P. H. Southcott, R. Herweynen and R. Fell
Hydro Tasmania is in the process of undertaking a Portfolio Risk Assessment of its 54 referable dams, of which 14 are concrete faced rockfill dams. One of the potential failure modes identified during the study so far is a concentrated leak developing in the face slab or joints of the slab, leading to failure of the dam. Current methodologies for assessment of piping failures through embankment dams are considered inadequate for this failure mode. This paper discusses an event tree methodology developed from the work of Foster and Fell (1999) and Foster et al (2001) to address this failure mode. The key aspect of this method is identifying the factors that influence the likelihood of initiating a concentrated leak through the perimetric, vertical and crest wall joints and through the face slab concrete. It is concluded that for the vast majority of well designed and constructed concrete faced rockfill dams that a concentrated leak leading to failure is very unlikely.
— OR —
Tim Waldron, K D Murray and Allan Crichton
The City of Hervey Bay is a growing tourist community that is located a comfortable 3½ hour drive north of Brisbane. To meet the growing water demands of the community, Wide Bay Water Corporation required the raising of its sole water supply – Lenthalls Dam.
At the time of the option study, Queensland dam owners were aware of their obligation to manage their dams to minimise adverse environmental impacts but detailed Environmental Flow Objectives were still being developed.
This required a solution for the raising of Lenthalls Dam that provided maximum flexibility while, at the same time, being cost effective.
A range of solutions and new technologies were investigated. Using a Risk Management methodology, the Crest Gate system developed in South Africa was adopted.
Subsequently, draft Environmental Flow Objectives have been set and the use of a gated system has been beneficial in meeting post-winter flow objectives.
Bill Hakin, Phillip Solomon, Peter Siers Bruce Goddard
Lyell Dam is located on the Coxs River near Lithgow NSW Australia. It was constructed in 1982 to supply cooling water to Delta Electricity’s Mt. Piper and Wallerawang power stations.
In 1994 the storage capacity of the dam was increased by 7,500 Ml by raising the embankment height and installing two 3.5m high inflatable rubber dams on an enlarged and slightly raised spillway sill. Two significant failures of the rubber dams in 1997 and 1999, led the dam owner to seek an alternative method of maintaining the increased Full Supply Level (FSL) whilst still providing spillway capacity for the design flood. Although the lost storage has a certain strategic value to Delta Electricity, the main reason for restoring the capacity to its former level was to preserve the environmental and recreational use of the reservoir for the local community.
Following a detailed review of options, Delta Electricity chose to regain the former FSL with the Hydroplus Fusegate System. Because of the freeboard available at Lyell dam it was possible to design the Fusegates such that none tip before the 20 000 AEP flood.
In order to derive accurate as-built levels and dimensions of the existing spillway, new laser scanning methods were utilised to create a digital 3-D model of its complex shape.
The water retaining concrete Fusegates were poured in-situ and designed without anti-crack reinforcement. This innovation was only possible by use of a special design mix and careful temperature control/monitoring during concrete placing.
This is the first installation of the Hydroplus Fusegate System in Australia. The paper examines the philosophy of approach and various unique methods used in the application of the System during the design and construction stages.
D. J. Dole, D. Dreverman and A. J. McLeod
The Murray-Darling Basin Commission is embarking on an ambitious project directed towards repairing the environmental damage to the River Murray, caused by a century of human intervention. Today the River Murray is one of the more highly regulated rivers in the world, with only a 27% natural annual median flow to the sea.
In April 2002 the Murray-Darling Basin Ministerial Council approved, in–principle, a program of structural works from Dartmouth Dam to the Murray Mouth, including the lower Darling downstream from the Menindee Lakes. The initial phase is estimated to cost $150 million over 7 years. At the same time the Council has authorised studies of the environmental, social and economic impacts of 3 scenarios involving recovery of 350 GL, 750 GL and 1500 GL per year from existing uses, for reallocation to the environment.
This paper describes some of the key projects in the portfolio of works under consideration, including:
The paper also outlines the extensive stakeholder consultation and community engagement processes which are fundamental to the success of the project, as well as the various means adopted to enhance the links between scientists and engineers involved in the project.
Chas Keys and Steve Opper
As the legislated ‘combat agency’ for dealing with floods, the NSW State Emergency Service has had considerable experience in planning for flooding on the state’s rivers and in developing arrangements to help keep people safe when floods occur. This experience has been put to use over the past decade in the particular context of managing floods caused or exacerbated by dam failure. Some of the complexities of the dam-failure planning problem are explored in this paper, specifically as they relate to warning and evacuation tasks and to the issue of preparing communities for the extreme flooding which dam failure can be expected to cause. The points are made that warning is not just about mechanical alerting devices, evacuation is not restricted to commanding people to move, and public education requires a sensitive comprehension of the problems of disseminating information about rare and difficult-to-believe events.
Tom Ryan, Charles Todd and Simon Nicol
The potential impacts of cold water releases on the downstream thermal regime include: reducing the seasonal temperature range (lowering of the maximum and raising the minimum); reducing the diurnal temperature variation; rapid temperature changes; and delaying the seasonal warming of stream temperatures. Recent investigations have identified at least 20 large dams within Victoria, that have the potential to release cold water from below the hypolimnium. A monitoring program is currently being implemented in Victoria to identify the occurrence of cold water releases and to estimate the extent of the downstream impacts.
Cold water releases have been shown to impact the biological processes within aquatic ecosystems and consequently reduce the natural productivity. The physiological development of native freshwater fish can be impacted in a number of ways. Growth and reproductive development of adult fish is impacted while the survival of eggs and larvae can also be retarded. As a result, the sustainability and viability of native fish populations are greatly compromised.
Using stream temperature data from the Mitta Mitta River downstream of Dartmouth Dam, the decline of the native fish populations, due to cold water releases, can be demonstrated under current operating conditions. The decline in population numbers can be further demonstrated with the use of a simple age-based population model for Murray Cod. The spawning opportunity and survival of egg and larvae can be improved for Murray Cod by increasing the overall spring release temperatures by 2, 4, 6 and 8 oC. The population model adjusted for these thermal improvements, results in increased survival prospects for the Murray Cod population.