Flood Design Criteria

DOE-STD-1020-2002

in these areas does not significantly affect the operation and safety of the SSC, then it may be

concluded that the design of the site-drainage system (i.e., for the 25-year rainfall) is adequate.

Conversely, if flooding does result in significant damage which impairs the operation or safety of

SSCs, appropriate measures must be taken to satisfy the performance goals. This may include

increasing the capacity of the drainage system, constructing detention ponds on site, or

hardening an SSC against the effects of flooding caused by local precipitation.

4.2

Flood Design Criteria

Unlike design strategies for seismic and wind hazards, it is not always possible to provide

margin in the flood design of a SSC. For example, the simple fact that a site is inundated (even

if structural damage does not occur), will cause significant disruption (e.g., down time during the

flood, clean-up). This is often unacceptable in terms of the economic impact and disruption of

the mission-dependent function of the site. Under these circumstances, there is no margin, as

used in the structural sense that can be provided when a site or SSC is inundated. Therefore, the

SSC must be kept dry and operations must not be interrupted in order to satisfy the performance

goals. Since a risk reduction cannot, in general, be specified, the hazard annual probability is set

to the performance goal probability of damage with the exception of Performance Category 1.

For Performance Category 1, a risk reduction corresponding to a factor of 2 is defined. This risk

reduction is based on the limited warning time that is required to evacuate personnel from an

area that may be flooded (Ref. 4-7).

The DBFL for Performance Category 1 can generally be estimated from available flood

hazard assessment studies. These include: the results of flood-screening studies, flood-insurance

analyses, or other comparable evaluations. For this Performance Category it is not necessary

that a detailed site-probabilistic hazard evaluation be performed, if the results of other recent

studies are available and, if uncertainty in the hazard estimate is accounted for.

For Performance Categories 2 through 4, a comprehensive site-specific flood hazard

assessment should be performed, unless the results of a screening analysis (see References 4-4

and 4-5) demonstrates that the performance goals are satisfied.

4.2.1 Performance Category 1

The performance goal for Performance Category 1 specifies that occupant safety be

maintained and that the probability of severe structural damage be less than or about 10-3 per

year. The mean hazard annual probability of exceedance is 2x10-3. In addition, event

combinations that must be considered are listed in Table 4-2.

To meet the performance goal for this category, two requirements must be met: (1) the

building structural system must be capable of withstanding the forces associated with the DBFL,

and (2) adequate time for warning must be available to ensure that building occupants can be

evacuated (i.e., 1 to 2 hours, Ref. 4-7). If the building is located above the DBFL, then structural

and occupant safety requirements are met.

4-9