The basic design strategy for SSCs in Performance Categories 2 to 4 (excluding local precipitation), is to construct the SSC above the DBFL. When this can be done, flood hazards are not considered in the design basis except that possible raised ground water level must be considered. The flood criteria have been established with this basic strategy in mind. Note that local precipitation is an exception since all sites must consider this hazard in the design of the site stormwater management system, roof systems, etc.
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Since it may not always be possible to construct a new SSC above the DBFL level, alternate design strategies must be considered. The following lists the hierarchy of flood design strategies:
1. Situate the SSC above the DBFL level, 2. Modify the flood, or
3. Harden the site or SSC to mitigate the effects of the DBFL such that the performance goals are satisfied, and
4. Establish emergency operation plans to safely evacuate employees and secure areas with hazardous, mission-dependent, or valuable materials.
If an SSC is situated above the DBFL, the performance goals are readily satisfied. If an SSC is located below the DBFL, alternatives can be considered to modify the magnitude of the flood or mitigate its effects such that the likelihood of damage and interruption of operations is acceptably low (i.e., performance goals are satisfied). In addition, emergency operation plans must be developed that establish the procedures to be followed to recognize/identify the flood hazard in a timely manner and provide for occupant safety and secure areas that may be
vulnerable to the effects of flooding. The implementation of emergency operation plans is not, in general, an alternative to satisfy the performance goals. While they are necessary to provide for occupant safety, generally they do not adequately limit the level of damage and interruption to facility operations.
Under certain circumstances the flood can be modified to limit the magnitude of the hazard. Alternatives include the construction of detention ponds that provide for the collection and controlled release of runoff on-site, modification of stream channels, etc.
The strategy of hardening a SSC or site and providing emergency operation plans is secondary to siting facilities above the DBFL level because some probability of damage does exist and SSC operations may be interrupted. If it is determined that a SSC may be impacted by the DBFL and thus must be hardened, the designer must determine the flood loads associated with the DBFL. The design of flood mitigation systems (i.e., exterior walls, flood-proof doors, etc.) must be conducted in accordance with the requirements specified in applicable regulations.
The evaluation of the site stormwater management system and roof design (i.e., drainage and structural capacity) differs somewhat from that for other flood hazards. First, all sites must be designed for the effects of local precipitation. Secondly, from the perspective of the
performance goals, the adequacy of the site stormwater management system is measured in terms of the impact of local flooding on SSCs at the site. For example, the initial design of the site stormwater management system may correspond to the 25-year rainfall 6-hour storm. If the DBFL for a SSC corresponds to a 5x10-4 rainfall, the site stormwater management system design clearly does not meet this criterion. However, at this point the only conclusion that can be reached is that the system (i.e., storm sewers, etc.) will be filled to capacity. The actual impact of the DBFL precipitation on the SSC is assessed by conducting a hydrologic evaluation for the site that accounts for natural and man-made watercourses on site, roof drainage, etc. The analysis may conclude that flooding is limited to streets and parking lots. If temporary flooding
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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.
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Where a structure cannot be constructed above the DBFL level, an acceptable design can be achieved by:
1. Modifying the flood or providing flood protection for the site or for the specific structure, such that severe structural damage does not occur, and 2. Developing emergency procedures in order to provide adequate warning
and evacuation capability to provide for the safety of building occupants. For structural loads applied to roofs, exterior walls, etc., the applicable regulations should be used.
4.2.2 Performance Category 2
The performance goal for Performance Category 2 is to limit damage and interruption of operations while also maintaining occupant safety. The DBFL is equal to the flood whose annual probability of exceedance is 5x10-4 per year including the event combinations listed in Table 4-2. For purposes of establishing the DBFL for Performance Category 2, a site-specific hazard assessment should be performed. This analysis must include the uncertainty in the hazard assessment in order to obtain an accurate estimate of the mean-annual probability level.
SSCs in this category should be located above the DBFL. For SSCs that cannot be located above the DBFL, an acceptable design can be achieved by the same measures described for Performance Category 1. Emergency operation plans must be developed to provide for occupant safety and to mitigate the damage to mission-dependent SSCs. These procedures may include installation of temporary flood barriers, removal of equipment to protected areas,
anchoring vulnerable items, or installing sumps or emergency pumps. As in the case of SSCs in Performance Category 1, applicable regulations should be used to incorporate flood loads in the building design.
4.2.3 Performance Category 3
The performance goal for Performance Category 3 is continued function of the facility, including confinement of hazardous materials and occupant safety. SSCs in this category should be located above flood levels whose mean-annual probability of exceedance is 10-4, including the event combinations shown in Table 4-2.
If SSCs in this category cannot be constructed above the DBFL level, a design must be developed that provides continued facility operation. The strategy must mitigate the flood (i.e., modifying the flood, hardening the facility, building a levee to prevent flood encroachment) to an extent that facility operations can continue. A higher level of protection is required for SSCs in Performance Category 3 as compared to Categories 1 and 2. Limited damage and interruption
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of operations may be acceptable for Performance Categories 1 and 2, however, for Performance Category 3 the DBFL must be mitigated such that the flood does not impact operations.
The design of Performance Category 3 SSCs that may be impacted by the DBFL should be based on the loads (i.e., hydrostatic forces) and other hazards (i.e., ice forces, debris) that occur. The design requirements in applicable regulations should be used to incorporate flood loads in the design. If mitigation systems such as watertight doors, sealants, etc. are used, manufacturer specifications should be applied. Section 4.3 describes design requirements for flood-mitigation systems such as levees, dikes, etc.
For SSCs that may be impacted by the DBFL, emergency operation plans must be developed to evacuate personnel not involved in the emergency operation of the facility, secure hazardous materials, prepare the facility for possible extreme flooding and loss of power, and provide supplies for personnel who may have an extended stay on-site. Emergency procedures should be coordinated with the results of the flood hazard analysis, which provides input on the time variation of flooding, type of hazards to be expected and their duration. The use of
emergency operation plans is not an alternative to hardening a facility to provide adequate confinement unless all hazardous materials can be completely removed from the site.
4.2.4 Performance Category 4
The performance goals for Performance Category 4 are basically the same as for Performance Category 3. However, a higher confidence is required that the performance goals are met. SSCs in this category should be located above flood levels whose mean-annual probability of exceedance is 10-5, including the combinations of events listed in Table 4-2.
4.3 Flood Design Practice for SSCs Below the DBFL Elevation
For SSCs located below the DBFL level, mitigation measures can be designed that provide an acceptable margin of safety. In practice, a combination of structural and non- structural measures (i.e., flood warning and emergency operation plans) are used. The design criteria for facilities that must consider flood loads are described for SSCs located below the DBFL level, in this section.