It's hard to imagine 26 inches of rain falling in just six hours, but that type of deluge was on the minds of engineers as they designed a $10 million rehabilitation of Lake Oneida Dam in Butler County.
The project is under construction and when it is finished, an earthen dam that dates to 1918 will be fortified with concrete and able to withstand nature's most wicked punches.
"The dam will have a shell of concrete over it," said Tyson Clouser, project manager for Pennsylvania American Water, which uses Lake Oneida as a drinking water reservoir. That will protect it in the very unlikely event that heavy rains cause the lake to spill over the top of the structure.
Penn America Water upgrades 2 dams in Butler
The Penn America Water upgrades to two dams in Butler are projects that will protect reservoirs serving 18,000 customers. (Video by Jon Schmitz; 10/22/2012)
The project was designed to comply with state Department of Environmental Protection standards, which establish what Mr. Clouser said is a threshold for "probable maximum precipitation" -- 26 inches in six hours -- and "probable maximum flood."
While that might seem excessive, there is historical precedent for a storm of that magnitude. In July 1942, 30.8 inches of rain fell at Smethport in McKean County in just 41/2 hours, believed by some to be the most intense rainfall ever recorded.
Lake Oneida Dam, along with the recently rehabilitated Thorn Run dam, collects water that serves about 18,000 customers in Butler City and 12 surrounding municipalities. The dam is 875 feet long and 33 feet high and can hold back more than 500 million gallons of water.
A 24-inch main below the dam takes water from the lake to Penn American's treatment plant in Oakland Township, just northeast of Butler. A 30-inch pipe can be used to draw down water during dry periods to maintain flows and aquatic life in Connoquenessing Creek.
Huge construction equipment prowled the isolated site last week as crews worked to build a new spillway. The top layer of soil has been removed from the dam to prepare it for a layer of compacted concrete. When that is in place, crews will top it with a new layer of soil so it can be landscaped.
"This will be one of the safest dams around, employing all of the latest technology and construction items," Mr. Clouser said.neigh_north
Jon Schmitz: email@example.com or 412-263-1868. First Published October 22, 2012 4:00 AM