The Porous Pavement
Curve Number
Thomas Ballestero, PE, PhD, PH, CGWP, PG, Federico Uribe,
Robert Roseen, PE, PhD, D.WRE, James Houle, CPSWQ
University of New Hampshire Stormwater Center
University of New Hampshire
Philadelphia Low Impact Development Symposium:
Greening the Urban Environment
25-28 September 2011
1
What is the Curve Number For Porous Pavement?
Who wants to know?!?
(What is your OBJECTIVE?)
2
The SCS (NRCS) Curve Number
Originally conceived to translate rainfall depth into runoff depth on agricultural watersheds…method worked best for large storms
This was then translated into a runoff hydrograph
3
Definition Sketch of SCS Runoff Hydrograph Characteristics
4
5
UNHSC Porous Pavement Sites
6
Permeable Pavement Sites
UNHSC Porous Asphalt Lot
UNHSC Porous Concrete Lot
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Typical Cross‐Section Construction
3-6”
PERVIOUS PAVEMENT
1-1/4” CRUSHED STONE CHOKER COURSE
BANK RUN GRAVEL
FILTER COURSE
3/8” PEA-GRAVEL RESERVOIR COURSE
4”
14”
4”
6”
SUBGRADE
NATIVE MATERIALS
Sub-base design matches that of the UNHSC Porous Asphalt Parking Lot
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UNHSC Porous Pavement Monitoring
 Compound weir
 Pressure transducer
 Datalogger
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REFERENCE
LOT
POROUS
ASPHALT
Tree Filter
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UNHSC Porous Pavement Hydrologic Data
 “Real time” flow monitoring…5‐minute time step
 “Real time” rainfall monitoring…5‐minute time step
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PC Flow Attenuation
1400
1200
4/1/08
1/1/08--6/30/08
3/31/08
1600
1400
Influent
Influent
Effluent
Effluent
Total
TotalVolume
Volume(liters)
(liters)
446,034
446,034
78,192
25,585
Influent
# #ofofFlow
FlowEvents
Events
15
16
85
Influent
Effluent
1200
1000
Effluent
Volume (gal.)
Volume (gal.)
1000
800
800
600
600
400
400
200
200
0
3/
25
/0
8
3/
18
/0
8
3/
11
/0
8
3/
4/
08
2/
26
/0
8
2/
19
/0
8
2/
12
/0
8
2/
5/
08
1/
29
/0
8
1/
22
/0
8
1/
15
/0
8
1/
8/
08
1/
1/
08
0
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PC Pollutant Removal
82% RE
94% RE
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Methods of Teasing CN from the Data
 Measure P and Q, invert basic SCS equation
 Measure P and outflow hydrograph (q), measure lag, estimate CN from lag equations
 Measure Q and qp, estimate CN from peak discharge equations
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Method 1 ‐ Depth of Runoff (Q)Method
P  Ia 
2
Q
Eq. 1.
P  Ia  S
Ia  0.2 S
Eq. 2.
Q: Total Runoff Depth (in)
P: Total Precipitation Depth (in)
Ia: Initial Abstraction (in)
S: Storage Parameter (in)
2
2

P  I a  P  0.2 S 
Q
Eq.3.

P  0.8S
1000
S
 10
CN
P  0.8S
Eq. 4.
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Method 2 ‐Lag Methods
 Study how the timing of the “runoff” is transformed
 Time of concentration
 Lag time
 Time base
 Peak time
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Method 2 ‐Lag
Methods
L S  1
Tlag 
1900 Y 0.5
0.7
0.8
5
Tc  Tlag
3
Eq. 5.
Eq. 6.
1000
S
10
CN
Tlag: Lag Time (hr)
Tc: Concentration Time (hr)
Y: Surface Slope (%)
S: Storage Parameter (in)
Eq. 7.
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Lag Methods
3 APPROACHES
LAG METHOD (A) – lag measured from precip peak and
In. Abs. runoff peak
T base
T precip
1. T base (Sánchez San Román [2009])
T base = T precip + T conc
Recession curve
0.06
6
0.05
5
0.04
4
Using Eq. 5 and Eq. 6 used into Eq.7., solve for CN
0.03
3
0.02
2
0.01
1
0
Runoff [gpm]
Rainfall [in]
T conc = T base – T precip
0
0
50
100
150
200
250
300
350
400
450
500
550
600
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Lag Methods
LAG METHOD (B) – measure lag from duration of excess
precipitation
In. Abs.
2. T peak (Sánchez San Román [2009], T peak
Folmar, Miller and Woodward [2007])
T precip
T lag
T peak = T lag + T precip/2
0.06
6
0.05
5
0.04
4
CN 
0.03
3
0.02
2
0.01
1
Runoff [gpm]
Insert Eq. 1 into Eq.3 and solve for CN:
Rainfall [in]
T lag = T peak– T precip/2
1000
1900 TLAG Y 0.5 1.423

