By Matt Gilbertson, P.E., S.E., Emanuel DeAndrade, P.E., and Sean Donlon, P.E.
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 historic 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 administered 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 mirror 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 construction 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.
The Design Flood Elevation (DFE) to which designers must adhere incorporates “freeboard” as a margin of safety to provide flood protection 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 freeboard 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 increase 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, building 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 hydrostatic pressures and reducing the risk of structural damage. Alternatively, 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 associated 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 permissibility of wet floodproofing is very limited and can create undesirable aesthetics due to restrictions on acceptable construction materials. Dry floodproofing can alleviate aesthetic concerns and allow more flexibility with respect to the use of finished space below the DFE; however, in practice, the implementation of dry floodproofing strategies can be complex and require detailed coordination between several disciplines including architectural, structural, civil, geotechnical, MEP/FP, and others. Dry floodproofing solutions are typically challenging and very expensive to implement, especially when designing for high levels of floodwater.
Changing Flood Hazards
The FEMA FIRMs are the most extensive source of flood hazard information within the United States. In order to align with minimum requirements of the NFIP, local requirements often adopt FEMA FIRMs as a basis for jurisdiction under local codes. The FEMA FIRMs depict present-day flood hazards based on the available historic data for the relevant flooding source, such as a riverine or coastal flood hazard. The FEMA FIRMs do not reflect projected future flood conditions related to climate change. Two trends are commonly identified when considering the impact of climate change on future flood risk: relative sea level rise and increased frequency of severe precipitation.
Coastal flood zones along the Atlantic, Pacific, and Gulf coasts are all likely to experience more flooding due to sea level rise. As the Global Mean Sea Level (GMSL) rises, the median water surface elevation increases, resulting in both more frequent low-level flood events and higher extreme flood elevations for major flood events (e.g., the 100-year or 500-year floods). Wave heights and areas subject to wave action will increase with elevated flood levels, compounding the effect of the increased flood elevations.
The latest scientific consensus suggests that climate change is changing the frequency and severity of heavy rainfall events and that these trends will continue into the future. As shown in Figure 2, the projected changes to future precipitation patterns vary widely across the country, with areas in the Northeast, Midwest, and Northwest expected to see the greatest increases in severe precipitation events. Although a direct connection between these trends and increased flooding has not yet been established, extreme precipitation is one of the primary factors contributing to both riverine flooding (those floodplains along natural watercourses) and urban flooding (areas subject to flooding related to inadequate performance of the local drainage system in extreme events).
Regulations for Enhanced Flood Protection
The timing and magnitude of the projections related to sea level rise and increased frequency of severe precipitation events contain a high degree of uncertainty. As a result, it is difficult for regulatory entities to adopt specific long-term design requirements to accommodate future conditions, as they may be significantly different from the current-day conditions. Local guidelines and policies that aim to encourage owners, developers, and designers to consider future flooding are not new; however, in recent years, some municipalities have begun to implement regulations that require construction projects to incorporate enhanced flood protection. Below is a sampling of these local regulations that have been introduced around the United States, although this is not an exhaustive list.
In 2017, Hurricane Harvey dropped approximately one trillion gallons of water on Harris County, Texas, and caused major flooding throughout the greater Houston area. Many rivers and watercourses crested at elevations that surpassed the mapped 100-year and 500-year levels. Twenty-two percent of the buildings within Harris County experienced flood damage during Hurricane Harvey, including many buildings outside of the map 100-year floodplain. After the floodwaters receded, regulators in Harris County and the City of Houston began drafting revisions to their floodplain development ordinances in an attempt to reduce the risk of flood losses for future development projects. The updated regulations, which became effective in September 2018, adopt a more stringent basis for the Design Flood event than the basic building code requirements. Whereas the prior regulations required designing buildings to protect against flooding at least 1 foot above the 100-year flood elevation, the updated regulations now require a minimum DFE of 2 feet above the 500-year flood elevation. In addition, the regulations impose flood-resistant design requirements on buildings located outside of the 100-year floodplain but within the 500-year floodplain.
As a low-lying coastal city, Norfolk, Virginia, is no stranger to flooding. While most coastal communities in the contiguous United States have recorded sea level rise over the past century, the historic rate of sea level rise in Norfolk is the highest on the Atlantic Coast. The observed sea level trend in Norfolk, about 1.7 inches per decade, is over twice the global average sea level rise rate (about 0.7 inches per decade). The rapid rate of relative sea level rise is the result of land subsidence, caused in part by groundwater withdrawal from aquifers, and contributes to more frequent coastal flooding. To combat systemic coastal flooding, the City of Norfolk adopted a revised zoning ordinance in March of 2018 that requires new construction or substantial improvements in the 100-year floodplain to provide 3 feet of freeboard above the mapped 100-year flood elevation. Like Houston, the new Norfolk provisions also regulate the 500-year floodplain, requiring new buildings to elevate or floodproof to 1.5 feet above the 500-year flood elevation or 1.5 feet above the highest finished grade immediately adjacent to the structure, whichever is higher.
Concerns about increased coastal flooding due to sea level rise also drove Boston, Massachusetts, to recently adopt more stringent flood-resistant design standards. However, Boston sought to first establish a target sea level rise scenario and better define the local flood extents and elevations for that scenario. Many guidance tools and policies for sea level rise are based on the approach of linear superposition in which a projected sea level rise quantity is added directly to a present-day flood elevation and compared with the local topography to determine future flood extents. However, as this approach does not consider changes to the dynamic coastal processes with elevated sea levels, it may not accurately reflect the future flood conditions. Therefore, Boston utilized a flood risk model, independent from the FEMA FIRMs and FEMA flood studies, to develop a map of future flood conditions considering rise. This map, intended to represent flood conditions during a 100-year storm event in 2070 with 40 inches of sea level rise, forms the new Coastal Flood Resilience Overlay District (CFROD). A new article of the Boston Zoning Code, Article 25A, regulates the CFROD, requiring buildings to be designed to this future flood elevation plus 1 foot of freeboard.
These new flood resilience initiatives do not appear solely in local zoning codes or building codes. Further up the Massachusetts coastline, the City of Newburyport has also experienced chronic coastal flooding issues. In an effort to minimize future flood-related damage, the City of Newburyport revised the local Wetlands Protection Regulations to include updated flood-resistant design requirements. The updated regulations consider the same future flood scenario as the Boston zoning provisions (100-year flood event with 40 inches of sea level rise) but do so through the regulatory jurisdiction of the local Conservation Commission.
The prevalence of enhanced flood regulations is likely to increase in concert with the projected effects of climate change on future flood events and the inherent uncertainty of those projections. These local regulations are implemented by municipalities using various avenues, as discussed above, and can have significant impacts on projects. It is easy to imagine a scenario where a prospective development project gains traction, only to later discover the more stringent local flood requirements, creating a major or even fatal disruption to the project. These requirements are often manageable but require early identification and careful planning for successful implementation.