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

Flood-related events, including hurricanes and intense storms, accounted for more than seven of 10 presidential disaster declarations in the period 2008-17.

Existing buildings surround us, from historically significant structures worthy of careful preservation to speculative commercial buildings that might not outlive their air-conditioning units. Many of these buildings were constructed without the benefit of modern building codes or licensed design professionals; many are not prepared for a future with intensifying climate challenges, such as flooding, higher temperatures or wildfires. How do building owners, design professionals and communities improve these buildings’ resilience if only 2 percent of the existing building stock is replaced each year? What will scale up the transition to sustainable and resilient buildings? The answer is in retrofitting these existing facilities.


The Mississippi Gulf Coast was landfall for HURRICANE KATRINA in 2005. Many structures that survived hurricanes Betsy and Camille were lost in Katrina because of the overwhelming storm surge from flooding. PHOTO: Allison Anderson

Existing buildings are susceptible to the same risks as new construction but may be less prepared to manage hazards because of factors related to their age or maintenance conditions: inadequate foundations, poor-quality repairs, lack of reinforcing, lack of a continuous load path, or deterioration and weathering.

There are several points in the life cycle of a building when it makes sense to consider features that mitigate hazards: in the initial design phase, during remodeling or alterations, when building functions change and when repairing damage from a hazard event. Small modifications, such as a new roof or HVAC system replacement, may trigger incremental upgrades, such as reinforcing the roof deck and installing an ice and water shield beneath a new roof covering or even adding a more efficient HVAC system to manage wider temperature ranges.

There is a measurable “resilience dividend” for investments aimed at increasing resilience without the immediate threat of a disaster, which yields short-term economic benefits. Every $1 invested by the federal government in pre-disaster mitigation saved society $6 in post-disaster recovery costs with the most favorable cost/benefit ratios addressing river flooding. For every $1 spent on improving building codes to better withstand disasters, society saved $4 with the most favorable cost/benefit ratio addressing storm surge. (Learn more about cost savings of pre-disaster mitigation in the Washington, D.C.-based National Institute of Building Sciences’ “Natural Hazard Mitigation Saves: 2017 Interim Report”.) When considering hazard-mitigation measures, building owners should know it costs an average of 50 percent more to rebuild in the wake of a disaster.

The first step toward improving resilience is to define the threats through a Vulnerability Assessment. It is rare that a structure faces only one threat; chronic stresses multiply over time in existing structures, and acute shocks can trigger failures of several systems at once. A vulnerability assessment, such as outlined in the Whole Building Design Guide, completed with the help of an architect, helps building owners and tenants decide if they can accept the risk climate presents to their assets. Making decisions about incorporating resilient features begins with answers to three questions:

  • Will this reduce the potential impacts from hazards?
  • How much damage can be avoided through this design strategy?
  • Are there other benefits associated with incorporating this strategy?

Flooding: Most Common U.S. Natural Hazard


In Bay St Louis, Miss., the DOWNTOWN CIVIC CENTER AND PARKING GARAGE uses parking to raise a community center well above the floodplain. This elevation also provides exceptional views to the Gulf of Mexico and barrier islands. Designed by unabridged Architecture PLLC, Bay St Louis, and built with flood damage-resistant materials, the building features a three-day uninterruptible power supply and photovoltaic array.
PHOTO: Eugenia Uhl

Every adaptation to climate is hazard- and site-specific. The most common hazards are floods (43 percent of disaster worldwide), storms (28 percent), followed by earthquakes, extreme temperatures, landslides, droughts, wildfires and volcanic activity. (Learn more from the Geneva-based United Nations Office for Disaster Risk Reduction’s report, “The Human Cost of Weather Related Disasters”.) Design strategies can Protect (robustly defend the building from a hazard) or Accommodate (design for interaction with the hazard in ways that reduce risks for occupants or damage to the structure). In acute conditions, building owners may elect to Retreat, or relocate to avoid hazard exposure.

Flood-related events, including hurricanes and intense storms, accounted for more than seven of 10 presidential disaster declarations in the period 2008-17. This is not just a coastal problem: The states with the most flood-related disaster declarations include Arkansas (No. 1) followed closely by Iowa, Tennessee, Oklahoma and Kentucky. (The Pew Charitable Trusts, Washington, provides more information about this statistic.) Architects must prepare for rainfall events that are more intense, more frequent and longer in duration. At the same time, we must consider the rapid growth in the amount of impervious surface that prevents stormwater infiltration and increases the velocity and quantity of surface water on sites and streets.

Storms arrive regularly in the South, for example. Weeks at a time there are incessant downpours that swamp yards and make rivers out of streets. Deluges of 4 to 6 inches a day are not unknown. As air temperatures climb because of climate change, this will only get worse; hot air holds more moisture than cooler air. Traditional building techniques in the South accounted for a degree of uncertainty: local builders sited structures away from low areas, raised the first-floor level, and sheltered windows and doorways with a wide porch or overhang. They placed inessential functions below the building and designed undercrofts with hard surfaces, like brick, that easily could be cleaned after an inundation. Occupants sandbagged doorways, wells and other essentials.

Flood protection hasn’t experienced a radical disruption; traditional techniques still form the basis of effective practice. Architects’ tools may be slightly more sophisticated than sandbags but the general approach remains the same. Proper siting for buildings is still the best defense against flooding. The optimal site location is a balance of variables: natural topographic ad- vantage but not on a steep slope and near the water for views and access but not close enough for overspills. Elevation remains the most durable protection against flooding, but most existing buildings do not have the luxury of elevating out of the encroaching floodplain: It is difficult if the building is not already elevated—and expensive, triggering a host of associated repairs, from plumbing to plastering.

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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