Assessing the True Value of Flood Control Reservoirs: The Experience of Folsom Dam in the February 1986 Flood

by Philip B. Williams, Philip Williams & Assoc. Ltd, San Francisco, United States,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering


Over the last 40 years considerable public investments have been made in large flood control storage reservoirs throughout the U.S. and internationally. These projects were typically designed to control floods in excess of the 100-year event. Although there have been only a few decades of operating experience for these projects, there is now sufficient evidence to show that the actual flood benefits achieved can be considerably smaller than promised in their design. The operating performance of Folsom Dam on the American River above Sacramento is illustrative. In the February, 1986 flood-variously estimated to be between the 40- and 70-year event-the reservoir was completely filled forcing releases higher than the designated floodway capacity and use of surcharge storage space. Possible catastrophe to Sacramento was averted by the early cessation of rainfall. This paper describes two different assessments of this event, both of which have profound implications for flood control policy in the U.S. One carried out by the Corps of Engineers significantly downgrades the capability of Folsom, thereby justifying the need for a new flood control reservoir upstream at Auburn. The second, carried out by author, focusses on the role of Folsom as part of a larger flood management system emphasizing the importance of efficient flood control operation.

Subject Headings: Floods | Reservoirs | Dams | Water storage | Hydraulic design | Investments | Rivers and streams | Systems management | California | United States

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