RAINFALL FREQUENCY ANALYSIS FOR NEW BRAUNFELS, TX

Transcription

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, [email protected]
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