FLOOD09459

• County: Statewide
• Basin: Statewide
• Title: Nine Fallacies of Floods
• Date: 1/1/1999
• Prepared By: National Center for Atmospheric Research
• Floodplain - Doc Type: Educational/Technical/Reference Information

1\4

ROGER A. PIELKE, JR.

literature is that society has learned through experience what actions will lead to
j reduction in vulnerability to floods, but that these 'thoughtful pa~t recommen:
jalions of how to attain flood mitigation had never been a~equately ~mpleme~ted
'Changnon 1996, p. 312). The central thesis of this paper IS that while exper~en~e
has provid~d sufficient knowledge of steps necessary to begin to reduce socIety s
vulnerability to floods, society lacks an accurate understanding of the nat~re of the
flood problem itself.* Because decision makers lack knowledge of the, policy prob-
lem, potentia] policy solutions developed by flood experts are, often lm~lement~~
unsystematically (if at all), overlooked in favor of false solutions, or Without t

leadership needed to ensure their success.. " .' wa of a
A number of important misconcepl1ons or falhlcles st~nd I.n the y .
better understanding of the nation's nood problem. The tallacles are no~ unl.
versal with many flood experts, decision makers, and sectors of the public ~s.
capin~ their seductive logic. But enough people ~~ fa~1 prey to these fal1aCle~
of floods so as to create obstacles to improved utIlIzatIon of the ~essons of ~x
perience. This paper uses three of these lessons to organize presentatIOn of the nme

fallacies:

We know the wrong things about the nature of the problem (Fallacies I, 2, 3
I. .
and 4),
2. We don't know enough about why and with what intensity we should act
(Fallacies 5. 6 and 7),
3, We know enough about what might be done (Fallacies 8 and 9).

Of course, there are other fallacies of floods (and disasters more generally) which
would expand the list presented here (e.g., Glantz, 1?76).. . '. "
The U ose of raising the fallacies in this paper IS to contnbute t~ a syst.emat~c
definiti:n ~f the nation's flood problem. It makes sense that. e~fectlve. ~ctlons In
t fl ds ought to be based upon an accurate and realIstic defimtlon of the
response 0 00 . . . fl h)' th
roblem. While this paper focuses on river floods (I.e., large-scale and as 10. e
o oited States, the fallacies have potential broader relevance to th~ extent to wh~ch
h . fI ce policy in other flood.prone regions of the world. Simply corr~ctlOg
~~~~~e~~neliefs will not in itself 'solve' the nation's flood problem, but I~ can
re~ove obstacles to the implementation of previousl~ unsee~ or un~er~ppreclated
policy alternatives. It is certain that the challenge ~f Impr~vlOg society s response
to floods will face obstacles so long as these fal1acles persIst.

One influential branch of the policy literature views policy solutions ~istinct from, policy prob-
. . d certain circumstances. See particularly the garbage can

~:;~l~~tc~:e~:~ ;~~t~;~)oa~~ t~~ ;1.icY ~d problem 'streams' of Kingdon (1984). A review of
the problem definition literature is found In Plelke (1997a).

NINE FALLACIES OF FLOODS

415

2. Flood Fallacies

2.1. FLOOD FREQUENCIES ARE WELL UNDERSTOOD

Flood experts use the terms 'stage' and 'discharge' to refer to the size of a flood
(Belt, 1975). A flood stage is the depth of a river at some point and is a function
of the amount of water, but also the capacity of a river channel and floodplain
and other factors. Hence, upstream and downstream levees and different uses of
floodplain land can alter a flood's stage. A flood discharge refers to the volume of
water passing a particular point over a period of time. For example. in 1993 51.
Louis experienced 'Lhe highest slagc we've ever had, but not the biggest volume'.

We've had bigger flows, but the stage was different because the water could
flow from bluff to bluff. Now we have communities in the floodplain. Every
time you do something on a floodplain, you change the flood relationship.
Every time a farmer plants a field or a town puts in a levee, it affects upstream
flooding. That's why you can't really compare flooding at different times in
history (0, R, Dryhouse quoted in Corrigan, 1993),

According to the World Meteorological Organization's International Glossary
of Hydrology, 'flood frequency' is defined as 'the number of times a flood above a
given discharge or stage is likely to occur over a given number of years' (WMO,
1993). In the United States, flood frequencies are central to the operations of the
National Flood Insurance Program, which uses the term 'base flood' to note 'that in
any given year there is a one percent chance that a flood of that mag.nitude could be
equalled or exceeded' (FIFMTF. 1992. p, 9-7), The 'base flood' is more commonly
known as 'the tOO-year flood' and is 'probably the most misunderstood floodplain
management teno' (FlFMTF, 1992. p, 9-7),
A determination of the probability of inundation for various elevations within
a community is based on analysis of peak flows at a point on a particular river or
stream. However, 'there is no procedure or set of procedures that can be adopted
which, when rigidly applied to the available data, will accurately define the flood
potential of any given watershed' (USWRC, 1981, p. I). For many reasons, in-
cluding limitations on the data record and potential change in climate, 'risk and
uncertainty are inherent in any flood frequency analysis' (USWRC, 1981, p. 2).
Nevertheless, quantification of risk is a fundamental element of flood insurance as
well as many aspects of flood.related decision making.
In order to quantify flood risk, in the early 1970s the National Flood Insur-
ance Program adopted the IOO-year-flood standard (FIFMTF. 1992, p, 8-2). The
standard was adopted in order to standardize comparison of areas of risk between
communities. Since that time the concept of the N-year flood has become a com-
mon fixture in policy, media, and public discussions of floods. Unfortunately, 'the
general public almost universally does not properly understand the meaning of
the teno' (FIFMTF. 1992, p, 9-7), Misconceptions about the meaning of the teno