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

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 ex - treme 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 own- ers, 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 In 2017, Hurricane Harvey dropped approximately one trillion gallons of water on Harris County, Texas, and caused major flooding through - out 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 up - dated 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 regula - tions impose flood-resistant design requirements on buildings located outside of the 100-year floodplain but within the 500-year floodplain. Norfolk, Virginia exhaustive list. Houston, Texas As a low-lying coastal city, Norfolk, Virginia, is no stranger to flood - ing. 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 ordi - nance 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

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 strate - gies can be complex and require detailed coordination between sev - eral disciplines including architectural, structural, civil, geotechnical, MEP/FP, and others. Dry floodproofing solutions are typically chal - lenging and very expensive to implement, especially when designing The FEMA FIRMs are the most extensive source of flood hazard in - formation within the United States. In order to align with minimum re - quirements 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 consider - ing the impact of climate change on future flood risk: relative sea level rise and increased frequency of severe precipitation. for high levels of floodwater. Changing Flood Hazards 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 eleva - tion 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 chang - ing the frequency and severity of heavy rainfall events and that these trends will continue into the future. As shown in Figure 2, the pro - jected 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 Figure 2 - Projected changes in intense precipitation (U.S. Global Change Research Program, Fourth National Climate Assessment, Volume II: Impacts, Risks, and Adaptation in the United States. Washington, DC: 2018)



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