Peter F Foster and Peter K Silvester
Clyde Dam, the largest concrete gravity dam in New Zealand, was constructed in the 1980’s on the Clutha River in New Zealand. Lake Dunstan, which is the reservoir formed by the dam, reached its full operating level in 1993, some 21 years ago.
This paper summarises the performance of the dam over this period, the changes in operations that have been undertaken and looks to future challenges. The performance and management of the landslides around Lake Dunstan that were remediated prior to lake filling is outlined. The large floods experienced in the Clutha River in the 1990’s highlighted aspects of the flood management procedures that needed amending to capture lessons learned and some modifications to appurtenant structures have been completed. Changes to the environmental management in moving from water rights to consent conditions under the Resource Management Act are addressed.
Over the last 21 years a sediment delta has progressed down Lake Dunstan, as expected, and a long term sediment management plan has been developed for both Lake Dunstan and Lake Roxburgh which is downstream of Clyde Dam. A summary of the plan is discussed. The seismic hazard at the dam site is currently under study to update the seismic assessment parameters for the dam.
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Leonard Wiliem, Rob Keogh, and David Thomas
Progressive rope creep on the steel ropes which hold 14 counterweights in tension on the seven spillway gates was monitored regularly. The 2011 annual inspection identified that the creep had taken the lower guide wheels of the suspended counterweights beyond the extent of the wheel guides.
A programed project to extend the guides was delayed due to Workplace Health and Safety concerns on confined access and working under a suspended load. A study was commissioned to deliver a safe method of extending the guides. Because regular testing and two flood events had proved the gates were functioning well, the risk of failure in gate operation during flood event was considered low and a lower priority was assigned to rectification work.
Callide Dam is a SunWater owned dam located in Central Queensland. It has a similar spillway gate mechanism as Coolmunda Dam. The only difference is that Callide Dam gates work in pairs with one counterweight attached to each gate.
In January 2013 due to heavy rainfall caused by the ex-cyclone Oswald, Callide Dam experienced a flood event which triggered a gate operation. During the draining phase, the gates operated abnormally sustaining damage to the structure and to the automatic gate opening mechanism. SunWater undertook investigations to identify the cause of the abnormal operation and found that the primary cause of the gate abnormal operation was due to jamming of the suspended counterweight on the end of the guides. This was due to cable stretched over 26 years of service to the extent that the lower wheel assembly was beyond the guide rails at the time of the flood event.
The event at Callide Dam was a wake up call for SunWater to re-evaluate the risk assessment for Coolmunda Dam. This re-evaluation recommended to assign the highest priority on the rectification of the wire rope creep issue on the radial gate as the risk of failure in gate operation during flood event was high.
This paper discusses the actions in re-evaluating the risks at SunWater’s Coolmunda Dam and the measures taken to quickly undertake remedial action on both dams and the challenges involved with each.
Stephen Newman, Rod Jacobs, and Dr John Yeates
Independence Group (IGO) is assessing the feasibility of re-commissioning a closed copper-zinc mine in Victoria. Due to the acid producing potential of the mine tailings if exposed to oxygen they are to be contained in a saturated condition not only during the life of the mine but well beyond closure and effectively in perpetuity. The tailings are to be stored in a saturated condition underground in the mining void however due to the limited volume available approximately half of the tailings produced over the mine life will require containment in a purpose built surface Tailings Storage Facility that would need to perform as a water retaining structure.
This paper describes key challenges with tailings management including demonstrating the viability of maintaining permanent saturation of the tailings and the long term integrity of the structure. Excessive poor quality seepage, piping and other failure modes have also been considered in the long term design of the closed Tailings Storage Facility. A surveillance program to provide early identification of potential issues has also been developed.
The design is consistent with ANCOLD guidelines and used a risk based approach to assess key issues associated with the extended design life.
Peter Hill, David Stephens, Kelly Maslin, Rachel Brown, Simon Lang, and Chriselyn Meneses
There has been a growing awareness of the potential dam safety risks associated with hydraulic structures in urban environments such as retarding basins, water quality detention basins and recreational lakes. This has required estimates of rare and extreme floods for urban catchments and there are a number of important characteristics of urban catchments which distinguish them from rural catchments such as impervious areas, lack of streamflow data, blockage of structures and complex hydraulics. This paper describes the key considerations for flood estimation in urban catchments and draws examples from a number of current flood studies for urban catchments in Canberra.
Suraj Neupane, Paul Southcott, and Tung Hoang
Conglomerate Dam has multiple cracks along the asbestos cement outlet conduits running through the embankment. The reservoir level has been maintained at 2m below the full supply level to reduce the amount of seepage, emerging on the downstream face, until the conduits are repaired and protect the embankment from slope instability and piping. Several methods were investigated under an options study to determine the most suitable internal lining method. Slip lining with polyethylene pipe was found to be the most suitable method in terms of technology as well as cost.
Shane McGrath, Phillip Cummins, and David Stewart
Dam owners and regulators now commonly use risk assessment techniques to assist with decision making for an individual dam or a portfolio of dams. In many cases risk assessment is used to select an optimal course of action in relation to ongoing safety performance of dams, including the achievement of public safety objectives. However, whilst it is an important tool, the use of risk assessment alone is not sufficient to establish that a dam is “safe”.
In modern organisations, business objectives are achieved through a systematic approach to management which described simply sets out what needs to be achieved, how the required outcomes will be delivered and audits the process and results.
In hazardous industries such as mining, chemical, nuclear and dams, it is necessary to reliably achieve business objectives such as product volumes, unit costs and workplace health and safety alongside public safety objectives. In the dams industry, dam safety management systems are now being implemented to document how the organisation satisfies its corporate and business objectives, governance responsibilities and risk management processes.
It is also common in hazardous industries that a “safety case” is required by regulators to demonstrate that the owner has identified what could go wrong at its facility, what controls are in place and that there is a system in place to ensure that the controls are reliable. Whilst dam owners may rely on a dam safety risk assessment to meet regulatory obligations and demonstrate due diligence, the results of risk assessments are not routinely documented sufficiently to satisfy a “safety case” and therefore will not fully meet the organisation’s requirements.
Many dam owners are also responsible for the safety management of other hazardous facilities, such as urban water and mining corporations which typically manage hazardous chemical installations and hazardous or toxic waste disposal. For such organisations, the corporate awareness and processes should already exist to extend the “safety case” philosophy to the management of their dams.
This paper sets out the importance of a dam “safety case”, the essential elements of a safety case and its relationship to the dam safety management system.