Pervious Concrete:

Hydrologic Design and Hydrologic Services

Stuart S. Schwartz

Center for Urban Environmental Research and Education

University of Maryland Baltimore County

Pervious Concrete Workshop

Waldorf, MD

9 December 2008

Overview

Stormwater Design Criteria

Pervious Concrete Design Criteria

Hydrologic Services: Effective Curve Number

Pervious Concrete & Environmental Site Design

Development Changes the Water Budget

From: http://www.buildingsoil.org/

Development Increases Runoff and Erosion

If imperviousness is the problem can pervious concrete be the solution?

Stormwater BMP Design•Uniform Sizing Criteria

•Pass-Fail Design

Pervious Concrete Design•Freeze-Thaw Criterion

•Drawdown Criterion

•Evaluate Hydrologic Services

Unified Sizing Criteria - “Pass-Fail”

•Rev – Recharge Volume

•WQv – Water Quality Volume

•CPv – Channel Protection Volume

•Qp – Overbank Flood

•Qt – Extreme Flood

Undisturbed subsoil

6” pervious concrete pavement

Clean Stone Subgrade ~30%

porosity

Basic Pervious Concrete System

Basic Pervious Concrete Section

Undisturbed subsoil

6” pervious concrete pavement

Underdrain

Clean Stone Subgrade ~30%

porosity

Overdrain

Pervious Concrete System Design Criteria

Risk-Based Operational Criteria

Freeze-Thaw Criteria: Pavement Saturation

Drawdown Criteria: Time Between Storms (e.g. Td=72 hrs)

Hydrologic Performance Criteria – “Partial Credit”

Infiltration & Detention

Effective Curve Number

Freeze-Thaw

Performance

Site Preparation:

Cleveland, OH

•Positive Drainage

•Demonstrate Cold Weather

Performance

•Pervious Concrete Detention

Tropical Storm Katrina

Storm Totals:

31Aug 2005 - 07:27 EDT

Cuyahoga Sustainability Network

Cold Weather Test Plot

Undisturbed subsoil

6” pervious concrete

Underdrain

Clean Stone Subgrade

~30% porosity

Undisturbed subsoil

6” pervious concrete

Overdrain

Clean Stone Subgrade

~30% porosity

Stormwater Design:

Freeze Thaw &

Drawdown

Overdrain to Minimize

Pavement Saturation

Underdrain to Provide

Positive Drainage &

Drawdown

Design Parameters

Maximum Subgrade Storage = s x ds

i – infiltration rate of subgrade soil

p,dp – porosity and depth of pavement

s,ds – porosity and depth of subgrade

DA – Total Contributing Drainage Area

PA – Pavement Area

IA – Infiltrating Area

Simplest design: DA = PA = IA

Rob Traver’s Instrumented Pervious Concrete Plaza

at Villanova University

PA < IA < DA

Villanova Stormwater Partnership www3.villanova.edu/VUSP/

Pervious Concrete Basic Design

Exfiltration – limited by subgrade soil infiltration rate

Runoff – overflow, overdrain, underdrain

Storage – subgrade voids (pavement voids***)

Design Example 1

i = 0.5 in/hr s = 0.3 p = 0.2 10-year storm P10 = 5.1 inches

Maximum Subgrade:

P10 / s = 17 inches

Undisturbed subsoil

6” pervious concrete

Clean Stone Subgrade

~30% porosity

Pervious Concrete Basic Design

Design Example 1

i = 0.5 in/hr s = 0.30 P10 = 5.1 inches P10 / s = 17 inches

Route 10 yr Design Storm

Max Elev= 9 ¼” Peak Drawdown = 23.5 hours 100% infiltration

0

0.2

0.4

0.6

0.8

1

0

2

4

6

8

10

0 10 20 30 40 50 60

De

pth

(ft)

Infl

ow

(cfs

)

Hours

Inflow Storage (depth)

Pervious Concrete Basic Design

Design Example 2 – 6 inch subgrade

i = 0.5 in/hr s = 0.3 p = 0.2 P10 = 5.1 inches P10 / s = 17 inches

Route 10 yr Design Storm

Max Elev= 10.3” Peak Drawdown = 23.5 hours FAILS FREEZE-THAW

Add drains to satisfy freeze-thaw criteria

Overdrains Add 2” overdrains with invert elevation at 5” (21 for 10,000 s.f.)

Max Elev = 6” Peak Drawdown = 18.5 hours 75.4% infiltration

Underdrains Add 1” underdrains with invert elevation at 0” (10 for 10,000 s.f.)

