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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