US Army Corps of Engineers

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USACE Methods for Quantifying 

Extreme Flood Hazards

Douglas J. Clemetson, P.E.

Chief, Hydrology Section

USACE, Omaha District

Omaha, Nebraska

29 January 2013
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1. Extension using Bulletin 17B Methods

2. Hydrologic Modeling using frequency based storm events

3. Regional Probability of the PMF

4. Stochastic Event Flood Model (SEFM)

5. Application of the GRADEX Method

6. Utilization of Paleoflood Information

Methods to Extend Frequency Curves
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0.9999 0.999 0.99 0.95 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.05 0.01 0.001 0.0001

Exceedance Frequency

1

10

100

1000

10000

100000

1E+006 Volume (cfs)

Legend

1-DAY COMPUTED

2-DAY COMPUTED

3-DAY COMPUTED

7-DAY COMPUTED

15-DAY COMPUTED

30-DAY COMPUTED

1-DAY WEIBULL

2-DAY WEIBULL

3-DAY WEIBULL

7-DAY WEIBULL

15-DAY WEIBULL

30-DAY WEIBULL

Cherry Creek Dam Volume-Probability Curves

Ppmf=.00016

V PMF (256,400 Acre-Feet or 129,268 cfs) pmf

Extension using Bulletin 17B Methods
http://www.nws.noaa.gov/ohd/hdsc/index.html BUILDING STRONG®

Hydrologic Modeling Using Frequency Based Storm Events
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Hydrologic Modeling Using Frequency Based Storm Events
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AEP=10−[(1−Ratio)×Range+MinValue]

Ratio = Historical Max Precip/PMP

Range = 10-3 to 10-5 = (-3)-(-5) = 2

Min Value = 10-3 = 3

AEP=10−[(1−Ratio)×2+3]

Regionalized Probability of PMF
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1965 Holly Storm Centered Over Cherry Creek Basin 

Pave = 13.56”
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Maximum Average HMR55A Transposed

Precip Precip (in) Vertical Horizontal Total Basin Ave

Storm (in) CC Basin Factor Factor Factor Precip (in)

1921 Penrose SW 1-23 12 8.21 0.99 1.00 0.99 8.13

1935 Cherry Cr 24 8.08 1.00 0.97 0.97 7.84

1935 Hale 24 8.85 0.89 0.95 0.85 7.48

1965 Plum Cr 14 5.10 1.00 1.00 1.00 5.10

1965 Palmer 16 10.49 1.00 1.00 1.00 10.49

1965 Holly 15 13.56 0.80 0.94 0.75 10.20

1981 Frijole Creek 14 8.14 1.00 1.00 1.00 8.14

1997 Fort Collins 12 3.84 0.97 1.03 1.00 3.84

1997 Pawnee 15 5.59 0.93 0.97 0.90 5.04

Cherry Creek Transposed Storms
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AEP=10−[(1−Ratio)×2+3]

Cherry Creek 24-hr PMP = 21.1”

Max Historical Precip = 13.56”  Ratio = 13.56/21.1 = 0.643

Max Transposed Precip = 10.49”  Ratio = 10.49/21.1= 0.497

AEP=10−[(1−.643)×2+3] = 0.000193  1 : 5,176

AEP=10−[(1−.497)×2+3] = 0.000099  1 : 10,139
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The red numbers represent PMP, Regional Precipitation, Ratio

21.1, 13.56, 64%

Regionalized Probability of PMF Applications
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Location

PMP

(in)

Regional

Rainfall

(in) Ratio

Return 

Interval

(years)

Addicks 49 15.3 0.31 23,742

Bald Eagle 25.3 11.1 0.44 13,260

Ball Mountain 24.6 11.3 0.46 12,059

Bluestone 20 7.34 0.37 18,450

Cherry Creek 21.1 13.6 0.64 5,176

Clearwater 25.4 8.5 0.33 21,415

Falls Lake 23.8 22.3 0.94 1,337

Folsom 29.6 14.1 0.47 11,254

Galisteo 16.8 7.5 0.45 12,798

Green Peter 17.9 14.5 0.81 2,398

Seven Oaks 44.3 15.8 0.36 19,350

Regionalized Probability of PMF Applications
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GRADEX Method
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P = 0.00014

P = 0.000016

PMF
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Method Annual Exceedance 

Probability

Return Interval

(years)

Bulletin 17B

(Inflow Probability) 0.00016 6,250

Regional Probability

(Maximum Centered Precip) 0.000193 5,200

Regional Probability

(Maximum Transposed Precip) 0.000099 10,000

GRADEX Method

(9 Gages w/ Historic storms) 0.000016 62,500

GRADEX Method

(7 Gages w/Historic storms) 0.00014 7,100

Average of all Methods 0.000122 8,200

Cherry Creek AEP of PMF
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With Minimum

PMF Probability

With Maximum

PMF Probability

Cherry Creek Hydrologic Loading 
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Questions/Discussion