Floodplain Development-Learning From the Great Flood of 1993

Real Estate Issues, Winter 2006/2007 by Ward, Richard C

NEARLY 14 YEARS AGO, THE NATIONAL NEWS WAS DOMINATED by stories of levee breaks and flooding along the Mississippi and Missouri rivers and their tributaries. In 2005, the story of the year was destruction on the Gulf Coast caused by hurricanes Katrina and Rita, compounded by the massive impact of two levee breaks that devastated neighborhoods in New Orleans. A year later, we saw serious flash flooding across the Northeast as a result of heavy local rains-up to 14 inches in places-with the entire state of Pennsylvania declared a disaster area.

Numerous other instances of flooding in areas along inland rivers impacted by flooding also have been documented, as has the continuing devastation of shorelands affected by hurricanes. Added to these traditional concerns is the fear of rising ocean levels from global warming that will affect not only coastal properties, but also inland riverfronts.

As quickly as these crises arise, they tend to fade from the public consciousness. More disturbing is the fact that we have seen few gains in terms of knowledge and commitment to changing land-use policies and practices to avoid repeat disasters. Instead, we remain destined to relive the past, perhaps with even worse consequences, as ongoing development occurs in flood-prone areas.

CONSIDERING THE PHYSICS OF THE RIVER

Any assessment of the phenomenon of repeated floodrelated catastrophes requires a brief review of the basics of floodplain development. A river system consists of two distinct components. The first and most obvious component is the water flowing within its banks. The area between the riverbanks is technically termed the "flood way," because it contains the runoff from the watershedthe area drained by the river-the vast majority of the time. When the volume of water coming downstream exceeds the capacity of the area between the riverbanks, floodwater overflows and spreads onto adjacent land.

Over the course of geologic time, hundreds and thousands of years, repeated flooding results in a build-up of alluvial soil-soil deposited by receding flood waters-on one or both sides of the normal river channel. This flat plain extends to the point where the land rises beyond the reach of the most severe flooding, often to a bluff where elevations increase steeply. This natural basin, the floodplain, is the second primary component of the river. However, because actual flooding occurs in this basin only periodically, there is a tendency to forget, or perhaps overlook, that this area is an integral part of the river system.

As long as there have been human settlements, people have sought to put floodplain land into productive use in support of human endeavors. This land is particularly attractive because it is flat, fertile and close to water. Perhaps most notable, in an urban context it also is less expensive than land outside of the floodplain. Conversion of floodplain land typically begins with the removal of bottomland forests to create crop or grazing land, followed by urban uses in many cases. The net effect of these changes is often that the volume and rate of runoff increase tremendously because there is a loss of natural land cover-forests, prairie grasses and brush lands-that has been replaced first by agricultural crops and grazing, and later by impermeable areas of pavement and buildings.

Once people invest in an otherwise flood-prone area, there is a natural desire to protect their investments. This leads to the building of levees, also referred to as dikes, to wall off rising floodwaters. When only one relatively small area of a floodplain is walled off from flooding by a levee, there is little impact on the river itself or on properties otherwise not so protected. The problem occurs when a system of levees is repeated along a much broader reach of the river. Then, when a dramatic increase in the volume of water is not allowed to spread out, the river rises higher and runs faster in the channel created by the levees. This effect is then intensified by higher volumes and rates of runoff from lands in the watershed.

VICIOUS CYCLE

Once the first levees are built and the area is altered by the economics and politics of urban growth, a vicious cycle starts. The value of flood-protected land rises dramatically, causing other property owners to want to "get on the bandwagon." More investment is made in the protected floodplain, so more people, businesses and governments have more at risk should the levee fail-and a growing stake in ensuring that it doesn't fail. Eventually, steps are taken to raise the levee even higher to enhance the perceived level of protection.

As more and more areas along the river system are similarly treated, there is a dramatic and corresponding loss of capacity to store floodwaters. The result is a river channel much like a large ditch with high levees on both sides and nowhere for the floodwater to go but up. At that point, the areas of protected floodplain also act as a bathtub, capturing and holding runoff from local streams that normally would flow into the river but cannot when the level of the river is above that of the floodplain.