California Department of Water Resources (DWR) engineers completely stopped the water flowing down Oroville Dam’s collapsing spillway on Monday in a desperate effort to clear intake pipes and restore hydro-electric production before flooding returns.
Over the last two weeks, engineers have been involved in a risky operation to clear a million cubic yards of cement chunks, debris and silt (about equal to five football fields at a height of 100 feet) by operating a dredging barge at the bottom of the 770-foot-high dam in what is called the Thermalito Diversion Pool. But the crews have been unable to clear intake pipes to restore 819 megawatts from the dam’s six hydro-electric generators.
Paul Preston of Agenda 21 Radio, who has continuously been broadcasting from Oroville all month, warned that the only reason Lake Oroville is down 62 feet from cresting over the dam has been due to DWR engineers running Oroville Dam’s main spillway at an average of 55,000 cubic feet per second (cfs), about double its 33,000 cfs rated capacity.
Water is supposed to run smoothly down a dam’s angled spillway in what is referred to in fluid dynamics as “laminar flow.” But it appears that a lack of maintenance at the 49-year old Oroville Dam allowed cracks to develop in between the spillway’s huge reinforced concrete plates. The uneven surfaces created “turbulence” in the water’s laminar flow.
As DWR increased water flow beyond the spillway’s maximum capacity, turbulence started acting like giant jackhammers on cement plates. As the plates cracked, vibration caused whirlpools that acted like massive drills and destroyed the spillway’s surface.
With both Oroville Dam’s main and emergency spillways failing earlier this month, Pacific Gas & Electric was forced to take down itsoverhead high-voltage power lines in order to allow helicopters fly in huge rock bags to plug the dam’s growing fissures. Since then, the dam’s work crews have been forced to rely on diesel generators to maintain electrical power.
Paul Preston reported the reason DWR engineers are temporarily shutting down 55,000 cfs of water flow on the crumbling spillway is to give dredging contractors a chance to carve a 1,500-feet-long by 150-feet-wide and 30-feet-deep emergency trench in the diversion pond to clear the Edward Hyatt power plant’s water intake pipes.
Rising temperatures later this week are expected to kick off an early spring snowmelt. But if dredgers are successful in clearing enough debris, diverting water into the power plant would have the added benefit of allowing another 14,000 cfs of water to be discharged continuously from Lake Oroville.