Improving American River Flood Frequency Analyses (1999) / Chapter Skim
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4 Climate and Floods: Role of Non-Stationarity
4 Climate and Floods: Role of Non-Stationarity
Pages 67-100
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From page 67... ...
Porparto and Ridolfi (1998) demonstrate that estimated flood exceedance probability can increase quite rapidly with time even in the presence of rather mild rising trends in the annual maximum flood.
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From page 68... ...
The 10 largest annual maximum floods In the 1905-1997 period on the American River in the Fair Oaks record occurred between late November and early 68 Improving American River Flood Frequency Analyses
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From page 69... ...
A composite of the average anomaly (departure from the full record) DJF sea level pressure for the 10 years with the largest annual maximum American River floods is shown in Figure 4.ib.
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From page 71... ...
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From page 72... ...
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From page 73... ...
Consequently, in terlus of flood potential and changes in flood frequency in the American River region, one needs to understand changes in the low frequency variability of the associated atmospheric flow patterns. These patterns are in turn related to oceanic temperature and ultimately oceanic circulation patterns, which are also related to atmospheric circulation patterns in an endlessly circular fashion, and hence to low frequency variability in ocean-atmosphere interactions such as the E!
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From page 74... ...
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From page 75... ...
A similar increase from the first to the second half of the century is seen in three-day amounts exceeding 14 inches. The three-day annual maximum basin precipitation for the American River basin estimated using the precipitation stations at Repressa, Auburn, Placerville, Nevada, Spaulding, and Tahoe shows similar trends.
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From page 78... ...
Although stations in many parts of the state exhibit this behavior, some areas show opposing effects. The 78 Improving America}' River Flood Frequency Analyses
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From page 79... ...
These analyses require long time series of homogeneous daily data, a situation difficult to find in many countries. For the United States as a whole, the study by Karl and Knight further shows little evidence of a stepped increase in the fractional contribution by heavier events at any point during the 20th Century.
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From page 80... ...
The low frequency projections of He SLP and surface temperature series were first converted to annual time series by picking off the value of the projection for the month of the annual maximum flood in a given year. The spectral coherence between these annual climatic time series and the annual maximum flood series were then computed.
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From page 82... ...
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From page 84... ...
They noted that weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that the observed decadal-scale winter temperature trends can explain the runofftiming trends.
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From page 85... ...
These shifts in atmospheric circulation are associated with concurrent shifts in both West Coast air temperatures and Norm Pacific sea surface temperatures, and with earlier snowmen and increased spring moisture fluxes in the American River basin. The investigations into the changing seasonality of flow, temperature, and precipitation were recently augmented by an analysis of trends in snow water equivalent (SWE)
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From page 86... ...
SOURCES OF SIERRA NEVADA CLIMATE VARIABILITY There are many possible physical sources of low-frequency variability of central Sierra Nevada climate behavior. These include quasi-periodic modes of ocean-atmosphere circulation, such as ENSO, as well as considerations related to global climate change due to increases in greenhouse gases and land use.
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From page 88... ...
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From page 89... ...
There is strong asymmetry in the response of the overlying atmosphere to tropical sea surface temperatures, and thus in cloudiness, precipitation, and heating of the atmosphere, and thus in teleconnections to and influences upon He mid-latitude jet stream. The convection and moisture sources in the western tropical Pacific are unusually active, and the interaction with equatorial excursions of the jetstream provides a western Pacific connection to the flow that eventually impinges on California.
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From page 90... ...
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From page 91... ...
Regimes of EI Nino/Southerr' Oscillation To the extent there may be a relation between floods and any of He phases of ENSO, such as La Nina, periods of extended predominance of one or the other ENSO phase could affect the frequency of Sierra Nevada floods. The record of ENSO does indeed show such behavior, most notably the period since 1976, when a major shift occurred in the Pacific climate (see Ebbesmeyer et al., 1991; Trenber~ and Hurrell, 1994~.
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From page 92... ...
Finally, an understanding of internal dynamic modes of the climate system with interdecadal time scales and their impacts on floods is essential if the potential effects of secular global climate change are to be sorted out from the last century of record. 92 Improving American River Flood Frequency Analyses
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From page 93... ...
, then the possibility exists to return to a prior regime, i.e., the one that existed during the first part ofthis century. Other Potential PDO Elects Not Involving ENSO lithe second way that the PDO could be relevant to central Sierra floods could be by modulating other connections, not related to ENSO, between the North Pacific and the Sierra Nevada.
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From page 94... ...
Although much of the focus has been on temperature, the realization has been slowly growing Hat other significant climatic adjustments to the altered radiation regime may be 94 Improving American River Flood Frequency Analyses
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From page 95... ...
With respect to human-induced changes in climate forcings, especially the radiative forcings associated with atmospheric composition changes, a widely held view is that such temporal trends are unidirectional and unlikely to change course in a century or two. Partly on the evidence of modeling experiments, it is likewise widely held that a steadily Increasing forcing will also lead to a steadily increasing response.
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From page 96... ...
are sufficient by themselves to produce regional circulation and surface temperature responses of the same magnitude as the changes that have been projected for changes resulting from greenhouse gas increases. In such regions as western Norm America worldwide land 96 Improving American River Flood Frequency Analyses
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From page 97... ...
Global climate change concerns related to greenhouse gas emissions over the last cenhuy may also be considered as a plausible factor in changing flood frequencies. The contribution of structured, oscillatory, interannual- to millennial-scale climate variability to changing flood potential in the region is also of considerable interest.
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From page 98... ...
climatic regime shifts may slowly or abruptly-significantly affect the local flood frequency curve for protracted periods, and (c) at this time, given the limited understanding of the low frequency climate-flood connection, the traditional independent identically cl~str~outeo approach to tiOOCl frequency estimation is recommended with the strong caution that the application of such a curve is likely to lead to significant biases or variability over any period of time.
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From page 99... ...
The existing static flood risk paradigm considers the estimation of a single flood frequency distribution from all available historical, regional,and paleoflood data and the application of the estimated distribution for an indefinite future period. A dynamic risk paradigm would call for the evaluation of potential flood risk over the duration of project operation, angler a regular flood frequency updating procedure.
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From page 100... ...
This relationship, coupled with "beliefs" as to scenarios for future climate derived from an analysis of the historical and paleoflood record and coupled general circulation models of the climate system, may be useful for assessing scenarios for future flood risk. A framework for formally conducting such analyses to better estimate potentially changing flood frequency distributions and their uncertainty is needed.
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