Floods
From ClimateWiki
The IPCC claims flooding has become more frequent and severe in response to twentieth century global warming. But it is important to establish whether floods are truly becoming more frequent or severe, and whether other factors might be behind such trends if they in fact exist. In this section we highlight studies addressing both questions.
To test for long-term changes in flood magnitudes and frequencies in the Mississippi River system of the United States, Pinter et al. (2008) “constructed a hydrologic database consisting of data from 26 rated stations (with both stage and discharge measurements) and 40 stage-only stations.” Then, to help “quantify changes in flood levels at each station in response to construction of wing dikes, bendway weirs, meander cutoffs, navigational dams, bridges, and other modifications,” they put together a geospatial database consisting of “the locations, emplacement dates, and physical characteristics of over 15,000 structural features constructed along the study rivers over the past 100–150 years.” As a result of these operations, Pinter et al. write, “significant climate- and/or land use-driven increases in flow were detected,” but they indicate “the largest and most pervasive contributors to increased flooding on the Mississippi River system were wing dikes and related navigational structures, followed by progressive levee construction.”
In discussing the implications of their findings, Pinter et al. write, “the navigable rivers of the Mississippi system have been intensively engineered, and some of these modifications are associated with large decreases in the rivers’ capacity to convey flood flows.” Hence, it would appear man has indeed been responsible for the majority of the increased flooding of the rivers of the Mississippi system over the past century or so, but not in the way suggested by the IPCC. The question that needs addressing by the region’s inhabitants has nothing to do with CO2 and everything to do with how to “balance the local benefits of river engineering against the potential for large-scale flood magnification.”
In a study designed to determine the environmental origins of extreme flooding events throughout the southwestern United States, Ely (1997) wrote, “paleoflood records from nineteen rivers in Arizona and southern Utah, including over 150 radiocarbon dates and evidence of over 250 flood deposits, were combined to identify regional variations in the frequency of extreme floods,” and that information “was then compared with paleoclimatic data to determine how the temporal and spatial patterns in the occurrence of floods reflect the prevailing climate.” The results of this comparison indicated “long-term variations in the frequency of extreme floods over the Holocene are related to changes in the climate and prevailing large-scale atmospheric circulation patterns that affect the conditions conducive to extreme flood-generating storms in each region.” These changes, in Ely’s view, “are very plausibly related to global-scale changes in the climate system.”
With respect to the Colorado River watershed, which integrates a large portion of the interior western United States, she writes, “the largest floods tend to be from spring snowmelt after winters of heavy snow accumulation in the mountains of Utah, western Colorado, and northern New Mexico,” such as occurred with the “cluster of floods from 5 to 3.6 ka,” which occurred in conjunction with “glacial advances in mountain ranges throughout the western United States” during the “cool, wet period immediately following the warm mid-Holocene.”
The frequency of extreme floods also increased during the early and middle portions of the first millennium AD, many of which coincided “with glacial advances and cool, moist conditions both in the western U.S. and globally.” Then came a “sharp drop in the frequency of large floods in the southwest from AD 1100-1300,” which corresponded, in her words, “to the widespread Medieval Warm Period, which was first noted in European historical records.” With the advent of the Little Ice Age, however, there was another “substantial jump in the number of floods in the southwestern U.S.,” which was “associated with a switch to glacial advances, high lake levels, and cooler, wetter conditions.” Distilling her findings down to a single succinct statement and speaking specifically of the southwestern United States, Ely writes, “global warm periods, such as the Medieval Warm Period, are times of dramatic decreases in the number of high-magnitude floods in this region” [emphasis added].
