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<br /> <br />416 <br /> <br />ROGER A. PIELKE, JR. <br /> <br />creates obstacles to proper understanding of the flood problem and, consequently, <br />the development of effective responses. <br />The IOO-year standard refers to a flood that has a one percent chance of being <br />ex.ceeded in any given year. It does not refer to a flood that occurs 'once every toO <br />years'. In fact, for a home in a IOO-year flood zone there is a greater than 26% <br />chance that it will see at least one I DO-year flood over a period of 30 years (and. <br />similarly, more than a 74% chance over 100 years). The general fonnula for the <br />cumulative probability of at least one flood of annual probability Pis (1- p)N ~ C <br />where N equals [he number of years from now, and C is lhe cumulative probability <br />over period N (P is assumed to be constant and events are independent from year <br />(Q year} By choosing values for P and C one can compute the number of years <br />that the cumulative probability (C) covers. <br />The concept and terminology of the 'I DO-year floodplain' was formally adopted <br />by the federal government as a standard for all public agencies in 1977 under Exec- <br />utive Order 11988.ln 1982 FEMA reviewed the policy and found that it was being <br />used in the agencies and, lacking a better alternative, concluded that the policy <br />should be retained (FIFMTF, 1992, p. 8-3). However, despite the FEMA review, <br />use of the concept of the I DO-year flood is encumbered by a number of logical and <br />practical difficulties (cr. Lord, 1994). <br />Pirst, there is general confusion among users of the \crm .\bou\ wh.~ it lY1C<U'\S. <br />Some use the term to refer to a flood that occurs every 100 years, as did the Mid- <br />western mayor who stated that 'after the 1965 flood, they told us this wouldn't <br />happen again for another 100 years' (lFMRC, 1994, p" 59). Public confusion is <br />widespread: A farmer suffering through Midwest flooding for the second time in <br />three years complained that 'Two years ago was supposed to be a I DO-year flood, <br />and they're saying this is a 75-year flood, What kind of sense does that make? <br />You'd think they'd get it right' (Peterson, 1995), <br />Second, the 'tOO-year flood' is only one of many possible probabilistic mea- <br />sures of an area's flood risk. For instance, in the part of the floodplain that is <br />demarcated as the' JDa-year floodplain' it is only the outer edge of that area that <br />is estimale<..llo have an annual probabiliLy of flooding of 0.01, yet confusion ex- <br />ists (Myers, 1994). Areas closer Lo the river have higher probabilities of flooding, <br />e.g., there are areas of a floodplain with a 2% annual chance of flooding (50-year <br />floodplain), 10% annual chance (lO-year floodplain). 50% annual chance (2.year <br />floodplain) etc., and similarly, areas farther from the river have lower probabilities <br />of flooding. The '100-year floodplain' is arbitrarily chosen for regulatory reasons <br />and does not reflect anything fundamentally intrinsic to the floodplain. <br />Third, the' I DO-year floodplain' is determined based on past flood records and is <br />thus subject to considerable errors with respect to the probabilities of future floods. <br />According to Burkham (1978) errors in determination of the 'IOO-year flood' may <br />be off by as much as 50% of flood depth. Depending on the slope of the 1100d <br />plain, lhis could translate into a significant eITor in terms of distance from the river <br />channel. A FEMA press release notes that 'in some cases there is a differenc~ of <br /> <br />NINE FALLACIES OF fLOODS <br /> <br />417 <br /> <br />only inches between the 10- and the JOO-year flood levels' (FEMA, 1996), Further. <br />researchers are beginning to realize an 'upper limit' on what can be known about <br />flood frequencies due to the lack of available trend data (Bobee and Rasmussen, <br />1995), <br />Fourth, the 100-year floodplain is not a natural feature, but rather is defined <br />by scientists and engineers based on the historical record. Consequently, while <br />the 'lOO-year floodplain' is dynamic and subject to redefinition based on new <br />flood events that add to the historical record, the regulatory definition is much <br />more difficult to change. For instance, following two years of major flooding on <br />the Salt River in Phoenix, Arizona, the previously estimated IOO-year flood was <br />reduced to a 50-year flood (FlFMTF. 1992. p, 9-7), What happens to the structures <br />in redefined areas? Any changes in climate patterns, especially precipitation, will <br />also modify the expected probabilities of inundation. For example, some areas of <br />the upper Midwest have documented a trend of increasing precipitation this cen- <br />tury (Changnon and Kunkel, 1995; Bhowmik et aI., 1994). Furthermore, human <br />changes to the river environment, e.g., levees and land use changes, can also alter <br />the hydraulics of floods. Finally, the extensive use of the term '100.year flood' <br />focuses attention on that aspect of flooding, sometimes to the neglect of the area <br />beyond the 100-year nood plain (Myers. 1994), <br />Wh.\t c.m be done? Givcn the pcrv.\sive use of the concept of the' lDO-yenr <br />flood' in flood insurance and regulatory decision-making it seems that adoption of <br />an alternative concept i~ unlikely. Nevt;~theless, there are a number of steps that <br />can be taken by those who use the concept when "dealing with poliC.y makers and <br />the public, First, we need to be more precise with language, The FIFMTF (1992) <br />recommends the phrase 'one percent annual chance flood' as a preferred alterna- <br />tive to '100-year flood', 'base flood', or 'one percent flood'. Another alternative <br />is 'national base flood standard' which removes reference to probability (Thomas, <br />1996, personal communication). Second, when communicating with the public and <br />the media, flood experts could take care to convert annual exceedances into annual <br />probabilities. And third, policy documents coold rely less on the' 1 DO-year flood' <br />to illustrate examples and propose policies, and at the very least explicitly discuss <br />floods of different magnitudes. <br /> <br />2.2. DAMAGING FLOODING IN RECENT YEARS IS UNPRECEDENTED BECAUSE <br />OF 'GLOBAL WARMING' <br /> <br />The phrase 'global warming' refers to the possibility that the earth's climate may <br />change because human activities are altering the composition of the atmosphere. <br />Scientists first raised this possibility more than a century ago, an9 in recent decades <br />policy makers have begun to express concern about the possibility of climate <br />change. Possible changes that have been discussed in the context of global warm- <br />ing include increasing or decreasing tropical cyclone activity, increased spread of <br />infectious diseases, change in mean global temperature and regional and local tem- <br />