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.
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M. Shirley, P. Hill, S. Hannon, B. Abernethy, H. Griffith and S. Gatti
There is an ever increasing focus on the impact of water resource infrastructure, and particularly dams, on downstream hydrology and hence ecology. Over the last few years this focus has led to the rapd development in the philosophy and techniques for estimating the requirements of water dependent ecosystems.
This paper outlines the application of a new framework for estimating environmental water requirements which results in a range of flows, rather than a single recommended flow. Furthermore, a range of strategies for providing this water to the environment will be explored.
The paper uses the current environmental flows study on the Onkaparinga River Catchment for the Onkaparinga Catchment Water Management Board to illustrate the issues and application of the methodology. The natural hydrology of this catchment has been impacted by pumping of water from the River Murray, a major dam (Mt Bold) and the diversion of flow at Clarendon Weir. This substantial multi-disciplinary study over 3 years is estimating environmental water requirements and the strategies for providing this water to the environment.
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.
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.
Mike Taylor, Paul Maisano and Rod Conway
Daylesford Dam forms an ornamental lake, known locally as Lake Daylesford, situated on Wombat Creek within the heart of Daylesford in Victoria. It is a focus of the local tourism industry and is vitally important to the Daylesford community as a recreational, social and environmental asset, with important heritage value.
On 24 October 2000, the 12m high embankment was overtopped following heavy rainfall and was in danger of breaching. This could have resulted in loss of the dam and lake, downstream damage to roads and the environment and possible loss of life. The overtopping of the dam prompted the Hepburn Shire Council, land manager for the dam, to initiate a safety review of the dam as well as the commissioning of a Dam Surveillance Program and a Dam Safety Emergency Plan.
The spillway is of the side-channel type with a 30m long concrete sill at the entrance discharging into a 5m wide unlined trough and chute. The existing spillway can only accommodate a peak flow of 24m3/s, which represents an AEP of less than 1 in 20. The required flood capacity in terms of the latest ANCOLD guidelines on spillway adequacy is for an AEP of 1 in 1 000 which equates to 120m3/s.
Following discussions with Hepburn Shire Council, and an evaluation of public usage of the Lake Daylesford area, it was assessed that the following constraints apply when considering options for increasing spillway capacity:
The proposed solution includes the following:
R.J. Nathan, P.E. Weinmann and P.I. Hill
Current practice for estimation of design floods is typically based on the “design event” approach, in which all parameters other than rainfall are input as fixed, single values. Considerable effort is made to ensure that the single values of the adopted parameters are “AEP-neutral”, that is, they are selected with the objective of ensuring that the resulting flood has the same annual exceedance probability as its causative rainfall. While this approach represents current best practice in Australia (and overseas), it does suffer from a number of limitations.
This paper describes the development and application of a Monte Carlo (or joint probability) framework which offer an alternative to the design event method. This technique recognises that any design flood characteristics (e.g. peakflow) could result from a variety of combinations of flood producing factors, rather than from a single combination. The approach mimics “mother nature” in that the influence of all probability distributed inputs are explicitly considered, thereby providing a more realistic representation of the flood generation processes.
The advantages of the technique are illustrated by application to a hypothetical dam located on a real catchment. The manner in which standard design inputs are incorporated are discussed, as is the relationship of the approach to current guidelines.