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.
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R.I. Herweynen and A.M. Hughes
Hydro Tasmania has a number of dams which were designed and constructed in the 1950-70s
with fully grouted, post-tensioned anchors. The method used was leading edge in its day,
however, it does not achieve the cable protection of modern methods which provide two barriers
against corrosion and are monitorable. Hydro Tasmania has developed and employed an
innovative program to ascertain the integrity and remaining life of the cables and to prepare
long term management plans for its cabled dams.
An international panel was set-up to provide guidance on the overall issue, assist in developing
a sound methodology for assessing the corrosion of the anchors and advise on long-term
monitoring. To focus the efforts, Catagunya Dam was adopted as the pilot dam, as the stability
of this dam is very much dependent on the integrity of the anchors. This paper will provide a
brief overview of the project to date but will focus in detail on the main components of the
corrosion assessment of the anchors, namely:
The paper also provides a brief summary of the instrumentation installed at Catagunya Dam to
assist with the long-term monitoring of the dam.
David J. Walland, Jeanette Meighen, Catherine Beesley, Karin Xuereb
The method for estimating Probable Maximum Precipitation in areas of Australia affected by tropical storms has been revised. The method that it replaces, designed in the 1970s is considered outdated and based on limited data.
The entire Bureau rainfall record has been examined objectively for the largest rainfall events. These events have been analysed and modified to enable storm transposition across a large region. The modifications are based on local topography, moisture and location. Once the storm data is transposed to a single location it can be meaningfully compared and used to construct an upper estimate on the possible rainfall. This estimate can then be used in conjunction with information about a specific catchment in order to estimate Probable Maximum Precipitation at that location.
Pieter van Breda, Alison White, and Greg Carmody
Site works on the $150 million Warragamba Dam Auxiliary Spillway project commenced in March 1999 and were completed in June 2002. Successful interaction with the local community, to achieve an equitable outcome, has been a feature of the communications strategy for the project.
The Auxiliary Spillway is located close to the village of Warragamba, a township of approximately 2,000 residents. The closest residence is about 200 metres from the site. The EIS and subsequent planning documents identified key localised environmental impacts that the project would impose. The main concern of local residents, including a local action group, was the impact on their amenity during construction of the Auxiliary Spillway, particularly in relation to noise, vibration, dust and traffic.
The conditions of approval for the project included a range of communication activities, of which the formation of a Community Liaison Committee (CLC) with an independent Chairperson was a key component. When the membership of the CLC was established the Sydney Catchment Authority (SCA) and chairperson agreed that it needed to fully represent the local community – and therefore included community representatives from Warragamba and two nearby villages, the Chamber of Commerce, the local action group, the local school, local council, the dam owner (SCA) and the project manager (AWT P/L).
The establishment of the CLC has proven to be very successful. It has been the voice of the community, with responsibility to act on behalf of the community and to keep them informed of progress on the project. When issues arose during the construction, the CLC were briefed on the particular matter. The CLC was instrumental in resolving these community issues and has allowed this $150 million civil project to proceed without community attributed delays.
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.
N. Vitharana, G. Bell, J. Jensen and J. Sinha
When the storage was enlarged in 1971, Wyangala Dam provided a storage of 1220Gl. The original concrete gravity dam was completed in 1936 with an initial storage of 37.5Gl. The enlargement comprised the construction of a central core earth and rockfill dam utilising the existing concrete gravity as an upstream “toe” dam. At its deepest section, the toe (concrete gravity) dam is 60m high with a base length of 40m. The rockfill dam is 85m and the full supply level is at 75m. Two cylindrical reinforced concrete intake towers were constructed utilising the crest of the toe dam as their bases.
Screening level analyses commissioned by The NSW Department of Land and Water Conservation have recommended that detailed seismic assessment of the toe dam and intake towers be undertaken. In 2001, GHD Pty Ltd undertook inelastic time-history analysis using site-specific seismic loadings. Toe dam was modelled together with the rockfill dam using a 2-dimensional model. Intake towers were modelled incorporating the composite behaviour of concrete and reinforcing steel with limited concrete strains to prevent the loss of cover concrete and the buckling of longitudinal steel. Time-history analyses supplements by conventional pseudo-dynamic analysis procedures.
This paper described the constitutive modelling, structural analysis criteria, evaluation of hydrodynamic and dynamic earth pressures and the findings.