AMEC work in

Edmonton

SWMM Municipal Water Template• Free government software

with simple GI tools

• Handles all aspects of site

stormwater analysis

• Pre-load local data

– Long-term rainfall & PET

– Inlet grate parameters

– Soil and GI media parameters

– Pollutant generation by land use

– Pollutant removal by BMP

AMEC is preparing a

template for Knox Co.

SWMM Define Prototypes1. Generic Section

3. Layer Specifics

2. Each prototype has

different layers

SWMM Place BMPs

• Simply define the area

for each BMP in each

subcatchment

Question 7:

• What about those difficult sites?

• Money for watershed projects!

Question 8:

• Great, where do I put them?

Difficulty of Location Choice

Channel

Improvement

Regional

Detention

Multiple

Bioretention

Cells

Pervious

Paving

Stormwater

Wetland

Flooding

Abatement

Volume/Quality

Abatement

GI Optimization

• Optimization routines

perform 1,000s of runs

to find the best mix

TN Optimization Trial

• SUSTAIN software

will be used to

optimize GI for a 600

ac. subdivision

• Results and lessons

learned will be shared

with MS4 Association

and AWRA

Let’s talk about:Questions?