C+S April 2022 Vol. 8 Issue 4 (web)

Flooding is one of the nation’s most costly natural hazards, but – despite the risks – owners and developers regularly construct new buildings and rehabilitate existing buildings in flood-prone areas. Most jurisdictions adopt building design criteria based only on his - toric data and present-day flood hazards, but some communities have recently begun to implement local regulations that require project teams to consider the effects of climate change on natural hazards, including future flood conditions. While even traditional flood mitigation considerations can present design challenges, these new resilience initiatives may require complex solutions that can often be disruptive to a prospective project if they are not identified and integrated during the early stages of a project. Building Codes and Flood Maps Modern building code requirements for flood-resistant design and construction have their origins in the provisions of the National Flood Insurance Program (NFIP). Enacted by Congress in 1968 and admin - istered by the Federal Emergency Management Agency (FEMA), the NFIP enables residents of participating communities to purchase subsidized flood insurance but, in return, requires those communities to regulate development in floodplains to reduce future flood-related property damage. Among other regulations, the NFIP requires new and improved buildings to conform to specific flood-resistant design and construction requirements. Most local building codes typically mir - ror the minimum requirements of the NFIP, so new construction or substantial renovations conform to or exceed the NFIP. More recently, some local regulations such as zoning codes, floodplain management ordinances, and wetlands bylaws have begun to impose requirements that are more stringent than the NFIP. The NFIP regulates construction in areas subject to a one-percent chance of inundation in any given year, also known as the “100- year” floodplain. These floodplains and the associated 100-year flood elevations are shown on FEMA flood hazard maps, officially known as Flood Insurance Rate Maps (FIRMs), and are usually referred to as a Base Flood Elevation (BFE). FEMA FIRMs (example shown in Figure 1) often depict additional flood hazard information, such as the 0.2-percent annual chance or the “500-year” floodplain. However, the NFIP, and therefore most local building codes, only regulate construc - tion within the 100-year floodplain for most residential, commercial, and mixed-use buildings. Nonetheless, floods exceeding the mapped 100-year flood sporadically occur throughout the United States. For example, some of the Texas rivers affected by Hurricane Harvey crested at levels exceeding the mapped 500-year flood elevations. FLOOD-RESISTANT BUILDING DESIGN AND LOCAL CLIMATE CHANGE RESILIENCE INITIATIVES By Matt Gilbertson, P.E., S.E., Emanuel DeAndrade, P.E., and Sean Donlon, P.E.

The Design Flood Elevation (DFE) to which designers must adhere incorporates “freeboard” as a margin of safety to provide flood protec - tion higher than the mapped 100-year flood elevation. Local building codes typically require 1 foot of freeboard above the 100-year flood elevation for most residential, commercial, or mixed-use buildings, while essential facilities usually require designing for 2 feet of free - board or the 500-year flood elevation. Freeboard is primarily intended to account for uncertainties in the flood analyses used to generate the mapped flood elevations. However, designers sometimes choose to increase the magnitude of the freeboard in the DFE to protect against more severe flooding than the mapped flood elevation, either to in - crease the reliability of the flood-resistant design for current-day flood risk beyond the 100-year, or to protect against future flood conditions. Building code provisions present a few options for flood-resistant building design. These options depend on the building location, build- ing use classification, and the intended function of finished spaces within the building. For buildings that are within a floodplain, but are not subject to coastal wave action, designers may choose to elevate the building so that the lowest occupied floor is above the minimum DFE. Areas below the DFE that are used solely for parking, storage, or building access may be “wet floodproofed” by purposely allowing floodwaters to enter the building, thereby eliminating unbalanced hy - drostatic pressures and reducing the risk of structural damage. Alter- natively, entirely non-residential buildings may be “dry floodproofed” by implementing a substantially impermeable building perimeter up to the DFE, provided that the building is capable of withstanding the as - sociated flood loads. Sites that are subject to wave action eliminate the dry floodproofing methodology as a viable option and require that the entire building structure, exclusive of foundation systems, is elevated entirely above the DFE. Each of the flood mitigation strategies has its own complications. Elevating provides a passive– and the most reliable– means of flood protection, but it is often difficult to achieve due to programmatic and accessibility considerations for ingress and egress. The permissibil- ity of wet floodproofing is very limited and can create undesirable Figure 1 - Portion of a FEMA FIRM for Houston, Texas, showing the 100-year floodplain (in teal) and the 500-year floodplain (in orange) (Federal Emergency Management Agency. Harris County Texas and Incorporated Areas, Panel 0865M. FIRM Flood Insurance Rate Map. Washington, DC: Federal Emergency Management Agency, 2019.


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