Far beneath the earthen embankment of Puddingstone Dam, a steel slab the size of a desk top helps to hold back the waters of Puddingstone Reservoir.
The 2-by-5-foot metal rectangle, installed as the reservoir's "bathtub plug" when the dam was constructed in 1928, sprang a leak in the mid-1970s. Now county officials say a "bad leak" of about 43,200 cubic feet of water a day is gushing around the plug's worn edges.
While the leak is only a drop in the bucket for the reservoir, which holds 282.5 million cubic feet of water, it has presented an engineering challenge that took months of planning to resolve and six years of scrutiny by the state Department of Water Resources.
County engineers cannot remove and repair the steel slab--actually a gate that can be opened and closed electronically from a control tower above--without draining the entire reservoir and causing environmental and recreational chaos at the popular reservoir which is the central feature of Frank G. Bonelli Regional County Park in San Dimas.
Removing the gate "would be just like pulling the plug, letting all the water out," said county Department of Public Works spokesman Roslyn Robson. "So it's not an option at all."
Nearly Identical Tower
Instead, the county plans to construct a second steel gate and control tower, nearly identical to the originals, to take over the job of plugging the reservoir.
The project, which will require months of underground construction work, the building of a special access road for trucks and bulldozers and $500,000 in county funds, is sending the county looking for a special company that can handle a big custom order.
The county will advertise for bids on the project beginning Friday, "but only a few companies in the country are expected to respond to this kind of job," said David Smith, a county engineer.
County officials are quick to point out that the leak, although big enough to create a small creek in the reservoir's drainage tunnel, does not threaten the dam's security and is not big enough to cause problems in Walnut Creek just downstream.
"We want everyone to know that it is not a safety problem, as people so often think when they hear the words 'leak' and 'dam' at the same time," said Robson. "The dam itself is not leaking, and there is no weakness of any kind" in the earthen embankment.
Empties Into Creek
Nevertheless, the small gate plays a key role in the county's operation of the heavily used fishing and boating reservoir. The gate has for decades sealed off the underground drain that leads from the reservoir's bottom to a small flood control channel that eventually empties into Walnut Creek.
Nearly every winter, county engineers said, the foot-thick gate is cranked up to release rainstorm waters that swell the shores of the reservoir, threatening to swamp the beaches and parks.
Each time the gate is hoisted, county engineers said, it drags heavily against the brackets that hold it in place, scraping away bits of steel.
"We've had years of use out of the gate, and it's simply worn down," Smith said.
After the leak was detected, scuba divers hired by the county were able to patch it on several occasions, but now the gaps are too large to repair, county officials said.
Patterned After Others
The repair plan, which engineers for the Department of Public Works have been working on since 1978, is similar to one used at other older dam sites in California where backup systems were installed after leaks were discovered.
Smith said the new steel gate and control tower will be constructed 180 feet downstream at the dry end of the drainage tunnel, a section normally used only when releasing flood waters.
The solution is relatively simple--and expensive. Smith said two costly items, essential to the repair project, drove the cost to the $500,000 mark:
- A new solid steel gate, about 5 by 6 feet and more than a foot thick, that can withstand "incredible water pressure" and will cost about $80,000.
- A steel tube, 180 feet long and nearly 6 feet across, that will be used as a reinforcing sleeve inside the drainage tunnel and will cost more than $100,000.
Tunnel to Be Strengthened
The steel tube will help the drainage tunnel withstand the high pressure introduced when reservoir water is released into the currently dry section of the tunnel, he said. In addition to installing the steel sleeve, the county will extensively reinforce the dry end of the tunnel with concrete, adding six inches to its already massive walls.
The new gate will take over the primary role as the reservoir's plug and the old gate will be left open, Smith said. The old gate will be closed only to seal off water if repairs are ever needed on the new gate, he said.
County engineers spent several months in 1979 designing the double gate system and redesigned several aspects of it to receive state approval. They are eager to find a company that can handle their unusual order, Smith said.
"There are a handful of companies out there that can build a steel gate that thick or a steel sleeve that long," he said. "We'll be hearing from the top companies when we advertise for bids."
The company selected for the job will have to wait out this year's storm season, beginning the repair work next May and finishing up in late 1986, he said.