P.I. HILL, R.J. NATHAN, P.E. WEINMANN, J.H. GREEN
The assessment of flood risk is important to the safe design, maintenance and operation of dams. Traditionally, a standards-based approach has been adopted, in which the adequacy of a spillway was assessed by its ability to pass the whole, or a specified fraction, of the Probable Maximum Flood (PMF). More recently, however, the ANCOLD Position Paper on Guidelines for Selection of Acceptable Flood Capacity for Dams has moved towards a risk-based approach, in which attention is focused on establishing the exceedance probability of the maximum flood that can be safely passed by the spillway.
The move to a risk-based approach has led to an increasing focus on the derivation of floods with very low probabilities of exceedance. The chapter in Australian Rainfall and Runoff that gives guidance on the estimation of extreme floods has recently been revised and issued as Book VI. The new guidelines reflect the move from a standards, to a risk based approach and also include recent developments in extreme flood estimation. These recent developments result in an improved estimate of floods in the large to extreme range and hence a more reliable estimate of hydrologic risk.
This paper illustrates the impacts of the new flood guidelines by summarising the results for 7 dams in Southeastern Australia. For the examples presented in this paper the impact of the new guidelines is to reduce the estimated hydrologic risk. The new guidelines have an important effect on the estimation of hydrologic risk and therefore the assessment and management of dams in Australia.
R J Westmore and P J Cummins
Wartook Reservoir is owned and operated by the Wimmera Mallee Rural Water Authority in western Victoria. The reservoir was constructed in the period 1887 to 1890 on the Mackenzie River within the Grampians National Park. It has a capacity of 29400 ML, is the sole supply of water to the City of Horsham, and also supplies stock, domestic and irrigation water to the Wimmera and Mallee Regions of Victoria.
The embankment is 1100 m long, 12 m high and is constructed of loose to medium density silty fine sands which are susceptible to liquefaction during a seismic event due to the combination of high pore water pressures and low density. Active seepage from the embankment and foundations render the embankment susceptible to failure by piping.
The outlet works were constructed of sandstone masonry and comprise a tower and cut-and- cover conduit buried within the embankment. Inflow of fine sands from the embankment into the masonry tunnel render the embankment susceptible to failure by piping through the joints in the masonry tunnel.
Design concepts for the rehabilitation of the embankment, outlet and spillways have been developed jointly between Wimmera Mallee Water and SMEC Victoria adopting a risk based approach. The design involves partial rehabilitation of the works, providing acceptable levels of risk to the Authority and community, at an economically justifiable cost.
David Watson and John Adem
For several years risk management has been promoted by the Victorian Department of Natural Resources and Environment – Water Agencies as the key mechanism for the effective and efficient business management of dams. As part of an extensive water reform program, the Victorian Government announced in October 1997, a financial assistance package for the water industry which included funding for dam improvements covering a majority of large dam owners in the State. One of the conditions for receipt of these improvement funds was the need for each water authority to undertake a Business Risk Assessment of all significant and high hazard dams under its responsibility.
This paper discusses the Business Risk Assessment document based on a framework developed by Water Agencies after consultation with the industry and expands on the following reasons why the document was produced:
Richard I Herweynen
For concrete gravity dams, when the foundation’s value of cohesion is low, it is very difficult to meet the sliding criteria proposed by ANCOLD. Low cohesion is generally associated with serious foundation defects. This was the case for Meadowbank Dam, with a foundation having persistent horizontal seams containing material of a clayey silt size classification. By adopting the ANCOLD strength reduction factors, it was found that a large number of ground anchors would be required to meet the ANCOLD sliding criteria. During original design, extensive laboratory and insitu testing was performed on the seam material. This paper proposes a methodology for arriving at less severe strength reduction factors based upon a statistical analysis of the strength parameters measured in the Meadowbank Dam foundation.
Additionally, a probabilistic approach using a Monte Carlo simulation is used to give further weight to this argument. This paper concludes that the probability of Meadowbank Dam failing due to sliding is very low and within acceptable limits.
A dam owner is often surprised to learn that his dam has been listed on a heritage register. This is often the first indication that the dam has heritage significance.
This paper discusses the different types of heritage listing and what the implications are for an owner. It suggests that a prudent owner will find out whether he needs a heritage conservation plan, particularly where redevelopment or remedial work at the dam is contemplated. The paper describes the content of a typical conservation plan for a large dam and how it is implemented.
Steve Everitt, Ron Fleming, Lelio Mejia
The Electricity Corporation of New Zealand Ltd (ECNZ) is strengthening its Matahina Dam which is an 80 m high, 400 m long rockfill dam impounding a 60 million cubic metre reservoir. The strengthening is to ensure the dam will withstand potential fault displacement within the dam foundation.
ECNZ’s management of the project is described from the design and consents phase through to construction. Key issues are discussed which have contributed to the success of the project such as management structure, the International Review Board, the design process and risk management.