Robert Wark, R.N.M. Nixon
Sediment inflows to Lake Argyle, the reservoir formed by the construction of the Ord River Dam, were seen as a significant threat to the Ord Irrigation Project when the scheme was being developed through the 1960s. Sediment monitoring was built into the operation of Lake Argyle when the Ord River Dam was completed in 1971. The paper describes the strategies that have been in place to assess sediment loads and monitor sediment build up in the reservoir.
Spectacular reduction in sediment flows has been achieved through developing a comprehensive catchment management program. The program commenced in the early 1960s and was adapted and modified as progress was made. The paper describes the steps taken to identify the areas of the catchment at risk, the measures implemented and the current status of the catchment.
A key feature of the catchment management program has been the willingness to critically review progress and adapt the program. A variety of sediment tracing techniques have been used to help confirm the sources of sediment in the catchment, and the paper describes these, and the broad range of results and how they have helped direct the work on catchment management.
Keywords: Sediment, monitoring, catchment management, Lake Argyle, Ord River
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The design of tailings dams under earthquake loading is quite challenging due to the nature of the tailings materials which are generally liquefiable under earthquake shaking. The design will be more complicated when the dam foundation is also liquefiable material. While assessment of liquefaction potentials is well developed in practice, assessment of liquefaction induced deformation varies from the simplest Newmark’s displacement method to the more complex effective stress dynamic analysis approach. It is generally accepted that the simplified method can be used for cases involving non-liquefiable materials. However, the use of this method for cases involving liquefaction may generally result in overly conservative designs to cater for the many simplified
assumptions in the method. With the advance of computer technology, time and cost are no longer obstacles for using the more appropriate method for estimating liquefaction-induced deformations of a tailings dams and achieving an optimum dam design.
This paper attempts to critically discuss issues in seismic design of tailings dams and provide an example of the use of the effective stress dynamic analysis method to estimate the liquefaction-induced deformations of a tailings dam and its importance in optimizing the design. The approach used is capable of estimating pore pressure response of liquefiable materials at any given time during the shaking. The effective stress analysis method used herein is embedded in FLAC software using a specially written FISH routine. Using this method, it can be demonstrated that although liquefaction is an issue, it does not necessarily mean that we must prevent its occurrence. As long as the deformation is acceptable, liquefaction is not necessarily a ‘show stopper’ for the project.
Keywords: liquefaction, seismic deformation, tailings dam design.
Eric Lesleighter, Peyman Andaroodi, Colleen Stratford
In January 2011 major flooding was experienced across a large part of Southern Queensland. The flood discharges through the Wivenhoe Dam spillway caused extensive erosion of the rock in the plunge pool. While not an issue in relation to the spillway structure’s security, the rock erosion experience was dramatic for a number of reasons. The paper presents details of the extent of erosion under head conditions that can be classed as moderate only when compared with many taller dams. The discharges over several days resulted in a pile of huge rock blocks downstream of the plunge pool.
The paper describes the plunge pool design dimensions, the geology, the hydrology of the releases, the hydraulics of the plunge pool, the surveys of the pool and rock mound, and moves on to discuss the mechanism of the fracturing and transport of the rock. Similar relevant experiences will be cross referenced, especially from details of recent experiences at the Kariba Dam and the study of remedies in the context of the dam’s actual safety.
From an actual major experience of erosion, and the sheer volume of rock that was lifted up and out of the plunge pool, the occurrence stands as a timely demonstration of what can happen in similar spillway situations, and suggests the type of awareness that spillway design needs to accommodate for energy dissipation facilities in unlined spillways plunge pool.
Keywords: Spillways, plunge pools, rock erosion, scour, plunging jets, pressure transients.
Karen Riddette, Chee Wei Tan, Alan Collins, David Ho
Due to a number of historical stilling basin slab failures around the world, modern basin slab stability assessment approaches now require allowance for hydrodynamic pressure fluctuations. Extreme fluctuations in uplift pressures have been found to occur in hydraulic jumps and plunge pools resulting in high-pressure pulses being transmitted via joints and drainage openings to the underside of the slab. If, peak uplift forces beneath the slab coincide with minimum pressure fluctuations on the top of the slab, the resulting pressure differential can be sufficient to lift a slab. As a result, simple static design based on tailwater depth and mean floor pressures is now considered highly non-conservative.
