tye w. parzybok, shauna bokn, r. jason caldwell, katie...
TRANSCRIPT
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
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
Close to all-time US record
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
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.
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
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
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)
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.
Based on MetStat 2016
17.43”
Based on MetStat 2016
30.56”
Based on Asquith 1998
46.88”364,000-year
(AEP = 2.8 x 10-6)
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
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
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
Skew and distribution are
largest contributors
Distribution used: GEV
▪ Customary for extreme
precipitation (and flood)
frequency studies
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
TVA Hydrologic Hazards Project
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
Tye W. Parzybok, CCM, GISP
CNBC
REAL – Houston, TX September 30, 2017
FAKE
Make sure to get your
information from reliable
sources!