Is the 100-Year Floodplain Now Obsolete?

The water is rising. So scientists working with the National Oceanic and Atmospheric Administration (NOAA) recently reported. Regardless of one’s point of view about ultimate causes, they note that the eastern and southern coastlines of the United States are increasingly subject to flooding as sea levels rise.

On January 30 last year, President Obama issued an executive order requiring that federal agencies employ strict building standards that take into account the likelihood of future flooding. That order followed a report from the US Army Corps of Engineers warning that increased flooding was likely on the Atlantic and Gulf coasts, as well as the National Climate Assessment’s findings that at least a trillion dollars of property damage would ensue from flooding brought about by a 2-foot rise in sea level.

Almost all climate models assume that rise. At the same time, some old tried-and-true measures used by climate scientists are being revised in the face of current realities. Storms and floods, for example, are spoken of in scientific shorthand as “100-year,” “500-year,” and even “1,000-year” events, meaning, in the case of the former, that there is a 1 percent chance of a significant flood occurring in any given year with a 100-year period.

Yet, the numbers can get confusing in this shorthand. Consider that in early August 2016, in Louisiana, a 1,000-year flood occurred as a result of two days of intense rain in which nearly 2 feet of water fell from the sky. This would be notable enough on its own, but this particular storm was bracketed by storms of similar ferocity, intense enough that by those old measures the region experienced no fewer than eight 500-year flood events in the space of just a couple of months. That would seem to render terms such as “100-year flood” nearly meaningless, especially if it’s assumed that there is some static standard attached to them.

Altogether, emergency-management specialists calculate, these storms were far more damaging than the previous recordholder, which caused more than $3 billion in damage in 1995—and then there was Hurricane Katrina in between. That speaks to the emerging reality that storms are indeed increasing in intensity, and with them, in the coastal regions of the eastern and southern United States, so are floods. Indeed, notes the Federal Emergency Management Agency (FEMA), “Flooding is the most common and costly type of natural disaster in the United States, and floods are expected to be more frequent and more severe over the next century due in part to the projected effects of climate change.”

FEMA has consequently proposed revising federal regulations for new buildings such that they must stand 2 feet freeboard—that is to say, two feet above flood level on a 100-year floodplain. That figure is twice as high as the current mandated level in most jurisdictions, a figure that in turn was set by a post–Hurricane Sandy reevaluation of the 100-year standard. The minimum rises to 3 feet freeboard in the case of “critical actions,” activities that take place in structures in which, for example, chemicals are stored. Such structures include laboratory facilities, data centers, elderly housing, and hospitals.

The new FEMA regulations set the 500-year floodplain as “the minimum floodplain of concern for critical actions.” More precisely, the FEMA plan mandates that “there will be no new construction or substantial improvement of structures unless the lowest floor of the structures (including basement) is at or above the level of the base [that is, 100-year] flood, and there will be no new construction or substantial improvement of structures involving a critical action unless the lowest floor of the structure (including the basement) is at or above the level of the 500-year flood.”

Federal guidelines already instruct agencies to avoid building in floodplains “to the extent possible.” The proposed FEMA plan requires that construction within the floodplain be analyzed with respect to the “natural environment, social concerns, economic aspects, and legal constraints,” as well as the effect the structure might have on the floodplain. If practical alternatives exist to building within a floodplain in the first place, then they should be preferred to any construction within a floodplain. The regulations, in other words, strengthen previous guidelines, and they add rigor to the slippery definition of what constitutes a 100-year floodplain.

Federal Lessors know that federal lease procurements restrict competition from buildings that are not outside the 100-year floodplain, and in some instances these procurement require locations outside the 500-year floodplain. With rising sea levels, floodplains are expanding in many coastal locations. Further, the freeboard requirements of FEMA’s proposed regulations may further restrict some buildings from competing for federal leases. They may even disqualify certain existing federal buildings.

The public comment period for the FEMA proposal closes on October 21.

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