Looking at the other side of the continent, Villarini and Smith (2010) “examined the distribution of flood peaks for the eastern United States using annual maximum flood peak records from 572 U.S. Geological Survey stream gaging stations with at least 75 years of observations.” This work revealed, “in general, the largest flood magnitudes are concentrated in the mountainous central Appalachians and the smallest flood peaks are concentrated along the low-gradient Coastal Plain and in the northeastern United States.” They also found “landfalling tropical cyclones play an important role in the mixture of flood generating mechanisms, with the frequency of tropical cyclone floods exhibiting large spatial heterogeneity over the region.” They additionally write, “warm season thunderstorm systems during the peak of the warm season and winter-spring extratropical systems contribute in complex fashion to the spatial mixture of flood frequency over the eastern United States.”
Of greater interest to the climate change debate, however, were their more basic findings: (1) “only a small fraction of stations exhibited significant linear trends,” (2) “for those stations with trends, there was a split between increasing and decreasing trends,” and (3) “no spatial structure was found for stations exhibiting trends.” Thus they concluded, (4) “there is little indication that human-induced climate change has resulted in increasing flood magnitudes for the eastern United States.”
Much the same was reported for Canada by Cunderlik and Ouarda (2009). They evaluated trends in the timing and magnitude of seasonal maximum flood events across that country, based on pertinent data obtained from 162 stations of the Reference Hydrometric Basin Network established by Environment Canada over the 30-year period 1974 to 2003. In spite of the supposedly unprecedented warming over the period of time they studied, the Canadian researchers report finding “only 10% of the analyzed stations show significant trends in the timing of snowmelt floods during the last three decades (1974–2003),” and they say these results imply “the occurrence of snowmelt floods is shifting towards the earlier times of the year,” as would be expected in a warming world. However, they note most of the identified trends “are only weakly or medium significant results,” and they add “no significant trends were found in the timing of rainfall-dominated flood events.”
With respect to flood magnitudes, the two scientists state the trends they observed “are much more pronounced than the trends in the timing of the floods,” but they note most of these trends “had negative signs, suggesting that the magnitude of the annual maximum floods has been decreasing over the last three decades.” In addition, they found “the level of significance was also higher in these trends compared to the level of significance of the trends in the timing of annual maximum floods.”
In Europe, Schmocker-Fackel and Naef (2010a) explored the relationship between climate and flooding from a paleo-perspective. Specifically, they collected and analyzed historical flood time-series of 14 catchments located in northern Switzerland, datasets for which stretched back a full five centuries. From these data the two Swiss scientists were able to identify four periods of frequent flooding in northern Switzerland, lasting between 30 and 100 years each (1560–1590, 1740–1790, 1820–1940, and since 1970). They report the first three periods of intervening low flood frequency (1500–1560, 1590–1740, and 1790–1810) were found to correspond to periods of low solar activity. However, they report, “after 1810 no relationship between solar activity and flood frequency was found, nor could a relationship be established between reconstructed North Atlantic Oscillation indices or reconstructed Swiss temperatures.” In addition, they determined “the current period of increased flood frequencies has not yet exceeded those observed in the past.” They also write, “a comparison with the flood patterns of other European rivers suggests that flood frequencies are not in phase over Europe.” In light of their several diverse findings, Schmocker-Fackel and Naef (2010a) thus concluded “the current period with more floods in northern Switzerland, which started in the mid 1970s, might continue for some decades,” even under conditions of “natural climatic variation.”
In a contemporaneous paper on Switzerland floods, also authored by Schmocker-Fackel and Naef (2010b), the two researchers further explored this subject by analyzing “streamflow data from 83 stations with a record length of up to 105 years, complemented with data from historical floods dating back to 1850,” in an effort to place the extreme flooding that occurred in their country in 1999, 2005, and 2007 in an historical construct. This expanded analysis indicated “in Switzerland, periods with frequent floods have alternated with quieter periods during the last 150 years” and “since 1900, flood-rich periods in northern Switzerland corresponded to quiet periods in southern Switzerland and vice versa.” As for the fact that over the same period of time “three of the four largest large-scale flood events in northern Switzerland have all occurred within the last ten years,” they report “a similar accumulation of large floods has already been observed in the second half of the 19th century.” In addition, they state, “studies about changes in precipitation frequencies in Switzerland come to similar conclusions,” citing the work of Bader and Bantle (2004).
