James Willey, Malcolm Barker, Javad Tabatabaei
Abstract: During successive flood events from the end of construction of Googong Dam in 1978 through to the late 1980s, erosion of in excess of 5,000 m³ of rock occurred in the partially unlined section at the downstream end of the spillway channel. Remedial works were undertaken in stages during the 1980s to stabilise the eroded chute and limit further erosion. A project is currently underway as part of the Bulk Water Alliance to construct remedial works in the spillway to repair erosion damage and increase the spillway capacity to safely pass the current estimate of the Probable Maximum Flood. The design was undertaken by GHD Pty Ltd as part of a separate engagement prior to the formation of the Alliance.
The recent work involved a review of the historical performance and prediction of future performance of the structure. A process involving the development and comparison of options and ultimately the detailed design of the preferred arrangement followed, including refinement and validation using a physical hydraulic model study.
This paper presents risk assessment techniques used throughout the project on a range of tasks including prediction of future spillway erosion damage and comparison of spillway remedial works options, assessment of construction flood risk and definition of instrumentation requirements for the dam and associated structures.
Keywords: risk assessment, remedial works, spillway erosion, rock erosion, construction risk, instrumentation.
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Richard John Kenny
Abstract: Physical hydraulic models have traditionally provided the input to spillway design.
However, the application of Computational Fluid Dynamics (CFD) is increasing in popularity.
The reliability of CFD depends on several complex issues including the physics of the flow regime, the mathematical formulae describing the fluid flow and the solution method. CFD reliability should be confirmed for each particular application, including ogee spillways (common discharge structures on dams).
A literature review has been undertaken to determine validation techniques and the reliability of CFD related to ogee spillways. There is a great deal of information available about the aeronautical, marine, automotive and mining industries but relatively little available about ogee spillways. The degree of validation for ogee spillways was found to vary significantly and raises the question of whether better validation can be achieved. Better validation may improve confidence in CFD techniques.
A project has been undertaken through Queensland University of Technology to model a prototype scale notional 2D ogee spillway using FLUENT CFD software under steady state conditions. The project tests the sensitivity of the CFD results to various parameters. The sensitivity of the inlet mass flow rate was tested for several CFD parameters. The investigation considered:
1.The grid spacing required for geometric convergence;
2.The effect of domain extent on the computational results;
3.The influence of a boundary layer and wall roughness;
4.The importance of the choice of turbulence model; and
5.The impact of inlet turbulence assumptions.
The recommendations of the sensitivity investigations were used for validation against the United States Army Corp of Engineers (USACE) design data.
Excellent agreement was found between the CFD predictions and the USACE values.
Keywords: Computational fluid dynamics (CFD), dam, flood routing, fluent, Navier-Stokes equations, Numerical/computational modelling, Ogee spillway crest, physical model, turbulence.
David Brett, Ben Hanslow. Rob Longey
Abstract: Mine tailings storages are among the largest man made structures in the world and often pose a considerable risk to the aquatic environment due to the nature of the stored materials. In particular, sulphide minerals are prone to oxidation when brought into contact with air and water. This leads to the formation of acidic conditions within the storages leading to dissolution of toxic metals, with seepage from these structures being known as Acid Rock Drainage (ARD). ARD is responsible for pollution of natural waterways in many areas of the world with some significant examples in Australia. Current practice in the mining industry is to attempt to exclude oxygen or water from tailings storages in order to prevent the oxidation process taking place. This involves capping of the storages with sophisticated soil covers or, where sufficient water is available, leaving a permanent water cover.
Mining operations have a relatively short life, usually around 5 to 10 years, although some can operate for over 100 years, as has the Mount Lyell Mine. Normal practice has been for companies to relinquish the mining lease on the cessation of mining, however governments are now realising the extent of liability involved with the “ownership” of large waste storage facilities. Bonds are placed by mining companies during operations, intended to cover the cost of “closure” of the mine. Often the major item covered by the bond is for the “closure” of the tailings storage facility. Following “closure”, the intention is that ownership of the lease, including waste storages, reverts to the State. State governments are now more aware of the potential liabilities in accepting the relinquishment of these leases and need to address the issues of their long-term management.
