San Lorenzo Creek Watershed Archive|
February 11, 2014
Cull Canyon Reservoir could be drained within the year, doomed by a seismically unsafe dam combined and so much silt filling the reservoir that the water level is only six feet deep. Because of these conditions, the Alameda County Flood Control District is proposing a plan to notch the spillway, allowing Cull Creek to flow naturally. The alternative, dredging the reservoir and strengthening the dam, is too costly, District officials say. District engineers want to notch the reservoir's weir, an underwater dam above its spillway. The notch would let the water flow downstream and restore Cull Creek to a more natural state above the spillway, with the area around it becoming a meadow.
"The first storm every winter, the reservoir fills up," said Hank Ackerman, program manager for the Flood Control District. "If we were to notch the weir, it could keep water from backing up in storms." The notch would not affect a separate popular swimming lagoon run by the East Bay Regional Park District.
Cull Canyon Reservoir was 30 feet deep when it was built in 1963. But about 12,000 to 20,000 cubic yards of silt and debris flow into the reservoir every year. "It's almost completely silted up," Ackerman said. At one time the reservoir was a popular fishing site. "East Bay Regional Park District used to stock it for fishing, but five or 10 years ago, it got shallow enough that there was a big fish kill on a hot August day, so they stopped," Ackerman said. "This last October, it dried up completely."
The reservoir never was very effective at flood control, Ackerman said. "It was constructed basically to put a road across over to the high school, which is now Canyon Middle School," he said. Heyer Avenue, a heavily traveled east-west road, runs along the top of the dam.
In 2006, because of concerns the dam could not withstand a major earthquake, a 12-inch pipe was punched through the concrete spillway, lowering the reservoir level and easing pressure on the dam. Bringing the dam up to current seismic safety standards would cost $8 million to $10 million, Ackerman said. If the county removed the silt, the dam would need reinforcing. "What makes it unsafe is the water behind it," he said.
Although the problem of silt has plagued the reservoir and dam for years, the Flood Control District's options were limited until now. The dam was built with funds from the Davis-Grunsky Act of 1960, and under the grant's terms the District had to maintain the dam. That grant expired in December 2013, freeing up the ability to modify the dam.
Dredging the reservoir would cost about $16 million and require 28,000 round trips by double bottom dump semitrailers to remove the estimated 450,000 cubic yards of sediment, Ackerman said. "We can notch it for less than $1 million," he said. "The state Division of Safety of Dams would no longer consider it a dam if it didn't hold back water."
The county's Department of Public Works may have a design for the notching by April, Ackerman said. The design would then be presented to the public, with work scheduled to start in late summer. The project would not completely restore Cull Creek to its natural state; the spillway would remain. "If we notch it, it would be something that would be reversible if the community later decided they wanted a dam," Ackerman said.
Source: Based on an article by Rebecca Parr in the Daily Review, Feb. 11, 2014