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 Definition Recurrence Interval The 100 year flood--What it means  Sacramento flooding issues Flood control methods Occurs when water overtops a stream’s banks and spills out onto the flood plain.http://waterdata.usgs.gov/ca/nwis/swWhy Not January 1 through Dec 31?California Stream Hydrograph020040060080010001200140016001/1 2/20 4/10 5/30 7/19 9/7 10/27 12/16DateDischarge, cfsAnnual Peak discharges: maximum momentary peak discharge (Q),in cubic feet/second (cfs), in each year of recordhttp://waterdata.usgs.gov/nwis10.00100.001000.0010000.00194719521957196219671972197719821987199219972002Station 11317000: Peak discharge 1947-2006Water YearPeak QAnnual flood series: a list of annual floodsWe need a measure of flooding for which we can associate a probability. If we use the maximum instantaneous streamflow during an annual period, we can determine an annual exceedance probability.If we have a record of 10 (or 50 or 100 or..) annual peak flows on a stream, can we use those 10 (or 50 or 100 or ..) recorded values to say something about how likely it will be next year (or any year) to have an annual peak bigger than the largest recorded value?Year Discharge (cfs)1998 91401999 81302000 10,1002001 22402002 27402003 44602004 32502005 15,4002006 27,200USGS site:11413000 N Yuba R Bl Goodyears Bar, CAYear Discharge (cfs) Rank2006 27,200 12005 15,400 22000 10,100 31998 9140 41999 8130 52003 4460 62004 3250 72002 2740 82001 2240 9Hydrologists list the dischargesin descending order and rank themNext we need to find the probabilitythat in any given year the instantaneous discharge will exceed the recorded discharge valueDischarge (cfs) RankProbability of Exceedance27,200 115,400 210,100 39140 48130 54460 63250 72740 82240 9Discharge (cfs) RankProbability of Exceedance27,200 1 1/9= 11%15,400 2 2/9= 22%10,100 3 3/9= 33%9140 4 4/9= 44%8130 5 5/9= 56%4460 6 6/9= 67%3250 7 7/9= 78%2740 8 8/9= 89%2240 9 9/9= 100%Are these probabilities realistic?Discharge (cfs) Rank Probability: (rank/ (n+1))27,200 1 1/10= 0.115,400 2 2/10= 0.210,100 3 3/10= 0.39140 4 4/10= 0.48130 5 5/10= 0.54460 6 6/10= 0.63250 7 7/10= 0.72740 8 8/10= 0.82240 9 9/10=0.9More realistic--method used by HydrologistsProbability: (rank/ (n+1))Recurrence Interval (R=1/ P) in years1/10= 0.1 1/ 0.1 = 102/10= 0.2 1/ 0.2 = 53/10= 0.3 1/ 0.3 = 3.34/10= 0.4 1/ 0.4 = 2.55/10= 0.5 1/ 0.5 = 2.06/10= 0.6 1/ 0.6 = 1.77/10= 0.7 1/ 0.7 = 1.48/10= 0.8 1/ 0.8 = 1.39/10=0.9 1/ 0.9 = 1.1Use your calculator to find the recurrence interval, or can rearrange the recurrence interval equation to: R= (n+1)/m where m= rank1,00010,000100,000 Discharge, cubic feet per secondANNUAL PEAK DISCHARGERecorded11413000 N YUBA R BL GOODYEARS BAR, CA1.11 1.25 2 5 10 25 50 100 Recurrence interval, yearsAnnual peak Q values plotted on lognormal probability paper1,00010,000100,000 Discharge, cubic feet per secondANNUAL PEAK DISCHARGERecordedBest Fit Line11413000 N YUBA R BL GOODYEARS BAR, CA1.11 1.25 2 5 10 25 50 100 Recurrence interval, years1,00010,000100,000 Discharge, cubic feet per secondANNUAL PEAK DISCHARGERecordedBest Fit Line11413000 N YUBA R BL GOODYEARS BAR, CA1.11 1.25 2 5 10 25 50 100 Recurrence interval, yearsExtraploated 100 Year Discharge1,00010,000100,000 Discharge, cubic feet per secondANNUAL PEAK DISCHARGERecordedLog Pearson DistributionBest Fit Line11413000 N YUBA R BL GOODYEARS BAR, CA1.11 1.25 2 5 10 25 50 100 Recurrence interval, yearsExtraploated 100 Year DischargeLimitations:• Extrapolating too far past the data;USGS rule of thumb—don’t extrapolatepast twice the record length• Not using the appropriate scales to plotprobabilities and recurrence intervals• Sometimes other probability methodsmay be more appropriatemnR1 Store lots of water at high elevation Drain water quickly Into flat area with a single, narrow outletFlood of 1862 1986 10 inches rain in 11 days Folsom Lake exceeds designed capacity Record releases from Folsom Dam Folsom Dam protection downgraded to about 70 year flood 1997 Record Flow Rates Sacramento spared from flooding▪ Modesto hit hard Paving increases amount and rate of surface runoff Sacramento Feather Yuba American Consumnes Mulkemne Stoney Cr. Cache Cr. Dams upstream 26 dams Levees River “bypasses” Hold 75% of Sac. River Water during floods Auburn Dam Proposed in 1960s Environmental opposition Recently active fault mapped at dam site Raise and strengthen levees Raise Folsom Dam Increase spillways on Folsom Dam Auburn Dam Proposed in 1950s Environmental opposition Raise and strengthen levees Raise Folsom Dam Increase spillways on Folsom Dam 1999 US Congress Appropriated funds to improve Folsom Dam 2000 Sacramento voters approved an increase in property taxes to improve flood control July 17 Drained 40% of Folsom Lake 40,000 cfs $20 M repair cost Fresh water in SF Bay Salmon and Bass fooled into beginning fall


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Sac State GEOL 10 - Flooding

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