Wednesday, February 21st, 2018

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)

“Oroville Dam Spillway – What Happened and Why?”

      – by Dr. Rob Campbell

In 1969, the Oroville Dam was constructed to provide drinking water storage, flood control and hydrostatic power for California. The dam is approximately 901 feet in height at the crest and has a capacity of approximately 3.5 million acre-feet, or 1.1 trillion gallons of water. Geologically, the Oroville Dam is constructed on Mesozoic volcanic rocks, which have been highly altered due to Mesozoic subduction zone deformation, which have been highly-weathered. Furthermore, the concrete spillway was constructed over thick layers of colluvium (ancient landslide deposits).

The local geology observed at the Oroville Spillway Repair project is consistent with metamorphism and metasomatism associated with porphyry induced epithermal deposition in the Miocene along the New Melones fault zone, specifically the Long Ravine and Cleveland Hill Fault zones. These metamorphic rocks include phyllite, slate, and meta-volcanic rocks. Sheeted dykes containing actinolite (tremolite) are juxtaposed against the Cleveland Hill Fault zone, along with localized serpentinized zones, which depict epithermal (hydrothermal) deposition zones.

In early February 2017, the concrete spillway cracked due to lack of maintenance, and severe erosion of the underlying colluvium. The breach of the emergency spillway, which was not concrete lined, created severe erosion and the collapse of the concrete spillway. California Department of Water Resources (DWR), increased outflow from the dam from 55,000 cfs to 100,000 cfs to stop the overflow on the emergency spillway. If the DWR did not stop the overflow, there was significant risk of dam failure.

Failure mechanisms on the Oroville Dam Spillway included the following: poor concrete-bedrock adhesion due to dust; lack of 3rd Party concrete inspections during original construction; areas of colluvium beneath spillway (ancient landslide deposits); no spillway maintenance (even though cracks were observed); poor reservoir management by DWR (inflow-outflow water management); inadequate and lack of engineering geologic investigations and design; Cleveland Hill Fault location, colluvium, and weathered bedrock; and lack of geological and engineering cooperation.

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