The significance of decadal- to centennial-scale climate variability is highlighted by a recent example of water resources planning.6 Flood control projects are designed to protect facilities from a design flood or flow. The level of protection (i.e., the risk of project failure) provided against the design flood is assessed through statistical analysis of the historical flood record. The economics of a new flood control project are determined by comparing the expected monetary benefits of reducing flood risk and the associated project cost. Flood insurance programs rely on a similar analysis. The variability of flood risk at decadal to centennial timescales and its implications for flood control are discussed here in the context of the American River near Sacramento, California.
Flood protection for Sacramento is provided by the Folsom Dam together with a system of levees. The dam was designed in the late 1940s, based in part on a flood record extending back to 1905. Since the dam's design, there have been six floods (not including the 1997 flood) on the American River larger than all previously recorded floods (see Figure 4.1). The estimated frequency of exceedance of extreme floods has correspondingly increased. It now appears that a large part of Sacramento may not even have 100-year flood protection. Should new flood mitigation projects be based on an assessment of flood risk from the