The use of risk analysis for dam safety is becoming more widespread. Dam owners are increasingly aware of the need for information on current and emerging practices from the world scene to determine how to proceed with the use of risk assessment. The paper summarises the findings of a Churchill Fellowship study into risk assessment for dam safety management. Dam owners, regulators and consultants from the United Kingdom, France, the Netherlands, Norway, Sweden, the United States and Canada were consulted for the study. Conclusions are drawn from the findings and recommendations made for future development.
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David Dole and Brian Haisman
The $75 million remedial works at Hume Dam on the Murray River near Albury have been of national significance. The rehabilitation program associated with the structure itself and with its appurtenant works is now in the final steps of construction. The authors summarise this program with an emphasis not on technical details, but on decision processes. Equity in this dam is, in effect, held in equal parts by three State governments and by the Commonwealth government. At the same time, in response to the national water reform agenda, the governments have agreed upon new cost sharing arrangements that more nearly reflect the value of services to each government. The particular problems of decision-making within this evolving inter-State environment are discussed.
Lessons from experiments with application of risk analysis are discussed. Finally, the matter of adequacy of the structure for extreme floods is still under consideration. Hume Dam will presently pass the Design Flood developed in accord with Book VI (1999) of Australian Rainfall and Runoff, and the Dam Crest Flood has been estimated to have an annual exceedance probability of 1:110,000. _ Retrofitting a spillway to pass the estimated Probable Maximum Flood will double the cost of remedial works and is estimated at 10 times the cost of similar capacity built into original construction. The authors discuss the public policy elements of this pending decision.
Steven Fox, Garry Fyfe
This paper describes some key details of the construction of the Lake Eppalock Main Embankment Remedial Works Project. This $8.25 million earthworks project was completed on a “live” storage to an accelerated program. As the dam owner Goulburn-Murray Water took the decision to directly manage the construction of these works with resultant benefits in timing, risk management and project management costs.
Jack Rynn, John Pix, Garry Grant and Albert Hill
Ground motions resulting from seismic activity can cause significant damage to existing dams. For this reason, monitoring of seismic activity is an important component of a dam safety management program. Similarly, the long term gathering of data on regional seismic activity provides a sound platform for structural adequacy checks of components of existing dams under seismic loading, as well as for future dam design. In this context, the South East Queensland Water Corporation Ltd (SEQWater) and its predecessors have been monitoring earthquake activity in relation to the Wivenhoe, Somerset and North Pine Dams in South East Queensland since 1977. In 1998, SEQWater upgraded the seismic instrumentation with a digital telemetered seismic surveillance system (DTSSS) six-station network to replace the original analogue seven-station network. This state-of-the-art instrumentation was supplied and installed by Nanometrics Inc., Canada through an international tendering process. This paper presents an overview of the DTSSS, results to date and future planning for an integrated strong ground motion accelerograph network.
Pieter van Breda, Peter Walton, Kate Lenertz and Tim Sheridan
The Warragamba Dam Auxiliary Spillway Project, designed to manage floodwaters up to a Probable Maximum Flood event, was approved by the NSW Minister for Urban Affairs and Planning on February 12, 1998. An Environmental Impact Statement prepared for this project predicted that noise, dust (suspended and deposited), blasting, vibration, water quality and revegetation would be the significant environmental issues requiring management throughout the construction phase.
The closest residents are approximately 200m from the construction activity. The works must not interfere with the operation of the Dam, which stores 80% of Sydney’s drinking water and the integrity of the existing infrastructure must be maintained at all times. The approved proposal was to emplace the 2.2Mm3 of spoil excavated to create the spillway in an area 25 ha by 20m high on top of a ridge on the left bank adjoining the Blue Mountains National Park. This created visual impact and rehabilitation challenges.
Although the contract for this project was primarily performance based, strict environmental clauses were incorporated to manage these priority issues. Noise and dust modelling were required from each pre-qualified Tenderer, to demonstrate capability of compliance with NSW Environment Protection Authority requirements. This formed part of the tender assessment. Criteria were also developed for revegetation, specifying numbers of endemic trees, shrubs and grasses per 400m2 of spoil emplacement in order to create a floral community similar to the existing adjacent National Park.
The implementation of these requirements and the development of a site Environmental Management Plan by the Sydney Catchment Authority, Australian Water Technologies and Abigroup Contractors, whilst maintaining productivity, has proven to be a working example of the benefits of Partnering.
P.W. Heinrichs and R. Fell
Ben Boyd Dam, a 29 m high earthfill embankment built in 1978, has had an unusual history. In 1979, a number of seeps developed during first filling with water 5 m below FSL indicating unexpectedly high pressures. Investigations concluded the coarse filter permeability was very low due to excess fines. Remedial works in 1982 included a drainage filter beyond the toe and a new stability berm above. New piezometers were installed, including several in the blanket filters in the existing dam. These later indicated up to 10.5 m head in isolated areas within the filter. Pump out tests partially lowered the water level in the standpipes but in 1995 the water level rose by 4 m back to its previous high level. All this during a period of relatively low rainfall. Stability analyses were carried out and further investigations in 1999 concluded that apart from general leakage from the foundation abutment into the filters, the rise in pressures was due to leakage from a riser hole from one of the nearby foundation piezometers. A potential for piping along the piezometer tubes within the dam was also identified.
This situation was managed without resort to costly capital works, because it was concluded that the pressures from the vertical riser were not a potential failure mode, and potential piping failure would be adequately handled by the existing chimney drain, intersecting the piezometer tubes trench. Any potential piping failure would also give warning signs which increased frequency of monitoring (now in place) would pick up in time to allow lowering of the storage.