Alberto Scuero, Gabriella Vaschetti, John Cowland
Waterproofing geomembranes have been used for new construction and rehabilitation of dams since 1959. Research for underwater rehabilitation with geomembranes started at the beginning of the 1990s. The first installation was made in 1997 at Lost Creek arch dam in USA, where a SIBELON PVC geomembrane system was installed partly underwater, to restore watertightness to the upstream face. Techniques for underwater cracks/joints repair, and for staged repair, were developed and first adopted in 2002 and 2010 respectively. The paper presents through some significant case histories the range of underwater applications available today. The paper also presents a new underwater technology, the Sibelonmat®mattress, that allows water-tightening canals without reducing water flow.The Sibelonmat®can be used in embankment dams, to waterproof the upstream. face or as upstream blanket
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Paul S. Meeks
In June 2008 a young girl kayaking at a hydroelectric control dam owned by Alcan in Quebec Canada, tragically drowned when she was swept through the open spillgates. The public safety boat barrier, installed the year before, failed to prevent this accident. In June 2015, Stephen Hembree took his daughter and 7 of her friends out for a pontoon boat ride on Lake Linganore to celebrate her 16th birthday. A short time later, Mr. Hembree was dead while his daughter and her friends were be rescued by helicopter as they clung to boulders in the spillway. Contrast these incidents to one in March 2017, when the public safety boat barrier installed by Alliant Energy at Kilbourn Dam was credited with preventing the loss of life after a woman fell into the river above the dam. What went wrong in the first 2 instances and what can we learn from the third incident? What steps can dam owners take to prevent accidents like these from happening?
The first two incidents represent preventable loss of life at a dam while the third incident proves how a proactive approach to public safety results in reduced liability for dam owners and lower loss of life. In the Alcan instance, the public safety barrier installed to prevent this very scenario was instead installed in a location that doomed the girl even before she set her kayak in the water. The second instance demonstrates how a dam owners lack of risk awareness coupled with a boat owners carelessness resulted in a fatality.
Using the incidents above, this presentation, modeled after the Canadian Dam Associations Guidelines for Public Safety Around Dams, will educate owners and operators how to identify “dangerous” zones above and below dams. We will consider the effects of surface water velocity of individual survivability and barrier effectiveness. Flow-3D models will be shown to illustrate the effect of barrier alignment and velocity to increase an individual’s ability to “self-rescue”. Lastly, we will integrate within the presentation practical guidelines for the use of signage, sign size, lettering height and message consistency. The presentation will conclude by examining lessons learned in the Alcan incident and presenting how a proper public safety barrier and signage plan would be implemented.
More people have died from accidents around dams than have died from dam failures. The Canadian Dam Association published its guidelines in 2011 and the result has seen a significant reduction in fatalities and injuries as a result of recreating around Canadian Dams. The United States Society on Dams (USSD), the Association of State Dam Safety Officials (ASDSO) and the Federal Energy Regulatory Commission (FERC) all have embarked on efforts, modeled in large part around the CDA Guidelines to bring Public Safety out of the dam safety toolbox so Public Safety is viewed as a separate managed system. This is being conducted in an effort to educate and alert dam owners, operators and recreational users to hazards and risks in and around dams.
Andrew Balme, Dan Forster, Tim Logan
The MW7.8 Kaikōura earthquake on 14 November 2016, ruptured over 20 faults during the initial shaking,which lasted nearly two minutes. A complex series of fault ruptures propagated northeast for nearly 180 km from the initial rupture location. Instrumentation from dams across New Zealand shows that whilst most dams did not suffer physical damage, piezometric responses were measured in dams and their foundations. Earthquake related changes in seepage regimes are not unusual and depend on the characteristics of the ground motions,and site specific characteristics that influence how a dam and its foundation respond to ground motions. The ability to measure a piezometric response in a dam or foundation is heavily influenced by the instrumentation network and method of monitoring. Data collected during events such as the Kaikōura earthquake provides valuable information for both characterising performance of a dam during the event, and assisting future analysis such as failure mode assessments. Careful consideration must be given to the scope of installed instrumentation and the frequency of monitoring in order to provide these benefits,and the robustness of the system to ensure it adequately survives the event.
