Flood Rules in Flux, Part I

Meet the 100-Year-Flood

Government agencies loom large in coastal and riverine flood regulation. But under the U.S. Constitution, the federal government can’t directly regulate building and development, even in the floodplain. The Feds’ authority over a coastline ends somewhere around the high tide mark, and its authority over rivers and streams is technically limited to navigable waterways. It’s actually states, counties, and towns that can make rules about where development is allowed or restricted in a flood zone.

Still, Uncle Sam wields major influence via the almighty dollar. Federal dollars come to the rescue in major flood disasters, and the federal government also backs the National Flood Insurance Program (NFIP). Founded in 1968, the program is now virtually the only available source of insurance coverage for flood damage to homes. But communities are only eligible for aid or flood coverage if they “join” the NFIP, which requires them to adopt Federal Emergency Management Agency (FEMA) rules that regulate development and construction in FEMA-designated flood zones. In addition, under FEMA’s Community Rating System, homeowners become eligible for flood insurance rate cuts if their communities adopt rules and policies that go beyond National Flood Insurance Program minimums.

One Percent Odds and 100-Year Floods 

On the ground, flood regulations resemble a game board with boundaries that are the edges of the officially designated floodplain. FEMA is responsible for publishing maps of the flood risk, drawing the boundaries of what is termed the “100-Year Flood,” or, alternatively, the “one percent chance” flood zone. Within this zone, the agency estimates the odds of flooding at one percent in any given year. It also indicates that FEMA thinks it’s likely that a flood will occur at least every 100 years.

The decision to make this “one percent chance” the official boundary criterion was made in 1968, when the National Flood Insurance Program was newly created; the concept has been revisited several times, but never modified. Critics have raised many valid challenges to the science and rationality of the 100-year flood concept, but as a workable compromise, the policy has held on tight.

”The one percent flood criterion is a compromise, arrived at through a democratic process, which achieves multiple ends, one of which is containing the growth in floodplain losses,” William Dawson, an Army Corps of Engineers official, said in 2004. Modifying the standard to make it either more restrictive or more flexible will not easy, he noted. “Adjusting a rule perceived as not achieving its assumed single purpose objective is one thing, while advocating overturning a democratic compromise is quite another.”

So what’s wrong with the 100-year flood rule? Plenty.

As a simple boundary, it’s too general. The National Flood Insurance Program only sees two options: a structure is either inside or outside the flood zone. In reality, though, structures that are close to the ocean or river and near a flood zone’s center may flood much more frequently than once every 100 years.

Elevations may also vary within the zone, so the same disaster could flood structures some structures by as much as 20 feet, while others in shallower areas could see just a foot or two of water. FEMA does take those elevations into account: the official standard uses the term “base flood elevation” to set the official flood depth of the 100-year flood.

When it comes to building and developing, though, the difference between a 10-foot flood that could happen every 10 years, and a one-foot flood that might happen every 100 years Is not always reflected in policy.

Additionally, many flood maps are inaccurate. Some places have accurate ground elevation data from sophisticated laser-imaging aircraft measurements. But many of the country’s floodplain maps rely on old-fashioned hand methods based on widely spaced field measurements. That means that homeowners (or their builders) may find a home listed as inside the floodplain even when it’s clear that the building sits far above any expected local flood elevation. Other times, structures that sit outside the official floodplain are flooded repeatedly.

As time progresses, even once accurate maps grow increasingly outdated and unreliable. Usually the real flood risk has grown greater than the maps reflect, because development has eliminated flood-absorbing fields and wetlands upstream, or because areas that used to lie below the flood elevation have been filled, which means water could be diverted into previously unflooded areas.

Engineer Cathy Kaake of the Southern Forest Products Association takes this last issue personally. “That’s the reason I got flooded in Hurricane Katrina,” says Kaake, who lives in Metairie, La. After her home was built, builders imported fill to elevate the grade and build homes on nearby building lots that were below the base flood elevation. When Katrina’s storm surge hit, water that would previously have gone into those lots instead flooded Kaake’s.

Today, post-Katrina floodplain rules in Louisiana and Mississippi are changing to avoid this situation and prohibit imported fill in many areas where it was once allowed. That’s just one of the many changes taking place in the Gulf and beyond.