Dam Seismic Retrofit
Crane Valley Dam
Crane Valley Dam is located about 40 miles northeast of Fresno in Madera County, California on the North Fork of
Willow Creek, a tributary of the San Joaquin River. The dam was built between 1902 and 1911 when very little was
known about seismic activity. The dam is a 145-foot high, hydraulic earth and rock fill embankment, with a thin central
concrete core wall. The dam is 1,880 feet long, and the crest of the dam is at 3,380 feet elevation.
Crane Valley Dam consists of two distinct dams separated by a rock knoll. The Main Dam is located in the main channel
of the creek, and has earth fill on the upstream side of the core wall and rock fill on the downstream side. The West Dam
is located adjacent to the main dam in a smaller side channel west of the creek, and has earth fill on both sides of the con
wall. The spillway is located on a saddle about 500 feet to the east ofthe dam.
Seismic Retrofit Project History
There are two main regulatory agencies involved with the Crane Valley Dam:
- The Federal Energy Regulatory Commission (FERC)
- The Department of Water Resources (DWR), Division of Safety of Dams (DSOD)
Division of Safety of Dams is responsible to review the stability of all dams in California. In light of improved design
approaches as well as new findings regarding earthquake hazards and hydrologic estimates, DSOD periodically
establishes new requirements for dams. In 2002, DSOD requested that PG&E revise previous seismic stability analysis
of Crane Valley Dam (BassLake). After completing studies on the dam stability, the results indicated that modifications
to the dam were necessary to meet current standards for seismic stability. Since 2002, PG&E has worked in concert with
DSOD and the FERC to conduct further analysis of the seismic stability and coordinate the development of a seismic
retrofit plan for Crane Valley Dam. Analysis showed that in the event of an earthquake, the dam could slump and allow
water to flow over the top, causing flooding downstream.
Since then, PG&E has developed a retrofit design to modify the Crane Valley Dam to increase the stability of the dam
structures to meet state and federal safety guidelines. The dam modification would consist of the addition of
approximately 253,000 cubic yards of rock fill to selected areas on the upstream and downstream faces of the dam.
In summary, the seismic retrofit project will include the following measures:
- Installation of rockfill buttresses on the sections of the dam that have hydraulic fill on both the upstream and downstream sides of the concrete core wall
- Tying back the top of the concrete core wall in the section of the dam where the downstream shell consists of a massive rockfill embankment
- Raising and re-grading the crest to elevation 3387, about 7 feet above present minimum crest elevation, to provide more freeboard
Key Facts / FAQs
What type of dam construction makes up the Crane Valley Dam?
Crane Valley Dam consists of two distinct dams separated by a rock knoll. The Main Dam is location in the main
channel of the creek, and has earth fill on the upstream side of the core wall and rock fill on the downstream side. The
West Dam is located adjacent to the main dam in a smaller side channel west of the creek, and has earth fill on both side~
of the core wall. The spillway is located on a saddle about 500 feet to the east of the dam.
What are the seismic improvements being made to the Crane Valley Dam?
In summary, the seismic retrofit project will include the following measures:
- Installation of rockfill buttresses on the sections of the dam that have hydraulic fill on both the upstream and downstream sides of the concrete core wall
- Tying back the top of the concrete core wall in the section of the dam where the downstream shell consists of a massive rockfill embankment
- Raising and re-grading the crest to elevation 3387, about 7 feet above present minimum crest elevation, to provide more freeboard
Will parts of the lake be closed off during the retrofit construction?
During parts of construction it is expected that an area by the dam will be closed. However, this work is scheduled to be
completed during normal drawdown of the reservoir and should not have an impact to recreation in the summer months.
Will local roads be closed during the seismic retrofit construction?
In order to safely and efficiently transport material and equipment to and from the construction site, a traffic control plan
will be developed in accordance with conditions required by the Madera County Road Department and other resource
agencies. Road safety signs will be installed and flag persons will be used in accordance with traffic control best
management practices (BMP). Along with public warning signs, temporary chain link fencing with gates will be
installed at both ends of the dam.
Bass Lake is being required to maintain a water level reduction to 10 feet below maximum water level for safety of downstream residents. Will this restriction be year round or just in the summer months?
As an operating reservoir, Bass Lake water level fluctuates throughout the year and during the winter months is often
lower than the level 10 feet below maximum lake level restriction. Hydro generation will continue throughout the
construction project and the 10 foot reduction restriction is specific to summer months.
How long will the Bass Lake water level reduction be in place?
The reservoir restriction will remain in place until completion of the dam seismic retrofit estimated to be 2012.
Will the Bass Lake water level be increased back to normal after construction is complete or will increasing the lake level have to wait for the re-vegetation to be completed?
PG&E anticipates the water level will return to normal once construction is completed pending approval by DSOD and FERC.
For more information, go to: www.cranevalleydam.com