Extreme Precipitation Frequency 

for Dam Safety and Nuclear 

Facilities – A perspective 

Workshop on Probabilistic Flood Hazard Assessment

January 29-31, 2013 Rockville, Maryland
Case Studies

Star Fort Dam, SC Trapped Rock Dam, NM

Anderson Ranch Dam, ID Altus Dam, OK
Star Fort Dam

• Owned by the National Parks 

Service, located in Ninety Six 

National Historic Site

• Flood control structure

• Built in the 1930s

• Drainage area is 1.15 mi2

Compute the PMF and flood 

frequency analysis for screening

Requires PMP and precipitation 

frequency
Star Fort Dam

PMP - the theoretically greatest depth of 

precipitation for a given duration that is 

physically possible over a given size storm 

area at a particular geographical location at a 

certain time of year.

0.5 0.1 0.01 1E-3 1E-4

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

24-hour Preciptation Depth (in)

AEP

Upper/Lower Precipation Depth

 Precipitation Depth

HMR 51 (published 1978)

PMP = 41” in 24-hours

NOAA Atlas 14

14.5” in 24-hours

for 1,000-year event

Compute the PMF from the PMP

Compute flood frequency up to 1,000-yr 

(upper limit of NOAA Atlas 14)
Current Understanding

New Storm Data for Eastern North Carolina

Caldwell et al. 2011

Hurricane Floyd, upon moisture 

maximization, exceeds PMP at 24-hr 

and 72-hr at large area sizes.

Compute the PMF from the PMP
Current Understanding

Resolution of NOAA Atlas 14 vs HMR 51

Compute flood frequency

Caldwell et al. 2011
Trapped Rock Dam

• Owned by the Zuni Pueblo 

Indian Reservation, Bureau of 

Indian Affairs

• Flood control structure

• Drainage area is 2.88 mi2

Flood Frequency Analysis

Requires Precipitation Frequency 

Analysis
Trapped Rock Dam

Australian Rainfall-Runoff method (ARR):

1. Assigns a probability to the PMP 2. Extrapolates between the credible 

limit of extrapolation and PMP using a 

2-parameter parabolic function in loglog space

Nathan and Weinmann, 2000

Credible 

limit

PMP
Current Understanding

Uncertainty on PMP

PMP
Anderson Ranch Dam

• Owned by Reclamation

• Storage structure

• World’s highest earthfill dam at 

the time of construction

• Drainage area is ~975 mi2

Detailed flood hydrology study for 

the Dam Safety Office

Requires precipitation frequency + 

storm patterns
Anderson Ranch Dam

Precipitation frequency using multiple 

methods: ARR and L-moments

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

72-hour Basin-Average Precipitation (in)

ANNUAL EXCEEDANCE PROBABILITY

.99 .50

Generalized Logistic

.90 .20 .10 .02 01 .002 10-3 10-4 10-5

ARR = red curve

L-moments = blue curve
Anderson Ranch Dam

Storm spatial patterns
Evolving Ideas

Radar derived rainfall for spatial pattern
Altus Dam

• Owned by Reclamation

• Irrigation storage structure

• Contributing area is ~1951 mi2

Detailed flood hydrology study for 

the Dam Safety Office

Requires precipitation frequency + 

storm patterns to run SEFM
Altus Dam

Precipitation frequency from L-moments
Altus Dam

Storm spatial patterns

May 22, 1972

% of 24x
Evolving Ideas

Temporal pattern (mass curves) from model 

data (CFS-R) 

Presented at the American Meteorological Society 

Annual Meeting, 27th Conference on Hydrology

Sept. 23, 1982
Summary

• Scaled approach (based on size of dam 

and consequences)

• In-house, site-specific work

• Precipitation frequency curves 

• PMP is used as a consideration

• Uncertainty

• Methods are evolving
Case Studies

Star Fort Dam, SC Trapped Rock Dam, NM

Anderson Ranch Dam, ID Altus Dam, OK