Tough lessons from recent floods.

• Author: Larson, Larry A.
• Position: Special Section: America Under Water - Cover Story

For centuries, the Mississippi and Missouri rivers have served a myriad of roles - as highways to settle the nation, commercial corridors to float logs and move farm crops to domestic and foreign markets, and water bodies to assimilate wastes. They provide water for irrigation and act as recreational corridors for countless millions of people each year. As diverse freshwater ecosystems, they provide water, wetlands, and habitat for fish, wildlife, and other species.

Over the decades, humans have worked in, played in, and moved into the floodplain of these mighty rivers. Towns were built on the banks to be next to the transportation and water the river provided. Because the soil in the floodplain had been made rich through centuries of natural flooding, wetlands were drained and tilled, and farms were established.

Summer homes built along the rivers often were flooded in the spring. In the beginning, these rough structures had little value. Damage was not considered a problem, and claims for disaster relief were rare. Over time, the structures became more costly, and people demanded more protection for their homes, businesses, and farmlands. Levees were built.

In rural areas, they usually were designed to protect only against low-level floods and were overtopped every 10-20 years. As the technology and cost of planting crops rose, farmers became more concerned with the need to protect their investments. The levees grew, gradually, through the work of individuals and groups of farmers who formed levee districts.

Urban levees often were erected during flood events by desperate residents working to protect their homes and communities. If the next flood rose higher, the levee was raised. If the waters caused substantial damage, people wrote their representatives in Congress and asked for help to solve their flooding problems. Congress provided funding for studies by agencies with programs designed to consider flood damage reduction alternatives such as the Corps of Engineers and Soil Conservation Service. The studies invariably resulted in recommendations to build new levees.

If standard analysis showed that the costs of a levee were outweighed by benefits to the nation, Congress would place funding for the project in the agency's budget. More levees were built, their construction borne entirely by Federal taxpayers until the 1970s. Today, Federal funds may cover 75-80% of the expenses, with state and local govemments responsible for the remainder.

It is important to remember that studies of flood loss reduction alternatives had common elements:

* Cost/benefit analyses looked at the value of land both before and after a project and assigned a positive or negative rating based on national benefits. The process favored structural initiatives like levees and dams since the value of lands protected from flooding often was higher after the project than before.

Alternatives, including the relocation of structures and restoration of wetlands in floodplains might have saved buildings and property from the risk of flooding. However, because they called for the removal of structures and had the effect of reducing the value of the affected lands, they lost out in the cost/benefit analyses. Moreover, floodproofing or elevating structures in place were viewed as complex and cumbersome since there was no single Federal agency or program to work with the community.

* The potential for a levee to increase flood elevations upstream and downstream seldom, if ever, were calculated. If they were, it did not deter construction.

To appreciate this potential, visualize a river 1,000' wide. All the water is moving downstream with velocity. Now assume that river is pinched to 500' wide. In order to pass the same flow, measured in cubic feet per second (cfs), the river must rise to a higher elevation at the pinch point and upstream. People with structures or levees of there own upstream, or across the stream, will experience higher levels of flood water.

Levees can cause an increase in downstream flood levels by reducing a river basin's...