The U.S. Army Corps of Engineers is a key federal agency in helping reduce flood damages across the nation. While several federal agencies play various roles in flood damage reduction, the Corps’ roles have traditionally focused on construction of levees and other river control works. Proper levee design must account for a variety of hydrologic, hydraulic, economic, and geotechnical considerations. Levees must be high enough to safely pass a large flood, but heights are limited by ecological and aesthetic considerations. For decades, the Corps used levee “freeboard,” an additional levee height above the estimated water surface of a given flood (e.g., the 100-year flood), for designing levees. Freeboard was used to help achieve specific design objectives, such as reducing the probability of a flood overtopping a levee. The standard “3 feet of freeboard” became an engineering tradition in the Corps and beyond.

Challenges to the freeboard concept emerged in the early 1990s from both within and outside the Corps. It was noted, for instance, that standard freeboard may not adequately account for geographic and hydrologic differences and thus may provide different levels of flood protection at different places. The Corps felt that risk analysis techniques held promise in addressing these issues, as these techniques aim to quantify and explicitly incorporate uncertainties in relevant hydrologic, hydraulic, economic, and geotechnical parameters into levee design. In the mid-1990s, the Corps abandoned the freeboard concept and adopted risk analysis as part of its flood damage reduction studies. There were, however, challenges to the new policy and, as a result, Congress asked the National Research Council to review and assess the Corps’ risk analysis techniques in its flood damage reduction studies.

This new report from the WSTB commends the Corps for adopting the new techniques and departing from the traditional freeboard approach. The risk analysis paradigm for flood damage reduction studies represents a step forward for the Corps and has been accomplished with a modest amount of human resources. The report’s recommendations are divided into four topical areas: risk analysis techniques, consistent terminology, levee certification, and floodplain management.

Risk analysis techniques. The report recommends that the Corps use annual exceedance probability as a key performance measure of engineering risk. Annual exceedance probability measures “the probability of getting wet” in any given year, considering the full range of floods that can occur.

The report describes the differences between natural variability and knowledge uncertainty. Natural variability refers to inherent variability in the physical world, while knowledge uncertainty refers to a lack of scientific understanding of natural processes and events. In the Corps’ risk analysis, it is not always clear that these different types of uncertainty are clearly distinguished and treated appropriately. It is thus recommended that the Corps differentiate between these uncertainties, as each affect the calculation of risk in different ways.

It is also recommended that the risks associated with flooding and the benefits of a flood damage reduction project be conducted on a structure-by-structure basis, rather than conducting risk analysis on damages aggregated over groups of structures in “damage reaches.”

Consistent terminology. Several terms describing risk and uncertainty are often used interchangeably within and between different Corps water resources programs. The report thus recommends that the Corps adopt a consistent vocabulary for describing risk analysis concepts, specifically distinguishing between risk, natural variability, knowledge uncertainty, and measures of system reliability.

Levee certification. The Corps and the Federal Emergency Management Agency (FEMA) jointly conduct a federal levee certification program. Before the Corps’ adoption of risk analysis, levee freeboard was the key criterion for levee certification. Abandonment of freeboard has resulted in complications to the certification criteria. The report recommends that levee certification focus on annual exceedance probability, rather than on the 100-year flood. The report recommends that a uniform criterion for the level of protection be adopted. While it is not obvious what that criterion should be, it is recommended that it be the historical median level of flood protection provided. The median annual exceedance probability of Corps flood damage reduction projects was estimated as approximately 1/230.

Floodplain management. Neither the Corps nor the U.S. Congress has identified an explicit goal for management of the nation’s floodplains. The report recommends that the goal should be to use the land for the greatest social benefit. Toward this end, the report recommends that the Corps explicitly address social and environmental benefits in its risk analysis. It also recommends that social and environmental effects within Corps flood damage reduction studies be quantified to the extent possible and included within the National Economic Development water resources planning alternative.