RAINFALL FREQUENCY ANALYSIS FOR NEW BRAUNFELS, TX

RAINFALL FREQUENCY ANALYSIS FOR
NEW BRAUNFELS, TX
(or Seems like we’ve been having lots of 100-yr storms)
Presented By:
SAUL NUCCITELLI, PE, CFM (LAN)
BLAKE KRONKOSKY, EIT (LAN)
JIM KLEIN, PE (CITY OF NEW BRAUNFELS)
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AGENDA
RECENT STORM REVIEW
HISTORY OF RAINFALL ANALYSIS
UPDATED ANALYSIS
1950 FLOOD IN NEW BRAUNFELS
CONCLUSIONS
Q&A
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1998 FLOOD
NWS APPROXIMATED RAINFALL DEPTHS
• PRODUCED >20” RAINFALL IN 24-HRS
• APPROACHING 500-YR FREQUENCY
• ~$750M DAMAGES, 31 DEATHS
ACROSS TEXAS
DECEMBER 1998 NRCS STUDY
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1998 FLOOD
GUADALUPE RIVER AT IH-35
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2002 FLOOD
NWS APPROXIMATED RAINFALL DEPTHS
• PRODUCED EXCESS OF 30”
RAINFALL IN 5 DAYS
• 30” IS SIMILAR TO YEARLY AVERAGE
RAINFALL DEPTH FOR COMAL COUNTY
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2002 FLOOD
• VOLUME ≈ 3 TIMES CANYON DAM’S
CAPACITY OVERTOPPING THE EMERGENCY
SPILLWAY
CANYON DAM EMERGENCY SPILLWAY
• CANYON DAM SPILLWAY FLOWS > 70,000 CFS
CANYON DAM GORGE FROM SPILLWAY EROSION
• CANYON DAM GORGE CREATED
• EXPOSED MILLIONS OF YEARS OF
GEOLOGIC TIME
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2002 FLOOD
SOUTH ACCESS ROAD AFTER FLOOD WATERS RECEEDED
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2004 FLOOD
• AROUND 6” RAIN IN 6 HOURS
NRCS DAM #3 AT BLIEDER’S CREEK AT FULL CAPACITY
• HEAVY RAINFALL DOWNSTREAM
OF CANYON DAM, BLIEDER’S CREEK,
AND DRY COMAL WATERSHEDS
NRCS DAM #3 AT BLIEDER’S CREEK AT FULL CAPACITY
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2004 FLOOD
DRY COMAL AT LOOP 337
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2010 FLOOD
GUADALUPE RIVER AT COMMON ST.
• EXCESS OF 8” RAIN IN 3 HRS
• FLOODING ALONG GUADALUPE
AND COMAL RIVERS
NWS NEXRAD IMAGERY
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2010 FLOOD
GUADALUPE RIVER AT IH-35
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2010 FLOOD
COMAL RIVER AT SCHLITTERBAHN
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QUESTIONS AND APPROACH
• WHY SO MANY BIG RAINS/FLOODS?
• ARE ALL THESE BIG STORMS ENOUGH TO CHANGE DEFINITION OF A
100-YR RAIN?