Max Elev = 6” Peak Drawdown = 12.5 hours 45.6% infiltration

Pervious Concrete Basic Design

•Site Specific Tradeoffs between Operational Criteria and Hydrologic Criteria

•Characterize Hydrologic performance of a Pervious Concrete System

Effective Curve Number (CN)

Curve Number parameterizes the non-linear relationship between rainfall and

runoff for a catchment

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Ru

no

ff (i

n)

Rainfall (in)

Curve Number

45 55 65 85 98 1:1

Effective Curve Number can be Estimated from rainfall-runoff data

Route range of storms and estimate best fitting CN

0

2

4

6

8

10

0 2 4 6 8 10

Ru

no

ff (i

n)

Rainfall (in)

Estimated CN = 76.8

1:1 Qsim CN Q

Design Example: 6” subgrade with underdrains

Effective Curve Number can be Estimated from rainfall-runoff data

9.25” subgrade 6” subgrade with overdrains 6” subgrade with underdrains

100% infiltration 75.4% infiltration 45.6% infiltration

ECN = 30.5 ECN = 60.1 ECN = 76.8

0

2

4

6

8

10

0 2 4 6 8 10R

un

off

(in

)

Rainfall (in)

Estimated CN = 76.8

1:1 Qsim CN Q

0

2

4

6

8

10

0 2 4 6 8 10

Ru

no

ff (i

n)

Rainfall (in)

Estimated CN = 60.1

1:1 Qsim CN Q

0

2

4

6

8

10

0 2 4 6 8 10

Ru

no

ff (i

n)

Rainfall (in)

Estimated CN = 30.5

1:1 Qsim CN Q

0

0.2

0.4

0.6

0.8

1

0

3

6

9

12

15

0 10 20 30 40 50 60

De

pth

(ft)

Infl

ow

(cfs

)

Hours

Inflow Storage (depth)

0

0.2

0.4

0.6

0.8

1

0

0.3

0.6

0.9

1.2

1.5

0 5 10 15 20 25 30 35

Ele

vati

on

(ft)

Infl

ow

(cfs

)

Hour

Inflow Elevation

0

0.2

0.4

0.6

0.8

1

0

0.3

0.6

0.9

1.2

1.5

0 5 10 15 20 25 30

Ele

vati

on

(ft)

Infl

ow

(cfs

)

Hour

Inflow Elevation

Pervious Concrete with ExfiltrationDelMarva Hydrograph 7.2" 24hr storm

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60 70 80

Time (hrs)

Dep

th (

ft)

0

0.2

0.4

0.6

0.8

1

Ru

no

ff (

cfs)

Subgrade Storage Depth Inflow

Pervious Concrete - Effective Curve Number = 58.5

6" PC 12" Subgrade 6" Orifice

0

2

4

6

8

10

0 2 4 6 8 10

P (in)

Q (

in)

CN Q 1:1 Q CN=98

~50 yr Storm

0.25 in/hr exfilt.

Draindown: ~48hr

“Just” encroaches

pavement

Completely Stores 2yr storm

Changes CN = 98 to Effective CN ~58.5

Pavement to “Meadow”

Elements of Prototype Design/Evaluation Procedure

• Perform site-specific infiltration of subgrade soils (MDE App. D)

• Subgrade storage – f(porosity)

• Size the subgrade for Freeze-Thaw protection & Drawdown

• Supplement Freeze-Thaw protection with overdrains as needed

• Modify Subgrade thickness for freeze thaw protection

• Supplement Drawdown time with underdrains as needed

• Quantify hydrologic performance of design with Effective CN

Pitfalls and Potholes

• Rule of thumb Designs (e.g. 6 inches of pavement over 6 inches of subgrade)

• DA : PA : IA

• Subgrade Soil Infiltration

Environmental Site Design:

Fundamentally Changes Regulatory Policy

•Structural BMPs: “Pass Fail” – Environmental Services

Presumed

•ESD: “Partial Credit” Towards Site Performance Criteria

Consistent Design Procedure

Risk-Based Operational Criteria - Freeze-Thaw & Drawdown

Hydrologic Performance Criteria - Effective Curve Number

ECN Quantifies Contribution to Environmental Site Design

•PC becomes a value engineering decision

•Standard Stormwater Computations*

•Verify Performance with Instrumented Test Plots

*with modification and margin of safety

Questions?

Porosity:

Typically 15-25%

Permeability:

60 - >1,000 in/hr

Compressive Strength:

500-4,000 psi

Pervious Concrete:• “No Fines” Concrete – Interconnected voids

• Most commonly used in low traffic areas

• Inlet to Stormwater Management System

• Hydrologic Design and Structural Pavement Design

Pavement Plot

•Poor Infiltration

•Pervious Concrete

Detention

Grass Plot

•2-3.5 in/hr at depth

UMBC Instrumented Test PlotsChesapeake Bay Trust

• Pavement Plot

•Grass Plot