A preliminary flood hazard analysis provides estimates of the probability of floods

DOE-STD-1023-95

probability value (10-5 to 10-3 (Kite, 1988). Furthermore, the results of any available existing

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-

STD-1022-94.

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 stage-discharge relationship (a relationship between flow

discharge and flood stage).

Stage-discharge relationships derived from historical floods, hydraulic

evaluation (e.g., Manning's equation, step-backwater calculation), and channel

geometry data. Uncertainty in estimating these relationships must be accounted

for (McCann and Boissonnade, 1988b).

Step 4:

Transform the probability-discharge frequency to stage frequency to determine

the probability of exceeding selected stage elevations using the stage-discharge

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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