In another paper from Europe, Diodato et al. (2008) conducted a detailed study of erosive rainfall in the Calore River Basin (southern Italy) “using data from 425-year-long series of both observations (1922–2004) and proxy-based reconstructions (1580–1921).” Their results showed pronounced interdecadal variations, “with multi-decadal erosivity reflecting the mixed population of thermo-convective and cyclonic rainstorms with large anomalies,” while noting “the so-called Little Ice Age (16th to mid-19th centuries) was identified as the stormiest period, with mixed rainstorm types and high frequency of floods and erosive rainfall.”
In the concluding section of their paper, the three researchers write, “in recent years, climate change (generally assumed as synonymous with global warming) has become a global concern and is widely reported in the media.” In regard to concern over floods becoming more frequent and more severe as the planet warms, however, Diodato et al. say their study shows “climate in the Calore River Basin has been largely characterized by naturally occurring weather anomalies in past centuries (long before industrial CO2 emissions), not only in recent years,” and there has been a “relevant smoothing” of such events during the modern era.
Working in southeast Spain, Benito et al. (2010) reconstructed flood frequencies of the Upper Guadalentin River using “geomorphological evidence, combined with one-dimensional hydraulic modeling and supported by records from documentary sources at Lorca in the lower Guadalentin catchment.” According to these scientists, the combined palaeoflood and documentary records indicated past floods were clustered during particular time periods: AD 950–1200 (10), AD 1648–1672 (10), AD 1769–1802 (9), AD 1830–1840 (6), and AD 1877–1900 (10). The first time interval coincides with the Medieval Warm Period, and the latter four fall within the confines of the Little Ice Age. By calculating mean rates of flood occurrence over each of the five intervals, one obtains a value of 0.40 floods per decade during the Medieval Warm Period and an average value of 4.31 floods per decade over the four parts of the Little Ice Age.
Czymzik et al. (2010) explored the relationship between level of warmth and degree of flooding as it may have been manifested in southern Germany over the past 450 years. In the opening paragraph of their paper, they observe “assumptions about an increase in extreme flood events due to an intensified hydrological cycle caused by global warming are still under discussion and must be better verified,” while noting some historical flood records indicate “flood frequencies were higher during colder periods (Knox, 1993; Glaser and Stangl, 2004), challenging the hypothesis of a correlation between the frequency of extreme floods and a warmer climate.”
Against this backdrop, Czymzik et al. retrieved two sediment cores from the deepest part of Lake Ammersee in southern Germany (48°00’N, 11°07’E), which they then analyzed via what they describe as “a novel methodological approach that combines microfacies analyses, high-resolution element scanning (µ-XRF), stable isotope data from bulk carbonate samples (δ13Ccarb, δ18Ocarb), and X-ray diffraction (XRD) analyses (Brauer et al., 2009).”
The six scientists determined the flood frequency distribution over the entire 450-year time series “is not stationary but reveals maxima for colder periods of the Little Ice Age when solar activity was reduced,” while reporting “similar observations have been made in historical flood time series of the River Main, located approximately 200 km north of Ammersee (Glaser and Stangl, 2004), pointing to a wider regional significance of this finding.”
