Post on 17-Feb-2019
The Porous PavementCurve Number
Thomas Ballestero, PE, PhD, PH, CGWP, PG, Federico Uribe,Robert Roseen, PE, PhD, D.WRE, James Houle, CPSWQ
University of New Hampshire Stormwater CenterUniversity of New Hampshire
Philadelphia Low Impact Development Symposium: Greening the Urban Environment25-28 September 2011
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What is the Curve Number For Porous Pavement?
Who wants to know?!?
(What is your OBJECTIVE?)
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The SCS (NRCS) Curve NumberOriginally 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
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Definition Sketch of SCS Runoff Hydrograph Characteristics
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UNHSC Porous Pavement Sites
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UNHSC Porous Asphalt Lot UNHSC Porous Concrete Lot7
Permeable Pavement Sites
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Typical Cross‐Section Construction
SUBGRADE
NATIVE MATERIALS
BANK RUN GRAVEL FILTER COURSE
PERVIOUS PAVEMENT 3-6”
1-1/4” CRUSHED STONE CHOKER COURSE
14”
3/8” PEA-GRAVEL RESERVOIR COURSE 6”4”
Sub-base design matches that of the UNHSC Porous Asphalt Parking Lot
4”
UNHSC Porous Pavement Monitoring
Compound weir Pressure transducer Datalogger
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REFERENCE LOT
POROUS ASPHALT
Tree Filter
UNHSC Porous Pavement Hydrologic Data
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“Real time” flow monitoring…5‐minute time step “Real time” rainfall monitoring…5‐minute time step
0
200
400
600
800
1000
1200
1400
1/1/08
1/8/08
1/15/0
8
1/22/0
8
1/29/0
8
2/5/08
2/12/0
8
2/19/0
8
2/26/0
8
3/4/08
3/11/0
8
3/18/0
8
3/25/0
8
Volu
me
(gal
.)
Influent
Effluent
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0
200
400
600
800
1000
1200
1400
1600
Volu
me
(gal
.)
Influent
Effluent
PC Flow Attenuation1/1/08 - 3/31/08
Influent Effluent
Total Volume (liters) 446,034 78,192
# of Flow Events 16 8
4/1/08 - 6/30/08
Influent Effluent
Total Volume (liters) 446,034 25,585
# of Flow Events 15 5
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PC Pollutant Removal
82% RE94% RE
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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Q P Ia 2
P Ia SEq. 1.
S 1000CN
10 Eq. 4.
Ia 0.2S Eq. 2.
.3.8.02.0
8.0
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EqSPSP
SPIPQ a
Q: Total Runoff Depth (in)P: Total Precipitation Depth (in)Ia: Initial Abstraction (in)S: Storage Parameter (in)
Method 1 ‐ Depth of Runoff (Q)MethodMethod 1 ‐ Depth of Runoff (Q)Method
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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 MethodsMethod 2 ‐Lag Methods
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Tlag: Lag Time (hr)Tc: Concentration Time (hr)Y: Surface Slope (%)S: Storage Parameter (in)
Tlag L0.8 S 1 0.7
1900Y 0.5 Eq. 5.
Tc 53
Tlag Eq. 6.
S 1000CN
10 Eq. 7.
Method 2 ‐LagMethods
Method 2 ‐LagMethods
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0
1
2
3
4
5
6
0
0.01
0.02
0.03
0.04
0.05
0.06
0 50 100 150 200 250 300 350 400 450 500 550 600
Run
off
[gpm
]
Rai
nfal
l [in
]
Lag MethodsLag Methods
1. T base (Sánchez San Román [2009])
T base = T precip + T conc
T conc = T base – T precip
LAG METHOD (A) – lag measured from precip peak and runoff peak
3 APPROACHES
T baseT
precip
Using Eq. 5 and Eq. 6 used into Eq.7., solve for CN
In. Abs.
Recession curveRecession curve
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0
1
2
3
4
5
6
0
0.01
0.02
0.03
0.04
0.05
0.06
0 50 100 150 200 250 300 350 400 450 500 550 600
Run
off
[gpm
]
Rai
nfal
l [in
]
2. T peak (Sánchez San Román [2009], Folmar, Miller and Woodward [2007])
T peak = T lag + T precip/2
T lag = T peak– T precip/2
LAG METHOD (B) – measure lag from duration of excessprecipitation
T peak
T precip
Insert Eq. 1 into Eq.3 and solve for CN:
In. Abs.
T lag
CN 1000
1900 TLAG Y 0.5
L0.8
1.423
9
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Lag MethodsLag Methods
0
1
2
3
4
5
6
0
0.01
0.02
0.03
0.04
0.05
0.06
0 50 100 150 200 250 300 350 400 450 500 550 600
Run
off
[gpm
]
Rai
nfal
l [in
]
3. T centroid (NRCS [2009], Folmar, Miller and Woodward [2009])
T lag: time from the centroid of excess precipitation to the peak of the hydrograph.
LAG METHOD (C) – Measure Lag from when ½ Q occurs
In. Abs.
T lag
CN 1000
1900 TLAG Y 0.5
L0.8
1.423
9
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Lag MethodsLag Methods
qp: Peak Discharge (cfs)qu: Unit Peak Discharge (csm/in) Am: Drainage area (mi2) Q: Runoff (in)
Method 3GRAPHICAL PEAK DISCHARGE METHOD
Method 3GRAPHICAL PEAK DISCHARGE METHOD
0
1
2
3
4
5
6
0 50 100 150 200 250 300 350 400 450 500 550 600
Run
off [
gpm
]
qp qu Am Q
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qp qu Am Q Eq. 8.
GRAPHICAL PEAK DISCHARGE METHODGRAPHICAL PEAK DISCHARGE METHOD
0
1
2
3
4
5
6
0 50 100 150 200 250 300 350 400 450 500 550 600
Run
off [
gpm
]
qu qp
Q Am
Eq. 9.
Area = 0.000201 mi2(5200 ft2)
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Method 3 – GraphicalPeak Discharge
Method 3 – GraphicalPeak Discharge
Tc estimated with Lag Method and qufound 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=0.1If Ia/P > 0.5 then Ia/P=0.5
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Method 3 – GraphicalPeak Discharge
Method 3 – GraphicalPeak Discharge
Knowing Ia/P and P, compute Ia. Then with Eq. 2 and Eq. 7,
obtain CN
S 1000CN
10 Eq. 7.
CN 1000
5 Ia 10
Ia 0.2S Eq. 2.
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CNMethod 1
CNMethod 2Method A
CNMethod 2Method B
CNMethod 2Method C
CNMethod 3
Average 74 11 6 6 51Median 75 8 2 3 13
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RESULTSRESULTS
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?
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time
flowwatershed hydrograph
pond hydrograph
REFERENCESREFERENCES
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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AcknowledgementsFunding Source:
Questions?
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http://www.unh.edu/erg/cstev/
or Simply Search for “UNHSC”