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Tye W. Parzybok, Shauna Bokn, R. Jason Caldwell,
Katie Ward, Alyssa H. Dietrich, Brian Crow
MetStat, Inc.
Fort Collins, Colorado
2017 Dam Safety, San Antonio, TX Sept. 2017
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What made the rainfall unique?
1. The large areal extent of extremely rare
rainfall
2. The storm efficiency caused 1000+ year
rainfalls over a large range
of durations – from 6-hours
to 5-days
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Close to all-time US record
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Detailed storm precipitation analysis with MetStorm®
Rain gauge data from numerous networks
✓ Total originally considered: 2,270
✓ Final gauge counts – 1,799
• Daily-reporting: 804
• Hourly-reporting: 995
5-minute Dual-Polarization radar-estimated precipitation mosaic
5-minute NEXRAD radar reflectivity
Hourly radar confidence grid
1-hour satellite-estimated precipitation
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MetStorm® Total
Storm (5-day)
Precipitation
51.65”
September 25-30, 2017
• Possible under-catch
due to high winds –
maybe as much as
20%
• Considered storm
center for the 120h
total but 1h, 6h, 12, etc
might be different
places.
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Average Recurrence Interval (ARI) of 100-years
=
Annual Exceedance Probability (AEP) of 0.01 (1/100) or 1.0 x 10-2
=
1% chance of occurring in any given year
ARI of rain does not equal ARI of flood
ARI of 250,000-years
=
AEP of 0.000004 (1/250,000) or 4.0 x 10-6
=
0.0004% chance of occurring in any given year
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Available Precipitation Frequency Analyses for putting the rainfall
into a probabilistic perspective
Document Durations Frequencies
NOAA Atlas 14 (available in 2018) NA NA
Technical Memo NWS Hydro-35 (1977) 5 – 60 minute 2 – 100 years
Technical Paper 40 (1961) 1 – 24 hours 1 – 100 years
Technical Paper 49 (1964) 2 – 10 days 2 – 100 years
Depth-Duration Frequency of Precipitation for Texas
USGS Report 98-4044 (Asquith 1998)
15-minute – 7-days >= 1 year
MetStat Precipitation Frequency Analysis (2016) 6- and 24-hour 1 – 1,000 years
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Depth-Duration Frequency of
Precipitation for Texas USGS Report 98-
4044 (Asquith 1998)
• Based on 38,120 cumulative years of
record
• Precip. Freq. = GEV(mean, L-skew, L-cv)
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Annual exceedance probabilities (AEPs) provided are estimated
using GEV parameters from USGS Report 98-4044 (Asquith 1998).
We recognize that these estimates contain considerable uncertainty,
but represent the only 3- and 5-day precipitation-frequency data
publicly available at this time. Precipitation-frequency estimates by
storm type/mechanism are required to more accurately place this
event into proper historical/probabilistic perspective.
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Based on MetStat 2016
17.43”
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Based on MetStat 2016
30.56”
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Based on Asquith 1998
46.88”364,000-year
(AEP = 2.8 x 10-6)
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Annual exceedance probabilities (AEPs) provided in this graphic are estimated using GEV parameters from USGS
Report 98-4044 (Asquith 1998). We recognize that these estimates contain considerable uncertainty, but represent
the only 5-day precipitation-frequency data publicly available at present. Precipitation-frequency estimates by storm
type/mechanism are required to more accurately place this event into proper historical/probabilistic perspective.
51.65”542,000-year
(AEP = 1.9 x 10-6)
Based on Asquith 1998
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Estimating Precipitation AEPs out to 10-9
Example Areal Precipitation-Frequency Relationship and 90%
Uncertainty Bounds for 48-hour Mid-Latitude Cyclone Storm
Precipitation Tennessee Watershed.
PMP ~ AEPs 10-4 to 10-7
Disconnect between PMP
“Limit” and shape of precip.
freq. curves
Attaining these AEPs
✓ Trading space for time
✓ Spatial mapping of
statistical parameters
✓ Include uncertainty
bounds
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
48
-HO
UR
P
RE
CIP
ITA
TIO
N (
in)
ANNUAL EXCEEDANCE PROBABILITY
10-910-3
Extreme Value Type 1 Plotting Paper
10-2 10-4 10-5 10-80.50.9 10-1 10-6 10-7
Holston River System - Cherokee Dam 3,425-mi2
Mean Frequency Curve Best-Estimate
90% Uncertainty Bounds 48-Hr PMP
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0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
48
-HO
UR
P
RE
CIP
ITA
TIO
N (
in)
ANNUAL EXCEEDANCE PROBABILITY
10-910-3
Extreme Value Type 1 Plotting Paper
10-2 10-4 10-5 10-80.50.9 10-1 10-6 10-7
Holston River System - Watauga Dam 467-mi2
Mean Frequency CurveBest Estimate
90% Uncertainty Bounds
Preliminary 2/16/2016
48-Hr PMP
PMP ~ AEPs sometimes to 10-9
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Skew and distribution are
largest contributors
Distribution used: GEV
▪ Customary for extreme
precipitation (and flood)
frequency studies
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USACE Hydrologic Hazards Development Project for the Trinity
River Basin in Texas
Late next year (2018) we will have the information to refine our
AEP analysis
PF estimates
for 4 storm types
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TVA Hydrologic Hazards Project
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500+ Year 24-hour Rainfall in Areas of the Southeast
Dry <1 yr 1yr 2yr 4yr 6yr 8yr 10yr 20yr 40yr 60yr 80yr 100yr 200yr 300yr 500yr
24-hour ARIs
ending @ noon
CDT Sept. 11, 2017
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Tye W. Parzybok, CCM, GISP
CNBC
REAL – Houston, TX September 30, 2017
FAKE
Make sure to get your
information from reliable
sources!