Oroville dam is the tallest dam in the United States, but it has an auxiliary spillway that consists only of a 1,730 foot long spillway lip, that spills directly onto bare hillside. Yuba County Water Agency (YCWA) raised concerns in 2002 about the damages that could occur from using the auxiliary spillway:
The discharge area below the emergency spillway is not armored and extensive erosion would take place if the emergency spillway were used. The spillway road and possibly high voltage transmission towers would be impacted. (p. II-1) Because the area downstream from the emergency spillway crest is an unlined hillside, significant erosion of the hillside would occur. (p. II-5) “The hillside between the emergency spillway and the Feather River would be subject to severe erosion when water flows over the spillway. Depending on the rate of flow, the erodable area . . . could range from 50 to 70 acres. The amount of soil, rock, and debris that would fall into the Feather River could be very large, depending on the depth of erosion. There could be damages to downstream structures, including the Thermalito Diversion Dam and Powerplant, Fish Barrier Dam, and highway bridges. If there is river channel blockage below the spillway, there could be impacts on operation of Hyatt Powerplant. (p. IV-3)
Yuba County Water Agency, Technical Memorandum on Lake Oroville Surcharge, August 2002
These concerns were raised with the California Department of Water Resources almost 15 years ago, during relicensing of Oroville dam by the Federal Energy Regulatory Commission (FERC.)
It will be stunning if this thing actually holds.What a stupid design not to be cement lined. Just incredible.