Sacramento, California is one of America’s more floodprone cities, having struggled with flooding issues for over 150 years. This report marks the second time that the WSTB has provided assistance to the Army Corps of Engineers on how to cope with American River flood risks. A major flood in 1986 served as the initial impetus for identifying an acceptable and feasible set of measures to decrease flood risk. Because of the complexities surrounding this issue, Congress subsequently requested the Corps to engage the NRC as an independent advisor. In 1995, the WSTB issued Flood Risk Management and the American River Basin: An Evaluation, which outlines a flood risk management strategy for the city.

Subsequent to release of the first report, in January 1997 the American River experienced a major flood, suggesting that it would be necessary to re-compute flood flow frequency relationships. In February 1998, the Corps published a revised flood flow frequency analysis for the American River at Fair Oaks. That analysis produced a flood frequency curve indicating that large floods are more likely than was previously thought. Based on the newly estimated 100-year flood discharge, the levees protecting Sacramento no longer provide protection against the 100-year flood. The revised flow frequency relationships had immediate policy implications and reduced the estimated level of protection provided by the flood control alternatives that were being considered for Sacramento.

The recommended flood distribution is based on both the systematic record of three-day rain flood flows estimated for Fair Oaks, California and on the historical record for 1848-1904, which includes an estimated large three-day flow associated with an 1862 historic flood. The method used by the committee is consistent in spirit with the guidelines set forth in Bulletin 17B, a 1981 federal report that specifies a uniform method for flood frequency analysis. Flood quantiles estimated in the NRC report are slightly less than those made by the Corps in 1998. If adopted, the 100-year flood estimate recommended in the report could result in removal of some areas of Sacramento from designated risk zones, by a slight margin. However, because the uncertainties in this estimate are large, it will be difficult to accurately determine flood risk of certain areas. The report made thus recommends the expeditious creation of a flood risk management strategy for Sacramento.

The report recommends the establishment of a new interagency research effort focused on flood risk assessment and management. The impetus for such action is clear: rising property damages and loss of life; 30 years of experience with the National Flood Insurance Program; aging federal policy and technical guidance; improvements in scientific methods of computing and modeling; new understanding of paleohydrologic and climate variability issues; and a growing data base and availability of information. This effort should emphasize research on coordinated flood risk reduction, including meteorologic, hydrologic and hydraulic, and policy and socioeconomic aspects of flood management.