AGENDA 21 RADIO

BY CHRISS STREET, NEWPORT BEACH, CA

Dam safety experts are warning that a three-day storm hitting Northern California could risk an overflow event at the partially rebuilt Oroville Dam spillway.

Northern California’s second largest March precipitation in 20 years and warm temperatures kicking off an early Sierra Nevada Mountains’ snow-melt has raised the water level at the 700-foot-deep Lake Oroville to just 36 feet below the point where dam safety operators must open the partially repaired Oroville Dam main spillway.

The Federal Energy Regulatory Commission and the California Department of Water Resources that owns the state’s second largest reservoir, intended to keep the water storage at or below 40 percent, or about 1.4 million cubic feet of the lake’s 3.5 million cubic foot capacity. But last month’s storms increased storage to 2.2 million cubic feet.

After last year’s Oroville Dam spillway failure, the California Department of Water Resources created the 2017-18 Lake Oroville Winter Operations Planto ensure public safety in the event of major storm events. The plan triggers more aggressive outflow at the Hyatt Powerplant and potential main spillway use if April water level reached 830 feet.

The current forecasts show the potential for inflows to raise the reservoir from the current 794 feet to near the 830-foot trigger elevation levels by the middle of next week.

Kiewit Construction Corporation is supervising a $710 million two-year repair of the dam and spillway infrastructure following last year’s near collapse of the dam and emergency forced evacuation of 188,000 Sacramento area residents. First phase construction shutdown in October for the rainy season and was expected to finish this summer.

California Division of Dams officials claim the spillway chute is in working order after first phase repairs and inspectors will be on site around the clock to monitor the performance of the spillway and correct any problems if the spillway floodgates are opened.

DWR spokesman Erin Mellon told the SFGate blog, “Hopefully we don’t need to use the spillway,” but if it is needed, “we’ve done a lot of hydraulic monitoring. We’re confident in the construction.”

Division of Dam engineers are not concerned about the structural integrity of the dam, but rather the sustainability of the main spillway that only has a temporary roller-compacted concrete surface, versus an extremely smooth permanent concrete surface. High-speed water-flow for a prolonged period over a rough temporary surface could cause the same cavitation friction that chewed up and destroyed the main spillway last year.

Kiewit Construction built a series of temporary cutoff walls along the half-finished main spillway as a precautionary measure in the event of an emergency overflow event. But last year’s 100,000-cubic-feet-per-second (cfs) overflow could take out the cutoff walls.

In anticipation of the coming rainstorm, DWR increased outflows Hyatt Powerplant outflows to 10,000 cfs. Maximum outflow capacity at the powerplant is 12,500 cfs and DWR can also open the River Valve Outlet to release another 4,000 cfs, before opening the spillway. But at the height of last years’ crisis, the spillway was releasing 230,000 cfs.

DWR has officially state and federal regulatory agencies, public safety organizations and the Butte County Sheriff’s office, and all local, state and federal elected officials about the increase in outflows and potential use of the spillway.