Improving American River Flood Frequency Analyses (1999) / Chapter Skim
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2 Data Sources
2 Data Sources
Pages 16-38
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From page 16... ...
These data can provide information about extreme flooding over much longer time frames than systematic records, and thus could increase the accuracy of the frequency analysis (Cohn and Stedinger, 19874. Regional studies of maximum precipitation and flood discharges can provide information about extreme floods that can be used to check the accuracy of estimated flood distributions, particularly when these distributions are extrapolated (i.e., applied to flood magnitudes much larger than observed in the systematic record)
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From page 17... ...
stresses the need to assess the reliability of extreme flood data, particularly data collected before 1950. Flood frequency analysis of systematic flood data typically assumes that the data are independent and identically distributed in time (i.e., temporally uncorrelated and stationary)
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From page 18... ...
There are three potential problems with the use of historical data in flood frequency analysis. First, estimates of peak flood discharges associated with historical stage information are subject to error; such errors can be reduced, however, by careful hydraulic analysis, and their impact on flood frequency analysis can be minimized by explicitly accounting for them in the analysis.
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From page 19... ...
Knowledge of the nonoccurrence of floods for long periods of time has great potential value in improving flood frequency estimates (Stedinger and Cohn, 1986~. The actual value depends on the correctness of the assumed probability distribution and of the assumption that flood flows are independent and identically distributed.
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From page 20... ...
Errors in the estimation of peak discharges and ages can be controlled by careful hydraulic and laboratory analysis. Furthermore, these errors can be quantified and incorporated Into the flood frequency analysis.
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From page 21... ...
1250 and 1450, during the transition from the medieval warm interval to the cooler Little Ice Age. All of these changes were apparently associated with changes in mean annual temperature of only about 1-2°C and changes in mean annual precipitation of <10-20%." Knox's evidence suggests that during the past 7,000 years, floods on upper Mississippi River tributaries have not behaved as independent and identically distributed random variables.
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From page 22... ...
22 ~ Cal I, ~ .s ° ~ ~ ~ o ~._ _ ~ ~ cn g ~.m ho ~ o ~ ~ _ ._ ~- ~ =^ ~ ~ oh ~ .e Cal ~ ~ ~0 ~ ° to .= ° Cal o 3 ~ =~ ¢ ~ ~ to ·s to .G o ~ .
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From page 23... ...
Unless the paleoflood record is grossly incomplete, this suggests that extreme floods in Arizona and southern Utah do not behave as independent and identically distributed random variables. The committee is interested in the use of flood frequency analysis to predict and mitigate future flood risk.
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From page 24... ...
In addition, critics have suggested that the frequency analysis should include the use of historical and paleoflood data and have argued that the results of the USACE flood frequency analysis are inconsistent with an existing flood envelope curve for California and with current estimates of PMF. These issues are explored in the remainder of this chapter.
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From page 25... ...
lo to to to to N (SI0)
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From page 26... ...
The only statistically significant (P<0.05) shift In American mean flood magnitudes relative to regional values occurred in or around 191S, well before 26 Improving American River Flood Frequency Analyses
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From page 27... ...
These are presumably within the range of gaging error, particularly when gage measurements represent extrapolations of the rating curve well beyond any discharges measured by current meter, as would be the case for an extreme flood. Cumulative Folsom Inflow 2000000 1 000000 O -1 0000~ 1 ~ ~ FOLSOM DAM CONSTRUCTED , 1 1 0 200000 400000 600000 800000 1000000 1200000 Cumulative North Fork Dam Inflow FIGURE 2.5 Cumulative peak annual inflows to North Fork Dam (North Fork of the American River)
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From page 28... ...
To construct a series of estimated basin average precipitations suitable for evaluating the homogeneity of American River flood records, daily data were assembled for the stations, shown In Table 2.1. Daily precipitation and temperature data in electronic format were obtained from the Western Regional Climate Center (WRCC)
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From page 30... ...
Although the impacts of these activities on flood hydrology are not well known, it is important to consider them when evaluating the relevance of historical and paleoflood data. Initial gold mining activities involved small placer claims along Sierra streams and probably had relatively minor ejects.
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From page 31... ...
As a result of the rapid population growth of California after World War II, timber harvesting rapidly increased, reaching 6 billion board feet per year by 1960. Grazing of domestic livestock, primarily sheep and cattle, has probably affected a larger proportion of the Sierra Nevada than any other human activity Grazing was minimal prior to about 1860, then increased dramatically until the early 19OOs.
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From page 32... ...
What are the implications of these various human activities with respect to the use of historical and paleoflood data for flood frequency estimation on the American River? The most obvious implication is that the enormous amount of mining sediment in the American River during the latter part of the 19th century makes it very difficult to accurately estimate historical flood discharges during that period, precisely the period when historical information is available.
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From page 33... ...
In any case, it is prudent to cautiously incorporate the historical data in the flood frequency analysis. Paleoflood Data As this report was being prepared, the U.S.
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From page 34... ...
If floods can be assumed to be independent and identically distributed in time, then all past information is equally 34 improving American River Flood Frequency Analyses
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From page 35... ...
Where paleoflood information is inconsistent with modern flood data (i.e., a systematic flood record) , the judgment might be not to use the paleoflood data in the flood frequency analysis.
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From page 36... ...
have peak discharges within 10% of the envelope discharge of 267,000 cfs; of these, the peak discharge of the 1997 flood was estimated to be about 10% larger. Given the precision of flood peak estimates, these four observations essentially lie on the Meyers envelope curve, indicating that the curve does not provide an upper bound for American River floods.
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From page 37... ...
. Using loss rates based on saturated soil for unfrozen ground and snow cover for frozen ground, the USER calculated one- and three-day probable maximum flood discharges of 575,000 and 401,000 cts, respectively, for regulated conditions upstream.
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From page 38... ...
· Meyer's envelope curve of maximum flood discharges is not especially useffi! to American River frequency analysis.
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