Baseline Risk Determination and Risk Management Formulation for the Isabella Lake Dam Safety Project

sabella Reservoir is located on the Kern River approximately 40 miles northeast of the City of Bakersfield near the confluence of the north and south forks of the Kern River, in Kern County, California. The Isabella project consists of two dams that provide flood risk management, irrigation water supply, power generation and recreation benefits to the region. The Auxiliary Dam is about ½ mile east of the Main Dam. The project is comprised of a 185 foot high earthfill dam, an ungated ogee concrete spillway, and a 100 foot high earthfill Auxiliary Dam. The reservoir has a gross storage capacity of 568,075 acre feet. Construction of the dams at Isabella Lake began in 1948 and was completed in 1953.
The Isabella dams have been determined to have an unacceptable level of risk by the U.S. Army Corps of Engineers (Corps), requiring urgent and compelling action by the Corps to reduce the probability and consequences that would result from a dam failure. There are three primary deficiencies (seepage/piping, hydrologic, and seismic) at the project which could lead to significant life loss in the event of a dam failure. The population at risk (PAR) is approximately 350,000 people in the city of Bakersfield and the town of Lake Isabella. In the event of a dam failure there could be significant impacts to Interstate 5, Highways 99 and 58; major railroads lines; and the California state water project (supplies water to the Los Angeles metropolitan area).
This paper includes an overview of the Isabella Project, the risk driving failure modes, and the potential risk management solutions for long term risk reduction. The approach and challenges to identify and understand significant dam safety issues (failure modes) will be presented, including seismic induced failure modes. The focus will be on the development of risk management measures and the formulation of plans that address the dam safety issues that were identified during the baseline risk evaluation. This paper will also discuss the phased approach taken to formulate and develop risk management plans, and lessons learned on the risk assessment and alternative development process.