Through a case study on the Waipapa Dam spillway stilling basin, this paper examines the use of CFD modelling to compute mean hydrodynamic slab pressures taking into account the location of the hydraulic jump and the effect of the impact blocks on the pressure distribution over the slab. By combining the CFD results with empirically-derived pressure fluctuations, uplift scenarios are applied in a FEA model to compute the maximum load in the slab anchors and examine the sensitivity of the stilling basin slabs to uplift failure.
Keywords: Stilling basin, hydrodynamic modelling, CFD, pressure fluctuation, slab stability.
Richard R. Davidson, Nate Snorteland , Doug Boyer, John France
The US Army Corps of Engineers (USACE) has embarked upon a monumental journey in applying risk-informed decision making in the management of the safety of the 650 major dams for which it is responsible. This process has shifted safety criteria from fully deterministic to a probabilistic basis. There has also been a shift from de-centralized district-based decision-making to centralized management of resources through the new Risk Management Center (RMC) and the Senior Oversight Group (SOG), a group of senior engineers and managers from across the USACE organization. The risk process began about five years ago with a portfolio prioritisation using screening-level risk assessments of the entire dam inventory, culminating in Dam Safety Action Classifications (DSAC) for each of the dams. Based on this risk prioritisation, Issue Evaluation Studies (IES) were initiated for the highest risk DSAC I and II dams, with each study including detailed failure mode and risk analyses for each dam. Because the Corps was relatively new to dam safety risk analyses, and their dam design history was one of following codified manuals of practice, various risk tools were prepared to provide guidance when assessing the risk of potential static, seismic and flood failure modes, as well as life loss and economic consequences of dam failure. Although these tools provided useful guidance to a relative large population of inexperienced risk estimators, many of these early risk assessments were flawed; they provided unrealistically high estimates of failure probabilities and the tools did not help estimators understand or explain each failure mode. To assist the RMC in bringing more defensible risk estimates to the table and improve consistency of the evaluations, the Quality Control and Consistency (QCC) review process was initiated about two years ago. The QCC process provides high level review of IES activities, including detailed reviews of risk analyses, by a small group of experienced dam safety risk estimators. Not only has this brought risk estimates into a more reasonable range, it has provided valuable training for risk estimators, and important checks and balances on the risk-informed decision making process for moving dam safety upgrade projects forward. The justification for a number of very expensive projects has been challenged and, in some cases, re-prioritised, and other projects have risen to the prominence they deserve.
Chi-fai Wan, Jason Hascall, Andrew Richardson, John Sukkar
Oberon Dam is the major headwork of the Fish River Water Supply Scheme providing bulk water supply to Oberon Shire and Lithgow City Councils, Sydney Catchment Authority, and Delta Electricity. The dam is owned and operated by State Water Corporation (SWC).
Located on the Fish River 2km south of Oberon in New South Wales, Oberon Dam was completed in two stages in 1946 and 1957. In 1996 the dam was upgraded to pass the 1993 Probable Maximum Flood estimate by raising the dam 1.77m and constructing a 50m wide auxiliary spillway on the left abutment. The upgraded dam comprises a 232m long, 35.3m high concrete slab and buttress section and a 165m long earth embankment section.
A typical buttress dam has its inclined upstream face made up of relatively thin reinforced concrete slabs supported by but not integral with the buttresses, making a relatively flexible dam structure vulnerable to earthquake damage.
As buttress dams evolved from concrete gravity dams, their structural design follows the same principles as applied to gravity dams. However, many buttress dams were designed over 60 years ago using outdated methods that did not consider earthquake loads. Current overseas and local design guidelines do not provide sufficient guidance for checking the seismic stability of existing buttress dams. For instance, the simplified seismic analysis, proposed by Fenves and Chopra to investigate the seismic response of gravity dams to earthquake loads in the upstream-downstream direction, is not applicable to buttress dams which are also susceptible to damage by earthquake loads in the cross-valley direction.
SWC engaged Black & Veatch to carry out a three-dimensional finite element analysis of Oberon Dam to better understand the structural behaviour of the dam under earthquakes. The analysis used both the response spectrum and time history approaches. Due to the uncommon design of Oberon Dam and the limited discussion found in the literature on the dynamic behaviour of buttress dams, the Authors would like to share their experience in the assessment of the hazard, and on the use of modern finite element modelling techniques to investigate the dynamic response of this type of dam.
Keywords: Ambursen dams, Buttress dams, Risk assessment, Time history analysis, Finite element