Remediation work begins at Ashton Dam in Idaho
Work began in July 2010 on the remediation work for Ashton Dam on the Henry’s Fork River in Idaho.
W.W. Clyde & Co. of Springville, Utah, will build a bypass tunnel around the dam. The tunnel will be constructed through bedrock, requiring drilling and blasting. It is expected to take five months to build, says Robert Atwood, project manager with owner PacifiCorp Energy. Other work to be performed includes removing and rebuilding the upstream face of the dam and constructing a low-level outlet tunnel.
Actual dam construction is to begin in 2011, and all work is scheduled to be completed in 2012.
The 7.85-MW Ashton project began operating in 1914 and consists of a rock and earthfill dam 60 feet tall and 226 feet long, with a 70-foot-wide concrete intake and 82-foot-long spillway. A roller-compacted-concrete cap was installed on the dam in 1991 to provide protection during flood flows. The powerhouse contains three turbine-generating units
In December 2004, observations by dam operators led to concerns about the condition of the structure. PacifiCorp lowered the reservoir about 5 feet to allow closer inspection and monitoring. In 2005, a potential failure modes analysis (PFMA) was conducted. One result was a recommendation for a more extensive investigation of the dam’s construction history and performance.
In 2007, PacifiCorp retained a board of consultants to assist in ensuring the continued safe operation of Ashton Dam. The board, which consisted of two geotechnical engineers and an engineering geologist consultant, assisted with a review of methods and materials used in the dam’s construction. The board also reviewed the performance of the dam since it was completed in 1914 and looked into the need for remediation measures to ensure continued safe operation.
PacifiCorp developed its remediation plan in late 2009. This involves removing 40,000 cubic yards of fine particle fill from the upstream side and installing replacement fill. During this work, PacifiCorp will draw down the reservoir by as much as 23 feet. Replacing fill is intended to stop the occurrence of sinkholes, which have affected the dam for several decades.
FEMA releases fact sheet on dam safety program
The Federal Emergency Management Agency (FEMA) offers a fact sheet on Dam Safety in the U.S. from the National Dam Safety Program. FEMA is the lead federal agency for this program.
The National Dam Safety Program makes federal funds available to the states, which are primarily responsible for protecting the public from failures of non-federal dams and pursuing initiatives that enhance the safety of dams posing the greatest risk to people and property, the fact sheet says.
The fact sheet discusses:
- Dam hazard classifications: high, significant, and low;
- Federal support for state programs, such as grants of about $3.5 million per year over the past eight years; research on risk assessment techniques and design and construction standards; safety training for dam operators, owners, and regulators; and development of technologies to help secure the safety of dams, such as the National Inventory of Dams database and the Dam Safety Program Management Tools software program;
- Creating federal policy by chairing two organizations: the National Dam Safety Review Board and the Interagency Committee on Dam Safety; and
- Partnerships that are key to program success, such as work with the Association of State Dam Safety Officials and the U.S. Society on Dams.
Seismic retrofit under way at Echo Dam in Utah
U.S. Bureau of Reclamation crews began drilling and installing dewatering and observation wells at Echo Dam in August 2010 to lower the groundwater table in the area immediately downstream of the dam to accommodate seismic retrofit work. This work is part of Reclamation’s Safety of Dams Program, which ensures dams are inspected for safety deficiencies and corrective actions are implemented if necessary.
Echo Dam is a 158-foot high zoned earthfill structure on the Weber River. Part of the Weber River Project, Echo Dam was completed in 1931 to help supply irrigation water to about 109,000 acres of land west of the Wasatch Mountains. Echo Dam is operated by the Weber River Water Users Association.
Seismic retrofit work was identified as necessary in a risk-analysis and corrective action study done by Reclamation and reviewed by a board of independent consultants. Work to lower the groundwater table will facilitate the planned excavation work downstream of the dam to replace potentially liquefiable materials in the foundation with stronger and denser material while also reducing potential risks associated with a significant seismic event in the meantime.
Drilling is scheduled to continue through the spring of 2011. Exploratory holes will also be drilled near the spillway and on the upstream face of the dam as part of the ongoing investigation.
USSD releases white paper on embankment dam aging
The U.S. Society on Dams (USSD) announces availability of a white paper, The Aging of Embankment Dams.
USSD’s Committee on Materials for Embankment Dams prepared the paper. William H. Allerton is committee chair.
This 16-page paper points out that there are aging processes and a dam may be developing aging characteristics that need to be examined, studied, and remedied to preclude a safety issue. For designers of new dams, it provides a checklist of conditions that could lead to early aging becoming a safety issue and for which designs could be developed to preclude or mitigate the effects of aging.
This white paper summarizes the chapter on embankment dams in the International Commission on Large Dams’ Bulletin No. 93, with additional comment from USSD.
– To print the white paper, visit the Internet: www.ussdams.org/aging.pdf.
Report on Red Willow Dam safety expected by late 2010
The U.S. Bureau of Reclamation issued an update in August 2010 regarding the activities concerning safety of dams issues at Red Willow Dam in Nebraska.
In October 2009, Reclamation crews discovered a sinkhole on the face of the dam. Subsequent investigations revealed cracking through the embankment, which prompted Reclamation to draw down Hugh Butler Lake behind the dam.
In February 2010, Reclamation initiated a corrective action study to develop and analyze alternatives for the repair of the dam. Reclamation expects to select a preferred alternative this fall. A modification report will be submitted to the Office of Management and Budget and Congress for consideration in early 2011.
Reclamation estimates that a contract to begin modification of Red Willow Dam could be expected in late 2011. Funds that are annually appropriated through Reclamation’s Safety of Dams Program would likely be available to complete the repairs.
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