RAINFALL FREQUENCY ANALYSIS FOR NEW BRAUNFELS, TX (or Seems like we ve been having lots of 100-yr storms)

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1 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)

2 AGENDA 1. RECENT STORM REVIEW 2. HISTORY OF RAINFALL ANALYSIS 3. UPDATED ANALYSIS 4. CONCLUSIONS 5. Q & A 1950 FLOOD IN NEW BRAUNFELS

3 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

4 1998 FLOOD GUADALUPE RIVER AT IH-35

5 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

6 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

7 2002 FLOOD SOUTH ACCESS ROAD AFTER FLOOD WATERS RECEEDED

8 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

9 2004 FLOOD DRY COMAL AT LOOP 337

10 2010 FLOOD EXCESS OF 8 RAIN IN 3 HRS GUADALUPE RIVER AT COMMON ST. FLOODING ALONG GUADALUPE AND COMAL RIVERS NWS NEXRAD IMAGERY

11 2010 FLOOD GUADALUPE RIVER AT IH-35

12 2010 FLOOD COMAL RIVER AT SCHLITTERBAHN

13 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

14 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

15 TP-40 NATIONAL RAINFALL ATLAS TP YR 24-HR RAINFALL FIRST NATIONAL STUDY (1962) BY NOAA FAIRLY SHORT RECORD OF GAGE DATA GUMBEL DISTRIBUTION

16 1998 USGS TEXAS RAINFALL STUDY NWS RAINFALL STATIONS USGS WRIR (STUDY) USGS SIR (ATLAS) REVIEWED NWS STATIONS MIN 274 HOURLY 865 DAILY DATA THROUGH 1994 UPDATED STATISTICAL METHODS

17 1998 USGS VS TP-40 RAINFALL CONTOURS

18 USGS 100-YR, 24HR DEPTH COMAL COUNTY 24-HR 100-YR DEPTH

19 NEW BRAUNFELS DEPTH-DURATION-FREQUENCY TABLE

20 RAINFALL STATION MAP

21 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

22 FILTER 1 NWS STATIONS

23 FILTER 2 ACTIVE STATIONS

24 FILTER 3 RECORD LENGTH 20 YEARS

25 FILTER 4 WITHIN 30 MILES OF NEW BRAUNFELS

26 STATIONS USED IN ANALYSIS

27 NWS STATIONS USED

28 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)

29 CANYON DAM -ANNUAL MAXIMA 1 HR TO 6 HR

30 NEW BRAUNFELS -ANNUAL MAXIMA 1 DAY TO 3 DAY

31 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

32 EXTREME VALUE STATISTICS

33 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

34 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

35 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

36 NEW BRAUNFELS STATION ANNUAL MAXIMA 1 DAY

37 NEW BRAUNFELS STATION ANNUAL MAXIMA 2 DAY

38 NEW BRAUNFELS STATION ANNUAL MAXIMA 3 DAY

39 PULLING THE STATIONS TOGETHER 1998 USGS STUDY FOR TEXAS 270,000 MILES 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

40 DDF RESULTS 1-HR

41 DDF RESULTS 6-HR

42 DDF RESULTS 24-HR

43 DDF RESULTS (PRELIMINARY)

44 DDF RESULTS (PRELIMINARY)

45 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

46 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