By their very nature, floodplains are the low, flat, periodically flooded lands adjacent to rivers, lakes and oceans and subject to geomorphic (land-shaping) and hydrologic (water flow) processes. As distinguished from the floodplain, a river' floodway is the dry zone typically between levees, which is designed to convey flood waters (making wonderful areas for bike trails and parkways because it is only periodically flooded).

Unfortunately, a river's floodplain has been viewed as completely separate from a river's active channel. The river and its floodway are usually the focus of construction and control, while fertile, flat and "reclaimed" floodplain lands are usually the focal points for other activities such as agriculture, commerce and residential development.

It is only during and after major flood events that the connections between a river, its floodway and its floodplain become more apparent. These areas form a complex physical and biological system that not only supports a variety of natural resources but also provides natural flood and erosion control. In addition, the floodplain represents a natural filtering system, with water percolating back into the ground and replenishing groundwater. When a river is divorced from its floodplain with levees and other flood control facilities, then natural, built-in benefits are either lost, altered, or significantly reduced.



 A "100-year flood" is defined as a flood event that has a one percent chance of occurring in any given year.

Following the Midwest floods of 1993, a Missouri farmer likened a 100year event to a bag full of marbles: If you have a bag with 100 marbles--of which 99 are white, and one is blue-every time you stick your hand in the bag and pull out the blue one you will find yourself with a 100-year flood. (Of course, each time you grab the blue marble you then have to put it back and shake the bag up before picking again. The laws of probability say that there is always a chance picking the blue marble one, two or even three times in a row. But flood probabilities are never known with perfect certainty: that bag of marbles could also contain two or three otha blue marbles.)

The choice of 100-year rather than 150, 250, or 500-year is an arbitrary one. It is not as if people living 15 feet outside of a designated 100-year floodplain are significantly safer than those living


For land use planning purposes, the regulatory floodplain is usually viewed as all lands within reach of a 100 year flood. The Federal Emergency Management Agency (FEMA) produces floodplain maps, defining what’s in and out of the 100-year (or “regulatory”) floodplain in order to implement the National Flood Insurance Program.

Unfortunately, as Friends of the River flood policy expert Ronald Stork has noted, the regulatory floodplain and the federal insurance program have fostered a planning culture that allows people to be blind to geographic realities.

“We need to have the entire floodplain mapped, not just an arbitrarily defined, 100- year flood zone,” says Stork. “Mapping the entire floodplain would allow people to understand the nature and extent of potential flooding, thus giving them a chance to decide to build, rebuild, buy or not buy.

It’s a theme also found in a 1992 federal floodplain management report: “Because the general populace may not have a complete understanding of the natural physical processes, such as hydrologic cycle and river hydraulics, and of geomorphology, they poorly grasp their vulnerability to flooding and the economic, environmental and social benefits of alternative strategies to avoid or reduce risk.

As Stork and others point out, once lands are defined as “protected” and beyond the reach of the 100-year flood  typically through the contruction and operation of dams and levees  they are no longer considered in the regulatory floodplain. By this gauge, flood insurance and prudent building techniques (such as elevating structures at least one foot above the base flood level) are no longer required; development is allowed to proceed as if the land had been raised above potential flood waters.

However, in many circumstances, simply because homes are no longer in the regulatory floodplain does not mean they are no longer in danger of being flooded. The much coveted status of being mapped outside of the regulatory floodplain provides the legal fiction that a community or home has complete flood protection. It lulls communities into sense of security because they could be in an area  by its geographic location, not its uncertain structural protection  that remains at risk from a foreseeable flood event.

Despite this fact, the bureaucratic action of defining the limits of the 100-year floodplain can, ironically, give the green light to local governments and developers to place more people and property in partially protected floodplain areas. Quite simply, it allows floodplain development in what otherwise might remain non-urbanized; areas that could provide critical floodplain storage during major storm events.

A good example of this problem can be found up and down California’s Central Valley where, by 1997, local officials had given their initial approval to a number of development projects that eventually could allow more than 250,000 people to take up residence on potentially vulnerable land.

within those ostensibly precise boundaries.

Furthermore, because areas like California have fairly short periods of record, what is defined as the "100-year flood" this year can easily be redefined next year if another large storm adds a new point to the statistical model developed to plot the frequency of historical and hypothetical storms. Since 1950 forinstance, floods have struck significant portions of the state about every ten years with near "100-year" events.

Relying on a 100-year designation for flood safety also allows people to forget that a slightly bigger storm (say a "ll0-year" flood) could have precisely the same disastrous consequences that they assume they're protected from in the 100-year event. Putting such definitive lines on a map can induce ill-advised development in a flood-prone area.