The following content was contributed by The Union Democrat. The original article can be found at the following link. http://www.uniondemocrat.com/localnews/4177145-151/twain-harte-lake-is-full-set-to-open#
Most of the
drilling for the project is done and contract workers are waiting for
made-to-order rock bolts 3-feet to 15-feet long, Tom Trott, general
manager for Twain Harte Community Services District, said Thursday.
“The rock is fractured and it has
potential to move by gravity down the hill,” said Scott Lewis, principal
engineering geologist with Condor Earth Technologies of Sonora, which
prepared a project report for Twain Harte CSD in January 2015. “Our goal
is to stabilize that rock to keep it from sliding.”
Sewer line connects to TUD
The sewer line serves about 500 homes, a
third of Twain Harte CSD’s customers, Trott said. It flows around The
Rock, across the face of Twain Harte Dam, below the lake parking lot,
and then into lines operated by Tuolumne Utilities District, Trott said.
The slab area the sewer line rests on is
about 150 feet long and 40 feet in slope-height, Lewis said. It’s 2-feet
to 10-feet thick. Some of the rock surface has moss and lichen in
“We’re only bolting the area of the
pipeline,” Lewis said. “We’re focusing on the foundation for the pipe to
keep it from moving.”
Upstream from the work project, the
private recreation reservoir called Twain Harte Lake is full, holding
about 140 acre feet of water. One acre-foot can flood a typical football
field 12 inches deep. Most of The Rock is fenced off and closed to the
public right now.
Concerns about Twain Harte Dam’s abutment
with The Rock, which cracked and prompted emergency draining of the
lake and a flash flood warning in August 2014, have been resolved, said
Dennis Wyckoff, general manager for the Twain Harte Lake Association.
Like other granodiorite domes in the
Sierra Nevada, the one known as The Rock sheds layers or slabs from time
to time in a natural process called exfoliation, due to uplift and
erosion over millions of years.
Condor said four “natural rock-stress
release events” happened on The Rock beneath the sewer line on Aug. 3,
Aug. 6, Aug. 20 and Sept. 4, 2014.
“The stress release caused ground motion,
the formation of new rock fractures at the ground surface, and buckling
of surficial rock slabs,” Condor’s January 2015 engineering report
states. The dates are based on information from Twain Harte Lake
Association staff, visitors, residents and Condor staff on-site
observations during the Aug. 6 event.
During inspections beginning Aug. 3,
2014, state Dams Safety Division staff observed cracking and offsets in
the left abutment rock dome and stress cracks in the left arch barrel of
The reservoir was judged unsafe to store
water, and the lake association was ordered to open the outlet and
dewater the reservoir, Dams Safety engineer Param Dhillon said.
Satisfactory remedial work was completed
at the dam April 29, 2015, and the owner was authorized for full storage
behind the dam, Dhillon said.
Opening May 28 this year
Opening of the lake was delayed past
Memorial Day last year, but Wyckoff and many Twain Harte business owners
were pleased to have the lake open again by mid-June 2015.
This year, Wyckoff and Twain Harte Lake
Association members are looking forward to having the reservoir open by
the Memorial Day weekend.
“We’re happy they’re doing this,” Wyckoff
said Thursday of the sewer line stabilization. “It’s a preventive
measure. Exfoliation like this can happen. It happens more frequently in
Yosemite where the cliffs are sheer. Here, this is a rounded dome.”
Wyckoff said Twain Harte Lake, which has
about 800 association members, will open this year on May 28. National
Dam Safety Awareness Day is May 31.