Nevada Irrigation District OKs design contract to improve Scotts Flat Dam spillway
Dam safety was a big topic for Nevada Irrigation District directors this week, with more than $1 million earmarked for projects at Bowman and Scotts Flat lakes.
Nearly $800,000 was approved Wednesday for a design contract to improve the nearly 70-year-old lower spillway at Scotts Flat Dam, reportedly the first major spillway repair project to seek approval from federal regulators since the 2017 Oroville Spillway failure.
Both spillways at the Oroville Dam crumbled and fell away during heavy rains in early 2017, prompting fears of a catastrophic dam collapse that forced the evacuation of 200,000 people downstream. The cost of repairs and other improvements reached $1.1 billion as of last fall with major construction activities set to be completed late this year.
In the wake of that incident, water districts across the state were required to perform potential failure analyses of their dam spillways, and Nevada Irrigation District was no exception.
Hydroelectric Manager Keane Sommers noted the original Scotts Flat Dam spillway was constructed in 1948 and has had problems since it was built, likely due to poor concrete quality in the original construction.
“We’ve done a lot of patching, but that has accelerated in recent years,” Sommers said.
Sommers said that while the old spillway does not meet current standards, a raised spillway crest was built in 1964 that will resist failure.
“The raised spillway is in excellent shape and is not a concern,” he said. “It is built on solid bedrock.”
Sommers noted that since its construction, the old spillway has repeatedly had portions of the chute slabs break off. Several sections came off during an extended “spill event” in 2017, he said, adding that damage was quickly repaired.
“We removed a significant portion of the top of the spillway,” Sommers said.
Since then, water district staff has performed extensive subsurface exploration and analysis of the old spillway, he said.
Under probable maximum flood conditions — which Sommers said would be a “darn near biblical event, the absolute worst flood a computer model can predict” — even if the lower sections of the spillway failed, the raised spillway crest would retain water.
Sommers stressed the old spillway is considered acceptable but needs to be upgraded to meet current standards.
In March, the California Division of Safety of Dams downgraded the condition assessment of Scotts Flat Dam and Reservoir from “satisfactory” to “fair” because of those deficiencies, and required every effort be made to rehabilitate the spillway by Oct. 31, 2022.
Six proposals were received for the design portion of the proposed upgrades, with Sommers noting a large variation in project approaches and in total costs, which ranged from $445,000 to $1.352 million. The winning bid was HDR/Schnabel, which was unanimously rated the top pick by the selection panel, at $790,883.
The board of directors also approved a $225,581 contract with Quest Structures for a seismic stability analysis of the Bowman South Dam.
The dam is a 105-foot-high and 567-foot-long concrete arch dam built in 1927 in a small canyon, which, along with Bowman North Dam, forms Bowman Lake. Federal and state agencies have classified Bowman South as an extremely-high-hazard dam due to the potential downstream impacts if it failed. Water district staff noted the dam is about 15 miles west of the Mohawk Valley fault, which can generate up to a magnitude 7.3 earthquake.
To contact Staff Writer Liz Kellar, email email@example.com or call 530-477-4236.
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