Flood mitigation design
Design parameters for flood mitigation measures will depend on whether the site is considered to be within a “flood hazard area” from a regulatory standpoint, insurance requirements, code requirements, local regulations, the risk category of the facility (I through IV), and the level of flood protection desired. ASCE 24 and FEMA technical bulletins provide flood design parameter guidance based on the Flood Design Class, elevations of the lowest floor and lower limits of horizontal structural members, and the FEMA Zone the facility falls within. This could result in designing protection to the design flood elevation (DFE), to 0.3 m (1 ft) above the base flood elevation (BFE), to 0.6 m (2 ft) above the BFE, or to the 500-year (0.2 percent annual chance) storm flood elevation.
Once the level of flood protection to be provided is established, vulnerable areas of the facility’s building envelope, such as windows, doors, louvers, wall penetrations, and sealant joints within this parameter, should be documented. In addition to flood depths, owners and designers must review the potential for stormwater and wind-driven rain that may impact the building envelope. For example, moisture could infiltrate the building walls through failed sealant joints. Equipment located on grade and/or in lower building spaces, and critical interior equipment/spaces should also be documented. This will allow for floodproofing to be designed to address the facility-specific vulnerabilities, both in the building facade and within the building interior. Part of the flood mitigation plan may include relocating critical mechanical and electrical equipment to upper floors of the building to reduce the potential operational impact or downtime during or after a storm event.
The type of system(s) selected for a site as part of this plan will depend on the frequency and extent of projected flooding, the facility’s vulnerabilities, the amount of notice anticipated prior to a flood event to allow mobilization of the system(s), available storage on site or near the facility for temporary system(s), and availability of personnel to mobilize the system(s) prior to an event. The two main protection options are wet floodproofing and dry floodproofing. As the names suggest, wet floodproofing involves building with flood-damage-resistant materials and construction methods able to accommodate flood waters and intentionally get wet, whereas dry floodproofing methods are intended to keep the site dry during flood events.
Wet floodproofing
Wet floodproofing is most suitable for new construction or major facility renovations where it is acceptable to allow area to flood up to the design flood elevation, given its use of flood-damage-resistant materials to accommodate flood waters that are not typical in building construction.
Flood-damage-resistant flooring may include concrete flooring with tile or vinyl-finished flooring glass block. Wood flooring or carpet would not be suitable given the potential for moisture damage during flooding. Flood-damage-resistant walls may include concrete, masonry, or appropriate lumber materials. Paper-faced gypsum board, fiberglass insulation, and other finishes susceptible to water damage should not be incorporated into the finished wall system. Rigid closed-cell insulation could be considered at/below the flood level. Metal doors and cabinets could be considered in lieu of wood doors and cabinets when selecting interior finishes.
Wet floodproofing also requires utilities, equipment, and other critical items within the building be raised above the selected elevation to be protected against flooding. To limit hydrostatic pressure on the building’s structure, wet floodproofing incorporates openings to allow floodwaters to enter/exit the wet floodproofed space. However, the anticipated hydrostatic load should be considered when designing structural elements or assessing the suitability of an existing building for wet floodproofing during renovations.
