Ike Report Draws Lessons from Success — and Failure
Six homes built to IBHS "Fortified" specificationssurvived Ike's storm surge. About halfway up the pilings are the remains of utility decks set slightly above the Base Flood Elevation (BFE). Though not intentionally installed as "breakaway" elements, these lower decks were torn off by waves. (Photo by Richard Reynolds)
When dawn broke after the midnight landfall of Hurricane Ike a year ago, virtually every residence on the Bolivar Peninsula had been swept away by the hurricane's overwhelming storm surge. Standing as lonely exceptions on the scoured beach, however, were 10 out of 13 homes built to the above-code standards of the Institute for Building and Home Safety's "Fortified... for safer living" program. (The other 3 homes, experts believe, were probably taken out when their pilings were struck by floating debris from other nearby houses.) A year of study later, IBHS has released a full report on Hurricane Ike. The report, " Hurricane Ike: Nature's Force Versus Structural Strength," draws on the lessons of the of the 10 surviving Fortified homes. But it goes beyond those examples to examine a range of issues in storm-resistant construction, making observations and recommendations relating to key topics, including elevation, wind-driven rain intrusion, roof attachment, and roof deck waterproofing. Also included is a guide for retrofitting existing homes to boost their resistance to wind, rain, and flood. The key difference between the 10 surviving Fortified houses and their vanished neighbors was the height of the buildings' first framed floor. The builder installed open utility decks for the homes at an elevation slightly above the area's 19-foot Base Flood Elevation (BFE), then framed the floor systems for the first occupied stories 8 feet higher than that. In the storm, the utility decks were lost to storm surge wave action — but the foundation pilings, and the homes' occupied space, survived.
IBHS argues that Ike demonstrated the limitations of building at or near the Base Flood Elevation (BFE), which is based on a statistically estimated 1% probability that water will reach the established elevation in any given year. While this likelihood is often described as a "100-year flood," wave action above the BFE is actually a strong possibility far more frequently than every hundred years. The report notes, "A '100-year flood' means that the level of flood water has a one percent chance of being equaled or exceeded in any single year. However, it is well recognized in the engineering community that coastal homes built to this level have a 26 percent chance of being flooded or demolished over the life of a 30-year mortgage. This chance increases to about 40 percent over a 50-year period." Building higher than the BFE improves the building's long-term odds of survival, explains IBHS: "For example, building to a 500-year flood elevation reduces the chance of surge exceeding the base elevation to about 10 percent over a 50-year period."
A scene of devastation surrounds the surviving Fortified homes. Nearly every house on Bolivar was destroyed by Ike's storm surge. Many, although sturdily built and properly attached to foundations, failed because their elevations were insufficient to keep them above the wave action of the surge. (Photos by Tim Reinhold)
Many houses on Bolivar had been built with first floor systems at, or slightly above, the area's official BFE. At dawn on September 13, 2009, virtually all of those houses were gone — just as thoroughly destroyed as structures set directly on the ground. Says IBHS: "When the vast majority of buildings are built at or slightly above the 1 percent annual probability of exceedance base flood elevation (BFE), all it takes is an event (i.e., Hurricanes Ike, Ivan, Katrina, or Rita) with surge levels a few feet above the BFE to wipe out the entire community."