Major Changes to ASCE 7-22 Flood Loads

To view the figures and tables associated with this article, please refer to the flipbook above.

ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures Supplement 2 was published in May 2023 and incorporates a complete revision of the flood load provisions of the standard. Prior to the supplement, the flood load provisions that are part of ASCE 7 Chapter 5, had been minimally changed since 1998. In contrast, communities in coastal and riverine areas have experienced extensive damage and loss from extreme storms with nine of the 10 costliest Atlantic hurricane seasons occurring since 2004. As a result, many jurisdictions have sought to increase flood standards locally. Figure 1 shows severe damage to two recently constructed houses, both of which were damaged from Hurricane Michael in 2018 when both were outside the 100-year floodplain. Building to the new flood provisions would have either made these houses more robust or they would have been elevated to minimize flood damage. With many lessons learned and new research conducted since 1998, the ASCE 7-22 Flood Load Subcommittee strongly felt that the standard needed to be replaced with up-to-date formulas and methods. The subcommittee developed a new approach for addressing the reliability of building design for flood considering climate change effects, particularly sea level rise. All references in this article to ASCE 7-22 flood loads are to Supplement 2.

The current design requirements in ASCE 7-16 for flood elevation only required designs to the 100-year flood (the 1% annual chance flood) plus some amount of freeboard. The use of the 100-year flood was largely based on the National Flood Insurance Program (NFIP) Base Flood Elevation (BFE) and did not meet the reliability targets of ASCE 7. This freeboard added to the Base Flood Elevation was a fixed amount based of the requirements of NFIP, ASCE 24, or the local jurisdictions; it was not risk-based and had no particular connection to the importance of the facility to the community. Dependent upon site conditions such as the topography and presence of waves, the freeboard provided inconsistent levels of protection above the 100-year flood. Moreover, ASCE 7-16 applied only to structures located within the 100-year flood area. Even as some attempts were made to increase elevation requirements via freeboard, none of this applied to structures immediately adjacent to the 100-year flood area, creating a ‘waterfall’ effect.

ASCE 7-16 and earlier editions recognized that the 100-year flood load did not meet the reliability targets of the standard and assigned a load factor of 2.0 to coastal A and V zones in an attempt to increase the reliability. This presented a similar issue to the addition of freeboard. The load factor of 2.0 scaled the magnitude of the horizontal flood force but did not scale the elevation of the flood water. The Flood Load Subcommittee wanted flood design to be based on risk and to be risk consistent across the country.

One of the most significant changes in ASCE 7-22 Supplement 2 is the change in the definition of the flood hazard area. Previously, structures within the 100-year flood area, or the Special Flood Hazard Area in the terminology of flood plain management, were subjected to ASCE 7. However, the new definition calls for the 500-year floodplain to be the new area of applicability for Risk Categories II, III, and IV structures. For Risk Category I structures, the 100-year floodplain remains as the designated flood hazard area. This change has significant implications for all new construction and substantial improvements to structures lying between the maximum extents of the 100-year and the 500-year flood areas.

The new approach can be described as going beyond the National Flood Insurance Program regulations and requirements. The new flood load provisions do not tie the loads to a fixed regulatory elevation but instead use a risk-based approach to determine the flood hazard and then use the hazard levels to calculate the loads for the selected flood hazard. The new definition in the revised standard for the design flood is “The flood corresponding to the design mean recurrence interval assigned by risk category in accordance with Table 5.3-1, including relative sea level change.” This revised definition includes references to two major changes in the ASCE 7-22 provisions. One is the link between flood design, flood return periods, and building risk category. The second is the inclusion of relative sea level change as part of the design flood. A reduced version of Table 5.3-1 is shown in Table 1.

The revised ASCE 7 Flood Load chapter now requires the use of a return period for design based on the Risk Category that ASCE uses to define use and occupancy classes. As evidenced by the amount of flood damage, losses, and claims in areas beyond the special flood hazard area, as shown in Figure 1, the regulatory minimum elevation that uses the 100-year event is no longer sufficient to reduce losses caused by flooding.

The solution to the problems of under-estimating the flood hazard and improving risk consistency is to require flood designs for each Risk Category to a specified flood return period. The risk categories and their corresponding mean recurrence intervals are shown in Table 1.
The advantage of using this method is that most buildings fall into Risk Category II and federally published Flood Insurance Studies (FIS) typically have 500-year flood elevation data in addition to the 100-year data and many FEMA Flood Insurance Rates Maps (FIRMs) also delineate the extent of the 500-year flood plain. The disadvantage of this method is that the 750- and 1000-year MRI elevations and flood conditions are typically not published in the FIS or on the FIRM. In order to determine these larger return period floods, scaling factors were developed for both riverine and coastal conditions to convert the 100-year flood elevation to the desired mean recurrence interval. The table of scaling factors is shown as Table 2. These scaling factors are also part of the standard Table 5.3-1.