A common concern for large spillways is erosion of the receiving plunge pool and potential impacts on the stability of the dam.Devils Gate Dam is an 84m high, double curvature arch concrete dam, located in northern Tasmania and constructed between 1968 and 1970.The full 134m long crest is designed as a free-overflow spillway and spill flows impact the downstream valley sides and plunge pool below, where energy is dissipated to reduce riverbank erosion downstream.To protect foundation rock,the plunge pool and large portions of the valley sides were concrete lined with 450mm thick reinforced and anchored concrete. During spill events the area is inundated by up to 12m of tail-water.In 2016 damage to the plunge pool concrete was discovered by divers during a special inspection of the impact areas, but poor visibility limited the understanding of the extent and severity. Subsequent investigations, including detailed sonar scanning, improved the understanding but it was not until the plunge pool was fully dewatered that the full extent of the damage was quantified.The damage commenced around 35m downstream of the dam arch and consisted of approximately 330 square metres of moderately to severely eroded concrete and exposed, deformed, and in some areas completely removed reinforcing bars. The most significant feature was a penetration through the concrete up to 2.5m into the foundation rock.A number of stressed anchor heads were also damaged or destroyed.A full appreciation of the damage necessitated the decision for immediate repairs given the impending power station refurbishment (commencing January 2018) which will subject the plunge pool to nine months of constant spill.This paper outlines the diving and sonar investigations undertaken in 2016, discusses the challenging tasks of dewatering the plunge pool and gaining access through the narrow canyon, and presents the physical works to strengthen the damaged areas.It discusses the difficulty of identifying and treating such damage, and serves as a cautionary tale for other owners who have fully submerged plunge pools downstream of spillways.
Barton Maher and Michael Peel
The Queensland Bulk Water Supply Authority (Seqwater) manages up to $12 billion of bulk water supply infrastructure and the natural catchments of the region’s water supply sources to ensure a reliable, quality water supply for more than 3million consumers across the region. Seqwater was formed on 1 January 2013 through a merger of three State-owned water businesses, the SEQ Water Grid Manager, LinkWater and the former Seqwater. Seqwater delivers a safe, secure and reliable water supply to South East Queensland, as well as providing essential flood mitigation services and managing catchment health. Seqwater also provides water for irrigation to about 1,200 farmers and offers community recreation facilities enjoyed by more than 2.5 million people each year.Seqwater owns and operates 26 referable dams which fall under the dam safety regulation in Queensland, 51 weirs, and two bore fields across the region. Twelve key dams across the region supply as much as 90% of South East Queensland’s drinking water.In 2011, Seqwater engaged a consultant team of URS (now AECOM) and SKM (now Jacobs) to undertake a portfolio risk assessment of the 26 referable dams and Mount Crosby Weir. At the completion of the project in December 2013 there were 12 dams with life safety risks assessed as being above the ANCOLD and DEWS Limit of Tolerability. A $6.2 million investigation was approved in 2014 to commence planning for the recommended dam safety upgrades and reduce uncertainties in the risk assessment.This program of work was completed in late 2016. The estimated costs of the identified dam safety upgrades exceed $900 million.Confronted with such a large capital program, Seqwater has instigated a number of key actions including:-benchmarking capital investment and rates of risk reduction achieved by other dam owners through a dam owners group-developing a dam safety investment policy to provide a clear guidance on the framework for prioritising and scheduling upgrades-undertaking targeted investigations to reduce uncertainty in the risk assessments including the use of detailed consequence assessment-preparing a prioritised schedule of planned upgrades to gain endorsement from Government and the Dam Safety Regulator. This paper presents the outcomes of the Portfolio Risk Assessment and key changes to the initial risk assessment following further studies. The basis for the dam safety investment policy is presented and the proposed prioritisation tools.The impacts of the risk assessment provisions in the most recent revision of Queensland Acceptable Flood Capacity Guidelines for Water Dams are also discussed. In particular,the application of the economic criteria for determining the minimum upgrade required by the Queensland Dam Safety Regulator and its relevance to other dam owners.
Dr Andy Hughes
Tailings dams continue to undergo failures at an unacceptable rate compared to water storage dams, including failures at operations owned by high profile mining companies.Tailings dams have often a different form and method of construction than water storage dams in that tailings dams continue to be raised over time as part of the mine operations and rise to considerable heights. These failures are often the result of a combination of design, construction and operations actions that are controlled by humans and must be better coordinated and managed in the future. The consequence of failure can be widespread flows of tailings and water over the landscape and water courses. This can have extreme consequences in terms of life loss, environmental damage, social license to operate, company value, and mining industry sustainability. Therefore,it is necessary that the mining industry strive for zero failures of tailings facilities. Any additional technology and information that enables an owner of a tailings dam to be more certain of its condition and thereby reduce the risk of failure is of tremendous value to reliable tailings and mine water management.The Willowstick method uses low voltage, low amperage, and alternating electrical current to directly energise the groundwater by way of electrodes placed in wells or in contact with seepage or leaks. This approach has been successfully used to identify water flow paths through, under and around tailings dam in plan and elevation.The Willowstick technology provides additional information to supplement the geological, geotechnical and hydrological, evaluations analyses and designs, and to further improve tailings dam safety by more robust designs if necessary. This paper, using several tailings dam case studies, illustrates the procedure, findings, and the benefits of the Willowstick methodology. The findings of many Willowstick surveys range from tailings dams where the methodology has confirmed the design evaluations, to tailings dams where new groundwater and leakage flow paths were identified. In the latter case, the dam designers were able to update the designs, based on the new information,to mitigate the identified risks and to improve the overall safety of the tailings dams in accordance with the goal of zero failure.