
 DOESTD102395
flood frequency analyses should be compared to the results of a preliminary flood hazard
analysis.
A preliminary flood hazard analysis provides estimates of the probability of floods and an
assessment of the uncertainty in the hazard estimate. Rivers or streams are the most common
sources of flooding. For this type of flooding, a simplified acceptable method to estimate the
probability that specified elevations at the DOE sites will be exceeded consists of the following
steps (McCann and Boissonnade, 1988a):
Step 1:
Compile, obtain and update a data base of peak discharge as described in DOE
STD102294.
Step 2:
Estimate the probability of exceedance of selected peak discharge levels with
associated uncertainty.
An acceptable methodology using streamflow data, and including uncertainty
estimates due to the statistical model selected and limited flood data is provided
by McCann and Boissonnade, (1986).
Step 3:
Determine the stagedischarge relationship (a relationship between flow
discharge and flood stage).
Stagedischarge relationships derived from historical floods, hydraulic
evaluation (e.g., Manning's equation, stepbackwater calculation), and channel
geometry data. Uncertainty in estimating these relationships must be accounted
for (McCann and Boissonnade, 1988b).
Step 4:
Transform the probabilitydischarge frequency to stage frequency to determine
the probability of exceeding selected stage elevations using the stagedischarge
relationship.
Existing dam failure analyses performed as part of emergency action plans shall be used if they
are available. Otherwise, acceptable simplified analysis methods to assess flooding due to dam
failure include those given by Hann et al., 1982 and McCann et al., 1985.
Acceptable hydraulic models to assess runoff or ponding include those given by Crawford and
Linsley, (1966) or HEC (1986).
The main results of a preliminary flood hazard assessment consists of the family of flood
hazard curves that describes the annual probability that specified flood elevations at the site
will be exceeded. A probability weight is assigned to each curve that quantify the uncertainty
in the analysis (see for example McCann and Boissonnade 1989). Based on the family of
hazard curves, a mean flood hazard curve can be calculated.
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