Flooding is the No. 1 Climate Hazard in the U.S.; Here is How Architects Can Prepare Existing Buildings

DRY FLOODPROOFING If elevation isn’t an option, commercial buildings can explore dry floodproofing. The Washington- based Federal Emergency Management Agency definition of dry floodproofing is to create a watertight enclosure, substantially impermeable to the entrance of floodwater (a maximum of 4 inches in a 24-hour period). (Learn more about dry floodproofing from FEMA.) The walls are reinforced to resist hydrodynamic forces, usually to a height of 3 feet; load calculations should address the lateral force exerted by water plus buoyancy, velocity, debris impact and waves, depending on the site. A waterproof coating seals the wall, including all joints and cracks, from water penetration. Doors and windows must be armored, as well, with new options: flood logs, flood gates, flood barriers or submarine doors with bulkhead thresholds. Backflow preventer valves must be installed below the Base Flood Elevation, or BFE, and sump pumps installed to prevent accumulation in low areas.


The ground floor of SAINT STANISLAUS SCHOOL in Bay St Louis, Miss., took on 10 feet of water in Katrina. Renovations after the storm included using flood damage-resistant materials, dry floodproofing the electrical and mechanical rooms, and changing the windows to meet missile impact-resistant standards. PHOTO: Ann Madden

Protecting an existing building from flooding begins outside the building envelope at the extended site. Floodwalls around the perimeter of a building include manual or passive barriers that deploy automatically against rising water: modular barriers, glass floodwalls, flip-up flood barriers (recessed in the ground when not in use), drop-down barriers (poised above an opening for quick response), and inflatable or water-filled barriers to fill gaps in a perimeter wall. These are ideal for retrofitting; floodwalls do not satisfy the National Flood Insurance Program requirements for new construction.

Strategies to accommodate water require planning ahead so the impact of flooding on a structure is minimized. Reducing impervious surfaces at parking, patios and paving intercepts stormwater; bioswales and retention direct water away from the building to a dedicated storage area where it can evaporate without affecting the building or its occupants. The realization that concrete and asphalt exacerbated flooding has led to new products for stormwater retention: storage, such as interlocking pavers and grids to construct durable surfaces that maintain permeability; french drains with perforated pipes and geotextiles to prevent sediment from clog- ging openings; constructed bioswales with water storage and densely planted vegeta- tion to reduce surface runoff by up to 99 percent. (Learn more from a study about the “Performance of Two Bioswales on Urban Runoff Management”, published by MDPI.)

WET FLOODPROOFING accommodates water by employing flood-resistant materials to resist damage while allowing flood- water to enter the structure; this allows a quick cleanup and recovery without significant gutting and renovation. Materials should be able to survive wetting and drying and be cleanable to remove harmful contaminants, mold and other indoor air quality concerns. Acceptable finish materials include ceramic tile, marine-grade plywood, water-resistant fiber-reinforced exterior gypsum sheathing and waterproof adhesives. The only insulation that meets the requirements is closed-cell spray foam. A cautionary note: Spray-applied prod- ucts are not generally acceptable for use in historic structures. (Discover more materials in FEMA’s report, “Flood Damage- Resistant Materials Requirements …”.)

Many communities have experienced changes to the extent of the floodplain, with corresponding changes to the BFE. This may not immediately affect buildings that are grandfathered in under previous floodplain regulations, but major renovations require updates. In addition, the potential savings from insurance or the anticipated costs of business interruption may lead building owners to undertake mitigation. FEMA guidelines for wet floodproofing include breakaway walls and floors below the BFE; alternately, flood vents are another way to allow hydrostatic pressure to equalize on both sides of a wall and reduce the chance of structural failure. Foundation vents remain closed until rising water activates a set of internal latches that open the vent; flood water flows through the engineered openings, designed to provide 1 square inch of opening per 1 square foot of enclosed space. Vents are located on opposite sides of a structure, within 12 inches of the adjacent grade.

Urban growth has driven development into areas that are unsuitable for permanent occupation. Retreat from flooding is a last-ditch solution for existing buildings considered “stranded assets.” If a structure has been subject to repetitive losses from flooding, it may be worth considering a buyout program (often available after a disaster event) and relocation to an alternate site. A cost-benefit analysis may identify the overall cost savings of a move, even if the initial costs are high: fewer interruptions, unimpeded access during storms, continuity of operations and—as more people transition out of the floodplain— better access to potential customers and service.

Improve Building Resilience Before a Disaster

Now is the ideal time to adapt to climate change. Waiting until after a disaster might seem like the most promising time to improve resilience, but there is a desire to return to a state of normalcy as quickly as possible. Design professionals must accept that even after a catastrophic event, communities are not a blank slate upon which to rewrite the future. Buildings, roadways and property lines still exist, even if they are not whole, and emotions run high.

Architects assume multiple roles in helping to retrofit buildings and communities for better resilience. As a resource for the community, we help residents envision the future and encourage broad participation in its creation. As an adviser to clients, we share information about predicted future conditions that may affect the project and recommend mitigation strategies. We use professional skills to design adaptation measures: conceptualizing; testing; and implementing projects to not just mitigate risks, but improve future performance. Architects are uniquely qualified to imagine and implement more resilient buildings and communities.

AIA Offers Continuing Education about Building Resilience

AIA offers continuing education about building resilience.

The Washington, D.C.-based AMERICAN INSTITUTE OF ARCHITECTS has released a nine-course continuing education series about Resilience and Adaptation. The series covers hazard mitigation, community resilience and adaptation. Course 7: Existing Buildings: Hazard Mitigation Retrofits may be of particular interest to this audience. Take the entire series to understand the best practices for mitigating risk for hazards, shocks and stresses, as well as adapting to changing conditions.

About the Author

Allison H. Anderson, FAIA, LEED AP
Allison H. Anderson, FAIA, LEED AP, is the founding principal of unabridged Architecture, Bay St Louis, Miss., a firm recognized for civic projects that are defensible against climate challenges. She also is the chair of the Washington, D.C.-based American Institute of Architects’ Resilience and Adaptation Advisory Group.

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