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<br />ROGER A. PIELKE, JR.
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<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
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<br />417
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<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-
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