Sacramento Flood Threat

Sacramento's risk of flooding is the greatest of any major city in the country.

Sacramento faces an unacceptably high risk of flooding for two primary reasons:

1. The cores of today’s levees are often the levees built by farmers and settlers as much as 150 years ago. Early levees were not constructed to current engineering standards, and little care was given to the suitability of foundation soils. These remnants of the past make today’s levees unreliable. To address this issue, levee improvements to strengthen levees and to make them less susceptible to seepage induced failures are a major portion of SAFCA’s efforts to reduce the risk of flooding in Sacramento.

2. The quantity of water flowing out of the Sierra Nevada mountains during large floods appears to be increasing. Folsom Dam was designed to reduce flood flows in the American River to a flow rate that could be safely carried by the downstream levees. Construction on Folsom Dam began in 1950. In designing the Dam, engineers used historic flow records and statistical analysis to predict the size and frequency of occurrence of large floods. From those predictions they determined how much space must be available in Folsom Dam to store excess flow from a flood with one chance in five hundred of occurring in any given year.

The graphic below shows the relative size of large floods over the past 100 years.

Note that the first storm occurring after beginning construction of Folsom was larger than any occurring in the prior 45 years. Since that 1951 storm, Sacramento has experienced four more, each somewhat larger than the previous. A comparative analysis run on the two periods (1905 to 1950 and 1905 to 2000) shows that a storm with 1 chance in 500 of occurring in any year based on the earlier period is approximately the same size as a storm with one chance in 50 of occurring using the entire 90 period. This apparent trend of larger and larger storms is the primary reason Sacramento’s risk of flood is unacceptably high.