The amount of flood information available from federally published flood studies varies depending on the amount and type of information collected and developed regarding the local floodplain and the date of the study. Table 2 assumes that 100-year flood elevation data is available and thus can be used to scale from. The revised standard addresses the issue of unavailable data, so the user knows how to proceed to obtain the required design flood elevation.

Flood velocity and wave heights are also part of the flood hazard. Scaling factors are also provided for those parameters for return periods greater than 100 years. These are shown in Table 3 and are in the standard as Tables 5.3-2 (velocity) and 5.3-3 (wave height).

These changes now require the user to select the Risk Category, determine the flood hazard level and calculate flood loads based on the design flood depth. In many instances, especially coastal, the designer may elect to elevate the building above the expected flood level instead of fortifying the building sufficiently to ‘resist’ the flood.

Another significant change is the inclusion of climate change effects in the determination of the design flood. The ASCE 7-22 equation for design stillwater flood depth is: df = (Stillwater elevation for design MRI—Ground elevation including effects of erosion) + Δ sea level rise.
The revised standard now requires that climate change in coastal flood plains be considered by adding the change in sea level rise to the flood depth. The user is to consider the SLR effect over a 50-year period, the minimum of this change effect is to be a straight-line extrapolation of the historical SLR. Other more conservative approaches may be used including projections made by the U.S. Army Corps of Engineers (USACE) a link to the Corps projections is provided in the standard. The Corps data makes projections for a low, intermediate or high rate of SLR until the year 2100. Those projections allow the user to make a choice of which rate is the most appropriate for their design condition or which rate best suits the needs of the client.

The new chapter also includes guidance on the amount of scour to consider for columns and walls under breaking and nonbreaking wave conditions.

There are changes to several flood load formulas in addition to the changes made in defining the flood hazard.

The calculations for hydrostatic loads are included in the supplement, and they require that the soil be considered fully saturated unless a seepage analysis in the geotechnical report determines otherwise. The calculations for hydrodynamic load are included in the supplement and the reduction in the maximum velocity can significantly reduce the hydrodynamic pressure as the velocity term is squared in the equation. There are new provisions for determining the impact of debris damming, which is the accumulation of debris between columns of an open foundation, which results in an increased area for the application of hydrodynamic loading.

The last significant change is the reduction of load factors used in load combinations that include flood. The LRFD flood load factor has been reduced from 2.0 in V Zones and Coastal A Zones to 1.0 for all zones; the ASD load factor has been reduced from 1.5 in V Zones and Coastal A Zones, to 0.70 for all zones. These reductions are possible because of the use of a higher return period for the design flood; the resulting loads on structural elements now achieve the target reliabilities assigned in ASCE 7 for each of the Risk Categories.

There are a lot of changes to ASCE 7 Chapter 5, and after years of minimal changes, it will likely take some time for the practice to incorporate all these changes into their designs. The new chapter is published in ASCE 7-22 Supplement 2 and is available for free download on the ASCE website. The recently published Building Designer’s Guide to Calculating Flood Loads Using ASCE 7-22 Supplement 2 by FEMA walks users through several examples and is also available for free download. FEMA continues to work on a Future of Flood Risk dataset and user tool so that in the future designers will be able to retrieve flood hazard data for various MRIs and risk probabilities. The changes to flood design that are now in ASCE 7-22, Supplement 2, should significantly strengthen the flood load resistance for structures designed to these new provisions.

William L. Coulbourne, PE, F.SEI, F.ASCE, Coulbourne Consulting, has 50 years of experience as a manager, designer, and building professional. He is a member of ASCE 7 standards committees on flood loads.
Daniel Cox, Ph.D, M.ASCE, is a professor at Oregon State University. His research focuses on community resilience to coastal hazards, including tsunami and hurricane surge and waves inundation.
Jessica Mandrick, PE, SE, M.ASCE, is Partner at Gilsanz, Murray, Steficek. She has a wide range of experience in multiple disciplines with specialties in education facilities, renovations, and buildings in the floodplain. Mandrick is also a member of the STRUCTURE magazine Editorial Board.