– 1998 TO 2010 HAD AT LEAST 4 LARGE EVENTS
• LAN’S ANALYSIS APPROACH
– REVIEW RAINFALL DATA
– FILTER DATA
– PERFORM STATISTICAL ANALYSIS
– COMPARE TO PREVIOUS VALUES
– RECOMMEND CHANGES, IF APPROPRIATE
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RAINFALL FREQUENCY TERMINOLOGY
• P = 1/T
• P = % ANNUAL CHANCE
• F = FREQUENCY (YEARS)
• P = 1-(1-F)N
• P = % PROBABILITY OF AT
LEAST ONE EVENT IN N #
YEARS
COMMON MISCONCEPTIONS
• 100-YR EVENT ONLY HAPPENS
EVERY 100 YEARS
• 100-YR RAINFALL IS SAME AS
100-YR FLOOD
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TP-40 NATIONAL RAINFALL ATLAS
TP-40 100-YR 24-HR RAINFALL
• FIRST NATIONAL STUDY (1962) BY NOAA
• FAIRLY SHORT RECORD OF GAGE DATA
• GUMBEL DISTRIBUTION
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1998 USGS TEXAS RAINFALL STUDY
NWS RAINFALL STATIONS
• USGS WRIR 1998-4044 (STUDY)
• USGS SIR 2004-5041 (ATLAS)
• REVIEWED NWS STATIONS
• 174 15-MIN
• 274 HOURLY
• 865 DAILY
• DATA THROUGH 1994
• UPDATED STATISTICAL METHODS
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1998 USGS VS. 1962 TP-40 RAINFALL CONTOURS
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USGS 100-YR, 24HR DEPTH
COMAL COUNTY 24-HR 100-YR DEPTH
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NEW BRAUNFELS DEPTH-DURATION-FREQUENCY
TABLE
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RAINFALL STATION MAP
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RAIN STATION FILTER
• FILTER 1 = NATIONAL WEATHER SERVICE STATIONS
– CONSISTENT WITH USGS METHOD
• FILTER 2 = ACTIVE STATIONS
– CAPTURE RECENT STORMS
• FILTER 3 = RECORD LENGTH ≥ 20 YEARS
– LONG ENOUGH TO MINMIZE DATA SKEW
• FILTER 4 = WITHIN 30 MILES OF
NEW BRAUNFELS
– RELEVANCE TO CITY
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FILTER 1 – NWS STATIONS
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FILTER 2 – ACTIVE STATIONS
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FILTER 3 – RECORD LENGTH ≥ 20 YEARS
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FILTER 4 – WITHIN 30 MILES OF NEW BRAUNFELS
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STATIONS USED IN ANALYSIS
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NWS STATIONS USED
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DATA PROCESSING
•SEPARATED THE YEARS
•DEVELOPED ANNUAL MAXIMA
•HOURLY DATA USED TO GENERATE THE
1,2, 3, 6, 12, & 24-HR DEPTHS
1 HR = 1 HOURLY DATA POINT
2 HR= Σ ( 2 CONSECUTIVE DATA POINTS)
3 HR= Σ ( 3 CONSECUTIVE DATA POINTS)
6 HR= Σ ( 6 CONSECUTIVE DATA POINTS)
12 HR= Σ ( 12 CONSECUTIVE DATA POINTS)
24 HR= Σ ( 24 CONSECUTIVE DATA POINTS)
48 HR= Σ ( 48 CONSECUTIVE DATA POINTS)
72 HR= Σ ( 72 CONSECUTIVE DATA POINTS)
•DAILY DATA CALCULATED SIMILAR TO HOURLY
1 DAY = 1 DAILY DATA POINT
2 DAY= Σ ( 2 CONSECUTIVE DATA POINTS)
3 DAY= Σ ( 3 CONSECUTIVE DATA POINTS)
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CANYON DAM - ANNUAL MAXIMA 1 HR TO 6 HR
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NEW BRAUNFELS - ANNUAL MAXIMA 1 DAY TO 3 DAY
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EXTREME VALUE STATISTICS
• REVIEW OF (HIGHEST OR LOWEST VALUE)
FROM A REOCCURING SAMPLE
• INTERVAL DATA
-HOURLY ANNUAL MAXIMA
(8,760 hr / YR)
-DAILY ANNUAL MAXIMA
(365 DAY / YR)
• INDEPENDENT RANDOM VARIABLES
• CLIMATE PHENOMENA SUCH AS :
