Sacramento >> The fifth memorandum from the independent board analyzing the Oroville Dam spillways design was released Friday, with recommendations to focus on the security of the emergency spillway foundation and methods of installing roller-compacted concrete.
The board said it agreed with most of the proposed design plans again, with a few discrepancies.
When water flooded down the emergency spillway Feb. 11, it was not a structure at all, but a ravine with trees, rocks and gravel. The new design calls for a reinforced concrete pathway.
It was the first time the structure was used and led to the evacuation of 180,000 residents surrounding Oroville.
Secant wall columns, which are overlapping concete-filled drill holes, will go into the “slightly weathered rock” under the emergency spillway, it says, noting that the rock is more erodable downstream.
“The potential for erosion to occur on the northern-most Emergency Spillway slope should be addressed in the design,” the memo reads.
DWR initially proposed focusing construction efforts on the top chute of the main spillway, but plans changed when further surveying showed the foundation was stronger at the top of the structure than its damaged lower portion. The memo states the current plan is to hold off on demolishing and replacing the upper chute for 1-2 more seasons.
Current plans call for reinforced concrete with training walls for the lower chute to be complete by November.
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