FLOOD09261

• County: Statewide
• Basin: Statewide
• Title: Flood Geomorphology
• Date: 1/1/1987
• Floodplain - Doc Type: Educational/Technical/Reference Information

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APPLIED PALEOFLOOD HYDROLOGY

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Following United States convention, the frequency
analyses 'employed by Kochel (Chapter 22) and by Webb
et al. (Chapter 24) utilized procedures outlined by the U, S.
Water Resources Council (1982), The SWD-PSI data are
treated as "historical floods" and combined with gauging
station records using a log-Pearson TYPe ill analysis, Recent
work by Stedinger and Cohn (1986) shows that the U. S.
Water Resources Council (1982) procedure is relatively
inefficient at extracting useful information from historical
and paleoflood data. Stedinger and Cohn (1986) advocate
the use of maximum likelihood analysis for paleoflood
data, and work on this new approach was in progress at
the time of this writing,

APPLIED PALEOFLOOD HYDROLOGY

There is an accelerating trend in the recognition and use
of paleoflood data by V,S. agencies. On April 4, 1986,
Colorado State House Bill No. 1186 was signed into law,
requiring, when appropriate, the uSe of geologic and veg-
etative studies to establish probable future hazardous surface-
water flows in relation to reservoir design and construction.
This act folIowed from the use of paleoflood data in eval-
uating flood hazards and dam safety in the Colorado Front
(Costa, 1978; Jarrett and Costa, 1982; Jarrett, 1987).
Paleoflood hydrology has recently been evaluated by
several high level advisory committees in the United States.
A National Research Council report "Safety of Dams"
advocates the analysis of physical evidence of large pa-
leofloods to provide objective evidence of the likelihood
and frequency of larger floods than can be documented by
gauged flow records (National Research Council, 1985,
p, 235). The report notes that stratigraphic and geomorphic
evidence of extraordinary paleofloods have the potential
of illustrating what size floods can occur. Such paleoflood
data should be used to demonstrate that calculated PMF
values, used in darn safety design, are credible and are
neither unreasonably large nor small (National Research
Council, 1985, p. 235). Because high-quality paleoflood
data cannot be obtained at all potential darn sites, paleoflood
hydrologic techniques cannot always be used to construct
a safety evaluation flood. Nevertheless, where such data
can be obtained, they demonstrate what magnitude floods
are and are not possible.
The Vnited States has generally adopted hydrometeo-
rological procedures for dam spil1way design. U,S. darns
must be designed to withstand the discharge of a probable
maximum flood (PMF), A PMF is determined by using a
rainfall-runoff model for a particular drainage basin re-
ceiving the probable maximum precipitation (PMP), This
latter concept is a key determinant in risk analysis for dams
(National Research Council, 1985). The Work Group on
PMF Risk Assessment (1986) was concerned with the fea-
sibility of assigning a probability to the probable maximum

flood (PMF). Although the committee concluded that no
method was adequate fOr that task, it observed that some
approaches in paleoflood hydrology might yield interesting
data. The report states, "'This method allows for an evaluation
of major flood events OVer the past hundreds to thousands
of years and may provide guidance on extending the fre-
quency curve beyond the range of the gage data:'
As a part of the process of selecting a high-level ra-
dioactive waste repository, the U.S. Nuclear Waste Policy
Act of 1982 requires the submission of site characterization
plans (SCP) for candidate sites, An SCP is also required
by the V.S. Nuclear Regulatory Commission for licensing
the disposal of high-level wastes in geologic repositories.
Hydrology is a paramount concern on the SCPo According
to the Deparnnent of Energy (1985, p. 20) the SCP will
describe the following:

. . , the flood history of the candidate area and site, . , .
The data used will be based of measurements from gaging
stations and on inferences from the geologic record. The
probable maximum flood and its relation to the planned
facilities will be estimated, and the potential for future
flooding of the site will be discussed, . , ,Geologic evidence
of Pleistocene and Holocene ftooding used to asseSs future
flood potential will also be described,

Paleoflood hydrology is part of a broader scientific en-
deavor that utilizes stratigraphic geology in natural hazards
evaluation (Baker, 1982). Studies of the Quaternary strat-
igraphic record may be used to reconstruct magnitudes and
times of earthquakes, volcanic eruptions, mass movements,
coastal hurricanes, and other cataclysmic natural phenomena.
Many of the same research techniques and scientific ques-
tions apply to all these phenomena. Critical needs for ad-
vancing this broader research endeavor are the same as
those applying to paleoflood hydrology:

1. Accurate transfer functions are needed that relate
the appropriate geological evideoce of the paleohazard
to the quantitative magnitude of the formative process.
2. The spatial applicability of the various hazard re-
construction procedures needs to be defined.
3. Regimes of the hazard in time and space, as might
be induced by climate, tectonism, or physiography,
need to be identified.
4. Better understanding is needed of the cataclysmic
geological processes responsible for the hazards.
5. The appropriate probability concepts need to be related
to the hazards of interest in risk analysis.

As pointed out by Wolman (1982), public policy has
not yet responded to make effective use of the whole broad
class of geologic paleohazard evaluation. The standard
benefit-cost procedures used in design considerations are
closely linked to frequency analysis based on sampling