-RAINFALL/FLOODING
-EARTHQUAKES
-TEMPERATURE
-DROUGHT
CONGRESS BRIDGE AT COLORADO RIVER AUSTIN, TX
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EXTREME VALUE STATISTICS
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EV- DISTRIBUTIONS
• LOG PEARSON TYP III
STREAM FLOW
• GUMBEL
TP-40
• GENERALIZED LOGISTICS (GLO)
1998 USGS 1-24 HR
• GENERALIZED EXTREME VALUE
(GEV)
1998 USGS 1-3 DAY
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STATISTICAL MOMENT
STANDARD MOMENTS
• Mr = E( ( X-M )r)
r = MOMENT ORDER (0, 1, 2, 3, 4…∞)
• CALCULATIONS REPRESENTING THE SHAPE OF DATA
– PROVIDE SUMMARY
– COEFFICIENTS TO STATISTICAL
• DERIVED WITH RESPECT TO “MEAN”:
M ( 0 ) = MEAN
M ( 1 ) = VARIANCE
M ( 2 ) = SKEW
M ( 3 ) = KURTOSIS
“AVERAGE”
“WIDTH”
“LOPSIDEDNESS” (+) = LEFT (-) = RIGHT
“SQUATTINESS ”
• SIMILAR TO PHYSICS:
X = F(t)
V = dX/dt
A = dV/dt
J = dA/dt
DISTANCE
VELOCITY
ACCELERATION
JERK OR JOLT
r (0) INTIAL EQUATION
r (1) 1ST ORDER
r (2) 2ND ORDER
r (3) 3RD ORDER
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L-MOMENT STATISTICS
• SIMILAR TO STANDARD MOMENTS
– SUMMARY TO DATA
– COEFFICIENTS TO STATISTICAL DISTRIBUTIONS
– CALCULATED FROM FINITE SAMPLES
• DIFFERENT FROM STANDARD MOMENTS
– TAKE IN ACCOUNT DATA RANKING IN ASCENDING ORDER
– DERIVED WITH RESPECT TO MOMENT ORDER (NOT THE MEAN)
– CALCULATION INTENSIVE “ REQUIRING AID OF COMPUTER”
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NEW BRAUNFELS STATION ANNUAL MAXIMA 1 DAY
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NEW BRAUNFELS STATION ANNUAL MAXIMA 2 DAY
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NEW BRAUNFELS STATION ANNUAL MAXIMA 3 DAY
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PULLING THE STATIONS TOGETHER
1998 USGS STUDY FOR TEXAS ≈ 270,000 MILES2
• 1998 USGS USED KRIGING TO PRODUCE CONTOURS
• USED ~850 STATIONS, GROUPED 5 AT A TIME
2011 LAN STUDY FOR NEW BRAUNFELS ≈ 30 MILES2
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RAINFALL CONTOURS NOT NEEDED
SMALLER STUDY AREA THAN 1998 USGS STUDY
1, 2 , 3, 6, 12-HR DURATIONS USED 7 HOURLY STATIONS
24-HR ; 2 , 3-DAY DURATIONS USED 7 HOURLY & 2 DAILY STATIONS
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DDF RESULTS 1-HR
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DDF RESULTS 6-HR
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DDF RESULTS 24-HR
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DDF RESULTS (PRELIMINARY)
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DDF RESULTS (PRELIMINARY)
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CONCLUSIONS
• ANALYSIS CURRENTLY UNDERGOING PEER AND CITY REVIEW
• DIFFERENT STATISTICAL METHODS FOR RAINFALL ESTIMATION
CAN SHOW VARIED RESULTS
• RAINFALL VALUES FROM THE 1998 USGS STUDY COULD BE
UPDATED IN AREAS WHERE EXTREME EVENTS HAVE BEEN
OBSERVED
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Q&A
• Definition of Statistics: The science of producing unreliable facts
from reliable figures.
Evan Esar
• Facts are stubborn, but statistics are more pliable.
Mark Twain
• Odds of this week’s satellite re-entry hurting someone in the world is
estimated 1 in 3,200. Odds of any one person being struck is 1 in 21
Trillion.
NASA, 9/21/11
Saul Nuccitelli, LAN, sanuccitelli@lan-inc.com
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