Tim Griggs and Richard Herweynen
The river diversion is an important aspect to be considered in the design of a dam. It generally consists of an upstream cofferdam, river diversion conduit and downstream cofferdam and allows the dam to be constructed in a dry section of river.
This paper reviews the diversion design adopted at three recent Australian roller compacted concrete (RCC) dams and comments on the effectiveness of the design in providing risk mitigation during the construction of each of these dams. The dams considered are Paradise Dam (2005), Meander Dam (2007) and Wyaralong Dam (2011).
Rather than selecting an arbitrary design flood for the diversion, a risk-based assessment was used that generally resulted in a relatively low design capacity. Even though there were cases where the diversion capacity was exceeded, it is considered that the risk based design process provided an economical diversion design for these recent Australian dams.
Keywords: Diversion, roller compacted concrete dam, RCC.
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Shane McGrath, Andrew Reynolds, Garry Fyfe, Chris Kelly, Steven Fox
Goulburn-Murray Water is a rural water corporation located in Northern Victoria. It has responsibility for 12 State dams and is also the constructing authority for the Murray Darling Basin Authority’s Victorian assets.
Over the past 15 years G-MW has been engaged in a dam improvement program across its portfolio. To date 14 individual projects have been undertaken at 11 dams. The total expenditure is $125 million.
Starting from a base level of data at its inception in 1997, the program has encompassed all facets required for a dam improvement program. From early prioritisation to set the investigation program, through design reviews and risk assessments to develop the upgrading program and subsequent implementation. Some elements of the program were at the leading edge of practice at the time and a range of experiences along the way were character building as dam safety investment challenged other corporate priorities.
This paper sets out the lessons learned in developing the methodology and implementing the program of works, particularly relating to corporate adoption of the program, organisational capability, investigations, risk assessments, design and implementation.
Gavan Hunter, Robin Fell, Chris Topham
Backward erosion piping is a failure mode that can affect water retaining structures with earthen cores of very low or no plasticity. Backward erosion involves the progressive detachment of soil particles as seepage through a core material exits to a free surface or unfiltered zone. In contrast to other piping failure mechanisms, backward erosion does not require a defect to be present for initiation, and is heavily influenced by the inherent characteristics of the core materials and the available hydraulic head. For dams with non-plastic or very low plasticity core materials, backward erosion can be a material contributor to the overall piping risk and warrants careful consideration during quantitative risk assessments of such dams. However, there is very little published literature for evaluating the potential for backward erosion piping, particularly in broadly graded soils. This paper concerns one such dam where backward erosion of the glacial till core needed to be assessed in the context of a detailed risk assessment for the facility. The backward erosion mechanism was tested in laboratory tests set up to model the situation in the core of the dam at a range of hydraulic heads. The paper describes the core material and objectives for the testing, presents the apparatus used, summarises the findings, and explains how they contributed to the risk assessment for the dam. Recommendations are also made for future similar testing and research needs.
Keywords: Backward erosion, piping, embankment dam, laboratory testing, quantitative risk assessment, glacial till.
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
David Stephens, Peter Hill, Rory Nathan
The estimation of incremental consequences of dam failure often requires consideration of coincident flows in downstream tributaries. In the past overly simplistic assumptions have often been adopted. Examples include an assumption that flows in downstream tributaries are negligible, equivalent to the 1 in 100 Annual Exceedance Probability (AEP) flood, the mean annual flood or the flood of record. Experience has shown that these assumptions often underestimate coincident flows, particularly for extreme events approaching the AEP of the Probable Maximum Precipitation. Additionally, the justification for adopting these techniques is usually driven by ease of use rather than the degree to which they represent the relevant physical processes at play. For some dams, these techniques may have a negligible influence on the overall consequence assessment. However, there are many dams for which an improved understanding of coincident flows using a joint probabilistic framework can result in significantly altered estimates of the natural flood and dambreak flood inundation zone. This can frequently lead to the consequences of the natural flood being larger than would otherwise have been the case, leading to a reduction in incremental consequences. Two examples of such situations are presented, including a description of the techniques used to estimate coincident flows and a discussion on likely influence of these flow estimates on incremental consequences. These examples are then used to draw some general principles for the types of dams at which an improved understanding of coincident flows is warranted.
Keywords: dam failure, coincident, joint probability, consequence assessment
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