Definitions of Structural and Nonstructural Measures

Adjustments to floods can be broadly classified into structural and nonstructural measures. Nonstructural approaches involve adjustment to human activity to accommodate the flood hazard (White, 1964; James, 1975; White, 1974) whereas structural methods are based on flood abatement or the protection of human settlement and activities against the ravages of inundation.

Structural change involves modification to the built environment to minimize or eliminate flood damage directly or flood channel construction changes. For example, see Figure 5. Structural measures are expensive. They may give the illusion of security but the record shows otherwise (Alexander, 1993). The security can be temporary. A flood can occur that is bigger than the design of the channel or levee, and changing priorities in flood control projects that require higher reservoir levels for recreation or water supply can diminish the efficacy of structural measures (Williams, 1998).

The failure of structural flood control works poses a significant threat to the lives of the people who live downstream from a massive structural project such as a dam. More than 2000 people died in 1969 in Italy when the Vaiont Dam collapsed (Blaikie et al., 1994). Because of stringent engineering standards and a system of inspections, the United States has seen few major failures. However, many structures are at the end of their design lives of 50, 75, or 100 years.

Structural flood control is still the dominant idea in many parts of the world. Following the 1927 Mississippi River floods, when river levees collapsed and 200 people died, 700,000 were displaced, and more than 135,000 buildings were damaged (Moore and Moore, 1989), the Army Corps of Engineers did not abandon its dream of controlling all floods. Rather, it proposed building large dams upstream to reduce flood peaks to the capacity of the floodway between the levees (Williams, 1998).

Until the 1970s, most flood loss reduction efforts involved structural solutions. Although nonstructural measures were discussed as alternatives, they were rarely implemented. The shift from mostly structural to mixed structural/nonstructural

Figure 5 Elevated home in West Virginia is a mitigation success story. Risk is greatly reduced to homes elevated before a flood. See ftp site for color image.

measures began in the 1970s and continues today. The mix of adjustments varies for cach situation, in Europe almost all measures that are taken have elements of combined structural and nonstructural measures. There has also been a move to be antistructural. Some dikes are being removed in favor of nonstructural or more environmentally sensitive techniques (Smith and Ward, 1998).

Nonstructural measures include floodproofing, land-use planning, soil bioengineering, warning systems, preflood mitigation efforts, and insurance. The simplest nonstructural measure is to acccpt the loss. Another nonstructural measure is to provide postflood relief. Protection of lloodplain residents and users, and the supply of relief when they suffer damage, are forms of hidden subsidy (Alexander, 1993). This category includes aid provided by the Red Cross, voluntary organizations, and governmental agencies.

Nonstructural measures include flood insurance and iand-use management, acquisition and relocation, floodproofing, preflood mitigation preparedness, outdoor warning systems, and soil bioengineering,

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