  9
L0.8


0
0
0
50
100
150
200
250
300
350
400
450
500
550
600
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Lag Methods
LAG METHOD (C) – Measure Lag from when ½ Q occurs
In. Abs.
3. T centroid (NRCS [2009], Folmar, Miller and Woodward [2009])
0.06
6
0.05
5
0.04
4
Rainfall [in]
T lag: time from the centroid of excess precipitation to the peak of the hydrograph. CN 
0.03
3
0.02
2
0.01
1
Runoff [gpm]
T lag
1000
1900 TLAG Y 0.5 1.423

  9
L0.8


0
0
0
50
100
150
200
250
300
350
400
450
500
550
600
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Method 3
GRAPHICAL PEAK
DISCHARGE METHOD
q p  qu Am Q
qp: Peak Discharge (cfs)
qu: Unit Peak Discharge (csm/in) Am: Drainage area (mi2) Q: Runoff (in)
6
5
Runoff [gpm]
4
3
2
1
0
0
50
100
150
200
250
300
350
400
450
500
550
600
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GRAPHICAL PEAK
DISCHARGE METHOD
q p  qu Am Q Eq. 8.
qp
qu 
Q Am
Runoff [gpm]
6
Eq. 9.
Area = 0.000201 mi2
(5200 ft2)
5
4
3
2
1
0
0
50 100 150 200 250 300 350 400 450 500 550 600
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Method 3 – Graphical
Peak Discharge
★
Tc estimated with Lag Method and qu
found with Eq. 9., find Ia/P from the Unit Peak discharge for NRCS type III rainfall distribution chart.
If Ia/P < 0.1 then Ia/P=0.1
If Ia/P > 0.5 then Ia/P=0.5
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Method 3 – Graphical
Peak Discharge
Ia  0.2 S
Eq. 2.
1000
S
 10
CN
Eq. 7.
Knowing Ia/P and P, compute Ia. Then with Eq. 2 and Eq. 7, obtain CN
1000
CN 
5 I a  10
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RESULTS
CN
CN
CN
CN
Method 2 Method 2 Method 2
CN
Method 1 Method A Method B Method C Method 3
Average
74
11
6
6
51
Median
75
8
2
3
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Natural state for Hinckley-Charlton soil (HSG – B/C)
= 60 - 72
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So….Which to Use?
 Events
 Peak Outflow from Underdrain

Peak flow method
 No net increase in benchmark storms
 Lag method (median)
 Long Term Simulation


Lag methods
Runoff depth method (~native soil)
 Watershed Simulation
 Seasonal CN
 Lag methods
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Philosophically Speaking…..
 What is the CN for a detention pond?
watershed hydrograph
flow
pond hydrograph
time
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REFERENCES
FOLMAR,N.D; MILLER, A.C.; AND WOODWARD, W.E; 2007. “HISTORY AND DEVELOPMENT OF THE NRSC LAG TIME EQUATION”. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION. VOL. 43(3): 829‐838. LEMAY, G; 2008. DETERMINING THE CURVE NUMBER (CN) FOR POROUS ASPHALT SYSTEMS –
INDIVIDUAL STORM VOLUMES. HONORS THESIS INDEPENDENT
SANCHEZ SAN ROMAN, F.J. 2009. “HIDROLOGIA SUPERFICIAL III”. ONLINE HTTP://WEB.USAL.ES/JAVISAN/HIDRO
UNITED STATES DEPARTMENT OF AGRICULTURE. NATIONAL RESOURCES CONVERSATION SERVICE. 1986. “URBAN HYDROLOGY FOR SMALL WATERSHEDS TR‐55”. ONLINE HTTP://WWW.WSI.NRCS.USDA.GOV/PRODUCTS/W2Q/H&H/TOOLS_MODELS/OTHER/TR55.HTML
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Acknowledgements
Funding Source:
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Questions?
http://www.unh.edu/erg/cstev/
or Simply Search for “UNHSC”
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