In Tasmania, Dam Safety legislation covers both water and also soil covered tailings storages, with the legislation requiring each type to meet various ANCOLD guidelines. In other jurisdictions this could well also be the case through common law requirements to meet common best practice. However, the current ANCOLD Guidelines are generally written around water storage dams and interpretation to include a waste storage facility is often not straightforward. As an example a tailings dam during operations with a water storage component is clearly a dam. Due to the environmental impact of failure it could well have a Significant or High-C hazard rating, which would require design for extreme floods and earthquakes. After closure, with say a soil cover and water diverted away, is it still a dam within an ANCOLD definition? Are ANCOLD guidelines relevant? The current ANCOLD (1998) Guidelines on Tailings Dam Design, Construction and Operation does not give specific guidance on these issues.
This paper explores these questions and suggests ways that ANCOLD could provide assistance with more guidance on the long term management aspect of tailings storages to assist designers, owners and regulators consider the closure phase.
Keywords: tailings, acid drainage, mine closure
Chris Topham, Paul Southcott, Tim Cubit
Abstract: Dee Dam is a 15 m high and 270 m long central core earth and rockfill dam on the upper reaches of Hydro Tasmania’s Tungatinah Power Scheme. The dam is assessed to have a High A hazard category. Hydro Tasmania’s portfolio risk assessment found that a risk based upgrade was warranted to protect against both piping and flood overtopping failure modes.
A $4.2M modernisation project was implemented in 2008 comprising the installation of a full height downstream filter with rockfill buttress, repairs to cracking in the diversion conduit and raising of the core for improved flood capacity. Lowering of the Dee Lagoon to facilitate full height excavation of the downstream shoulder of the dam was impractical to Hydro Tasmania for production, environment, cost and stakeholder reasons. Hence, careful analysis and risk management was required to ensure the safety of the dam during the construction of the above works against a full storage.
This paper presents the risk objective for the upgrade work, modelling and analysis undertaken to assess dam safety during the works. A wide range of construction risk mitigation measures were employed prior to and during the upgrade works. The response to latent conditions and potential dam safety incidents that occurred during construction are described. The continuous adaptation of the construction methodology to suit site conditions encountered during the project is also presented. The project was successfully completed in June 2008. The approach of detailed investigations and design coupled with a strong risk based approach during the construction proved to be effective in managing the dam safety risks of construction work with a full reservoir.
Keywords: earth and rockfill dam, filters, construction risk mitigation, dam safety, dam safety incidents.
The author has been involved in the design of a number of dams and spillways and specifically the design of several ACT lakes and ponds working either for the development authorities or as a consultant to them. This paper, a memoire, describes seven innovative spillways utilised in the ACT, five of which he was directly involved with. The use of dual spillways has been a common feature of the designs and this has been a very economic approach. As well as some use of fuseplug spillways, a labyrinth spillway and the safety of embankment overtopping, this paper may provide some useful ideas for new developments.
Rick W Schultz PE
Abstract: Dams serve many purposes: flood control, fresh water intakes for municipalities/power plants, navigation, and recreation. Throughout history risk of catastrophic failure of dams has always been an issue which engineers have had to deal with. The potential for loss of life and property damage of residents living and working downstream of dams continues to grow as more and more development occurs downstream of dams.
The U.S. Army Corps of Engineers is embarking on a complete dam safety evaluation program which encompasses over 600 of its projects throughout the United States. This program is used to evaluate the risk and reliability of dams and will assist in making decisions for budgeting and repairs of dam projects throughout the Corps.
Corps of Engineers dams have a wide variety of ages, environments, structural and mechanical designs, and geological conditions.
This paper will discuss the condition assessment tools which are being developed to determine the reliability/probability of failure of the mechanical and electrical components and systems on dams. These tools, when combined with other tools such as geotechnical, structural, and hydraulic, are used to give an overall reliability of a dam.
Keywords: probability of failure, Weibull formula, characteristic life of mechanical/electrical components shape parameters, fault trees.