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)
AGENDA 1. RECENT STORM REVIEW 2. HISTORY OF RAINFALL ANALYSIS 3. UPDATED ANALYSIS 4. CONCLUSIONS 5. Q & A 1950 FLOOD IN NEW BRAUNFELS
PRODUCED >20 RAINFALL IN 24-HRS APPROACHING 500-YR FREQUENCY ~$750M DAMAGES, 31 DEATHS ACROSS TEXAS 1998 FLOOD NWS APPROXIMATED RAINFALL DEPTHS DECEMBER 1998 NRCS STUDY
1998 FLOOD GUADALUPE RIVER AT IH-35
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
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
2002 FLOOD SOUTH ACCESS ROAD AFTER FLOOD WATERS RECEEDED
2004 FLOOD AROUND6 RAIN IN 6 HOURS NRCS DAM #3 AT BLIEDER S CREEK AT FULL CAPACITY HEAVY RAINFALL DOWNSTREAM OFCANYON DAM, BLIEDER S CREEK, AND DRY COMAL WATERSHEDS NRCS DAM #3 AT BLIEDER S CREEK AT FULL CAPACITY
2004 FLOOD DRY COMAL AT LOOP 337
2010 FLOOD EXCESS OF 8 RAIN IN 3 HRS GUADALUPE RIVER AT COMMON ST. FLOODING ALONG GUADALUPE AND COMAL RIVERS NWS NEXRAD IMAGERY
2010 FLOOD GUADALUPE RIVER AT IH-35
2010 FLOOD COMAL RIVER AT SCHLITTERBAHN
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
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
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
1998 USGS TEXAS RAINFALL STUDY NWS RAINFALL STATIONS USGS WRIR 1998-4044 (STUDY) USGS SIR 2004-5041 (ATLAS) REVIEWED NWS STATIONS 17415-MIN 274 HOURLY 865 DAILY DATA THROUGH 1994 UPDATED STATISTICAL METHODS
1998 USGS VS. 1962 TP-40 RAINFALL CONTOURS
USGS 100-YR, 24HR DEPTH COMAL COUNTY 24-HR 100-YR DEPTH
NEW BRAUNFELS DEPTH-DURATION-FREQUENCY TABLE
RAINFALL STATION MAP
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
FILTER 1 NWS STATIONS
FILTER 2 ACTIVE STATIONS
FILTER 3 RECORD LENGTH 20 YEARS
FILTER 4 WITHIN 30 MILES OF NEW BRAUNFELS
STATIONS USED IN ANALYSIS
NWS STATIONS USED
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)
CANYON DAM -ANNUAL MAXIMA 1 HR TO 6 HR
NEW BRAUNFELS -ANNUAL MAXIMA 1 DAY TO 3 DAY
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
EXTREME VALUE STATISTICS
LOG PEARSON TYP III STREAM FLOW GUMBEL TP-40 GENERALIZED LOGISTICS (GLO) 1998 USGS 1-24 HR GENERALIZED EXTREME VALUE (GEV) 1998 USGS1-3 DAY EV- DISTRIBUTIONS
STANDARD MOMENTS STATISTICAL MOMENT M r = 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) DISTANCE r (0) INTIAL EQUATION V = dx/dt VELOCITY r (1) 1ST ORDER A = dv/dt ACCELERATION r (2) 2ND ORDER J = da/dt JERK OR JOLT r (3)3RD ORDER
SIMILAR TO STANDARD MOMENTS L-MOMENT STATISTICS 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
NEW BRAUNFELS STATION ANNUAL MAXIMA 1 DAY
NEW BRAUNFELS STATION ANNUAL MAXIMA 2 DAY
NEW BRAUNFELS STATION ANNUAL MAXIMA 3 DAY
PULLING THE STATIONS TOGETHER 1998 USGS STUDY FOR TEXAS 270,000 MILES 2 1998 USGS USED KRIGING TO PRODUCE CONTOURS USED ~850 STATIONS, GROUPED 5 AT A TIME 2011 LAN STUDY FOR NEW BRAUNFELS 30 MILES 2 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
DDF RESULTS 1-HR
DDF RESULTS 6-HR
DDF RESULTS 24-HR
DDF RESULTS (PRELIMINARY)
DDF RESULTS (PRELIMINARY)
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
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