Twain Harte Lake alert
Cracks in rock next to dam prompt evacuation advisories
More than 130 residents downstream of Twain Harte Lake were placed under an evacuation advisory Thursday afternoon as multiple local and state agencies investigated cracks in the granite slab that anchors the dam’s southern side.
An evacuation advisory was initially issued as a precaution for people in the areas of Good Shepherd Drive and Lucky Strike Trail in Upper Crystal Falls below the dam, before it was extended about 1:30 p.m. to areas east of and along Sullivan Creek.
The Tuolumne County Sheriff’s Office warned of a rise in water levels downstream as the private, man-made recreational lake was partially drained to alleviate pressure on the dam.
Sheriff’s Sgt. Niccoli Sandelin, spokesman for the agency, said the cracks in the granite slab, known to lake goers as “The Rock,” were discovered by first responders investigating reports of a loud “explosion” noise in the area about 10:30 a.m. Thursday.
Sandelin said investigators did not find any explosive devices or evidence of explosions in the area of the dam and determined that the sound people heard was likely caused by cracking in the granite slab that anchors the dam’s southern side.
Dore Bietz, the county Office of Emergency Services coordinator, said that managers with the Twain Harte Lake Association were confident that releasing the water would alleviate any concerns of a larger dam failure.
The lake has a maximum capacity of 142 acre feet, or enough water to cover 142 acres to a depth of one foot.
Other agencies involved with responding to the incident included the Twain Harte Community Services District, Twain Harte Fire Department, Tuolumne Utilities Dis
trict, Cal Fire, California OES, the state Division of Safety of Dams, and Pacific Gas and Electric Co.
It marked the third time since 2014 that cracking in the granite slab has caused concerns about a potential dam failure, though it was the first to occur outside of summer and during a storm that brought heavy rainfall and cold temperatures to the area.
The first time cracking in the slab was discovered was the morning of Aug. 3, 2014, and was followed by three more subsequent events over the next month.
Hikers and people near the lake at the time of the 2014 incident reported hearing loud booms and seeing a cloud of dust above the slab. Small popping noises in the areas of the cracks persisted throughout the day as officials investigated.
It was determined that the cracking in 2014 was a rock stress release event, or exfoliation, which is a term geologists use to describe the natural process of pieces of granite breaking away from a larger dome.
“Natural rock stress release events are caused by years of weathering of an unprotected rock surface and fluctuations in temperatures,” the Twain Harte Lake Association's website says. “The resulting rock fractures, known as exfoliation joints, form onion-like patterns where it appears that layers of the rock are peeling away — just like an onion.”
The 2014 caused damage to the left dam abutment that resulted in several cracks and a small leak.
All of the water was immediately drained from the lake after the 2014 event, with the ensuing
repairs to the granite slab and dam costing nearly $900,000. The project, described as the largest project ever undertaken by the association since the dam was built in 1928, was funded by a combination of the association's reserve funds and a special assessment paid by members.
In addition, the lake that serves as a significant economic driver for the Twain Harte economy was
closed for the remainder of the summer that year and didn't reopen until late June 2015.
Another exfoliation event occurred on July 22, 2016, and resulted in the lake being closed for the second time during summer, though it reopened on Aug. 6, 2016, after the area was cordoned off and safety protocols were put in place.
The association's web
site says the dam is now monitored twice yearly by the state Division of Safety of Dams, and regular movements of the granite slab are monitored by the U.S. Geological Survey, as such natural stress release events are expected to continue into the future.