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Review panel confirms Corps' high-risk assessment of Isabella Dam

An external peer review panel commissioned by the Corps of Engineers has confirmed the Corps' finding that Isabella Dam on California's Kern River has a high risk of failure.

Isabella was one of six dams the Corps identified as having a high risk of failure during an initial screening of 130 dams in 2005 and 2006. (HNN 5/2/07) Built in 1953 as a flood control and water conservation structure, Isabella Dam also controls flows released to Southern California Edison's 12-MW Borel project (No. 382).

The external panel's Isabella Dam Consensus Report, released Nov. 9, found the Corps' Class I designation �Urgent and Compelling� is appropriate for Isabella Dam for several reasons:
o A possibility of piping along the outlet conduit of its auxiliary dam;
o Evidence the auxiliary dam's drain blanket is not performing as intended;
o Studies finding the Kern Canyon Fault, under the auxiliary dam's right abutment, is active;
o Evidence the upper 20 feet of the auxiliary dam's foundation is loose and might be subject to loss of shear strength during seismic loading;
o Hydrologic studies that indicate the spillway to be inadequate; and
o Extremely high consequences of failure.

The panel recommended maintaining the current reservoir pool restriction of elevation 2585.5 feet, 20 feet below normal pool. It called for other short-term risk reduction measures including: testing of new piezometers; evaluation and installation of devices to monitor the main and auxiliary dams and the Kern Canyon Fault; assessing underlying alluvium to determine its effect on seismic stability; updating the emergency action plan; and conducting emergency exercises.

The panel also recommended long-term risk reduction measures including completion of on-going studies of major rehabilitation of the auxiliary dam and evaluation of overall earthquake performance of the main dam.

Corps officials presented information on seismic and seepage investigations at the auxiliary dam to the Kern County Grand Jury on Nov. 15. The grand jury investigates all aspects of local government to ensure effective government and judicious handling of public monies.

The Corps said its studies of the fault are expected to be completed in December. A Kansas City-based Corps geotechnical drilling team finished drilling at Isabella in November. That included drilling ten holes up to 200 feet deep to collect soil samples. More drilling might occur in January.

The Corps also is conducting a thorough seepage study of the auxiliary dam. It also invited geophysicists of the U.S. Geological Survey and an international group of dam safety experts to examine the facilities.

The agency also awarded an indefinite delivery/indefinite quantity contract to a joint venture of URS Corp., Kleinfelder Inc., and Geomatrix Consultants to study designs to meet dam safety requirements.

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