Working in the United Kingdom a couple years earlier, Hannaford and Marsh (2008) noted “recent flood events have led to speculation that climate change is influencing the high-flow regimes of UK catchments” and “projections suggest that flooding may increase in [the] future as a result of human-induced warming.” Utilizing the U.K. “benchmark network” of 87 “near-natural catchments” identified by Bradford and Marsh (2003), Hannaford and Marsh conducted “a UK-wide appraisal of trends in high-flow regimes unaffected by human disturbances” to test such speculation. They found “significant positive trends were observed in all high-flow indicators ... over the 30–40 years prior to 2003, primarily in the maritime-influenced, upland catchments in the north and west of the UK.” However, they state, “there is little compelling evidence for high-flow trends in lowland areas in the south and east.” They also note, “in western areas, high-flow indicators are correlated with the North Atlantic Oscillation Index (NAOI),” so “recent trends may therefore reflect an influence of multi-decadal variability related to the NAOI.” In addition, they state, longer river flow records from five additional catchments they studied “provide little compelling evidence for long-term (>50 year) trends but show evidence of pronounced multi-decadal fluctuations.” Finally, they note, “in comparison with other indicators, there were fewer trends in flood magnitude” and “trends in peaks-over-threshold frequency and extended-duration maxima at a gauging station were not necessarily associated with increasing annual maximum instantaneous flow.” In light of their several observations, Hannaford and Marsh conclude, “considerable caution should be exercised in extrapolating from any future increases in runoff or high-flow frequency to an increasing vulnerability to extreme flood events.”
In another paper from Europe, Matthews et al. (2009) conducted detailed investigations at three alpine slope-foot mires located in the valley of Leirdalen in an area known as Sletthamn, above the treeline among some of the highest mountains in southern Norway, where they say “exceptionally detailed radiocarbon-dating controlled chronologies of Holocene debris-flow events have been reconstructed.” This allowed them to analyze “the frequency and timing of debris flows since c. 8500 cal. BP which, in turn, are related to climatic variability, extreme climatic events and site conditions.”
The results of this exercise revealed “no obvious correlation between debris-flow frequency and a relative warm climate.” In fact, they write, “debris-flow frequency was lowest post-8000 cal. BP during the Holocene Thermal Maximum” and most of the “century- to millennial-scale phases of enhanced debris-flow activity appear to correlate with Neoglacial events,” one of which was the Little Ice Age. In addition, they write, “the Sletthamn record agrees quite closely with a compilation of other debris-flow records from widely distributed sites in east and west Norway.” What is more—citing the work of Berrisford and Matthews (1997), Stoffel and Beniston (2006), Pelfini and Santilli (2008), and Stoffel et al. (2008)—they report “there appears to be no consistent upward trend in debris-flow frequencies over recent decades,” when one might have expected them to be growing in both number and magnitude if the model-based claims were correct. Given these findings, the Norwegian and U.K. researchers conclude there is little real-world evidence “for the association of higher debris-flow frequencies with an increasingly warm climate.” In fact, they state, “the evidence suggests the opposite.”
Panin and Nefedov (2010) identified “a series of alternating low-water (low levels of seasonal peaks, many-year periods without inundation of flood plains) and high-water (high spring floods, regular inundation of floodplains) intervals of various hierarchal rank” for the Upper Volga and Zapadnaya Dvina Rivers of Russia. The two Russian researchers report “low-water epochs coincide with epochs of relative warming, while high-water epochs [coincide] with cooling epochs,” because “during the climate warming epochs, a decrease in duration and severity of winters should have resulted in a drop in snow cover water equivalent by the snowmelt period, a decrease in water discharge and flood stage, and a decrease in seasonal peaks in lake levels,” noting “a model of past warming epochs can be the warming in the late 20th century, still continuing now.” They also report finding, “in the Middle Ages (1.8–0.3 Ky ago), the conditions were favorable for long-time inhabiting [of] river and lake floodplains, which are subject to inundation nowadays.” In addition, their results indicate that of this time interval, the period AD 1000–1300 hosted the greatest number of floodplain occupations.
Interestingly, Panin and Nefedov state this last period and other “epochs of floodplain occupation by humans in the past can be regarded as hydrological analogues of the situation of the late 20th-early current century,” which they say “is forming under the effect of directed climate change.” This relationship clearly implies the current level of warmth in the portion of Russia that hosts the Upper Volga and Zapadnaya Dvina Rivers is not yet as great as it was during the AD 1000–1300 portion of the Medieval Warm Period.
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Definition
From Wikipedia, the free encyclopedia
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.
While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.
Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed in natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding.
The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities to destroy civilization as an act of divine retribution, and are featured in the mythology of many cultures.
References
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