Guidelines on Selection of Acceptable Flood Capacity for Dams (2000)
The application of risk assessment involves a major philosophical addition to, and enhancement of the deterministic standards developed over many years for appropriate safety of dams. These guidelines are issued to provide more appropriate and consistent guidance within a risk process, for dam safety evaluation under floods. In applying them, it must be remembered that they are guidelines only to what is considered current acceptable practice and allow owners and practitioners flexibility to exercise professional judgment in all aspects. Indeed, without the application of such judgment, the procedures themselves could lead to results that have serious shortcomings.
This is a rapidly developing area with ongoing research in many related aspects. In view of this ANCOLD will review the guidelines periodically and, to that end, seek comments from users at every opportunity.
ANCOLD published its first Guidelines for Design of Dams for Earthquake in 1998. The Guidelines were prepared to bring together knowledge about earthquakes in Australia following the devastating
Newcastle (1989 magnitude Mw 5.4) and the Tennant Creek (1988 magnitude Mw 6.6) earthquakes, and improved analytical methods to predict the behaviour of dams subject to earthquake.
When the 1998 Guidelines were issued, it was recognised that over time there would be improved data and tools to help the designer. This has indeed been the case and ANCOLD decided that it would be timely to update its Guidelines to incorporate the significant advances made in the understanding of earthquakes, seismic hazard assessments, analysis and design.
The Working Group convened to produce these updated Guidelines, replacing the 1998 Guidelines, was composed of representatives from dam owners, State dam safety regulatory agencies and private consulting practices. The draft Guidelines were made available for comment by ANCOLD members and international review of the Guidelines was undertaken by eminent practitioners in the subject matter from the United States, New Zealand and Switzerland.
Because of the seismic hazard uncertainty and the associated structural response, these Guidelines
encourage the use of risk-based methods for assessing existing dams and for the design of new dams. However, the deterministic approach is also covered for those owners who prefer to use it.
This guideline is the culmination of a great deal of voluntary work by convenor, Mr Steve O’Brien and his working group. It is a significant development for dam engineering in Australia and will be a valuable resource.
As with all ANCOLD Guidelines, this guideline is not a design code or standard and has been produced for the guidance of experienced practitioners who are required to apply their own professional skill and judgement in its application. Users must keep abreast of developments in the design of dams and appurtenant structures for earthquake and take those developments into account when using these Guidelines.
The Guidelines will again be reviewed when knowledge and practice have developed to a point when an update is required. Accordingly, ANCOLD welcomes comments from users and other interested parties.
Shane McGrathLearn more
Chairman of ANCOLD Inc.
Within about ten years of the 1991 edition of the ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams being published the Limit State design approach it proposed fell out of favour with Australian dam engineers. The ANCOLD Executive recognised this situation and resolved that the Guidelines be revised to reflect the preferred design approach amongst Australian dam engineers. A Working Group and a separate Expert Review Panel were formed in 2005 to review the Guidelines with the intention of preparing a document that was not only more in line with the current thinking but would also be widely adopted and used by practicing dam engineers.
It has not been an easy task for the Working Group and the Expert Review Panel. Although the working stress method, with associated Factors of Safety, approach was agreed amongst the team, it took considerable time and effort to reach consensus on the details, in particular chapters four through to six. The issue that took most effort to resolve was how the Factors of Safety relate to the level of certainty of the inputs, including knowledge of the foundations, loads, strength of materials and assumed mechanically feasible failure surfaces. The notes to Table 6.4.1 and the discussion in Appendix A highlight this issue.
One thing the drafting team agreed on early, and unanimously, was the importance of the concrete gravity dam design team having a thorough understanding of the foundation conditions, the development of a detailed geological model of the foundations and an understanding of the kinematically feasible failure mechanisms. In keeping with this principle, this revision of the guidelines has the section on foundations at the front of the document. There is a strong recommendation that the design team has expertise covering foundation geology and rock mechanics as well as the dam engineer.
These Guidelines provide a basis for design of concrete gravity dams in most situations. However, they require that the user will be a professional dam engineer with significant experience who is able to use sound engineering judgement in the application of the guidelines. It is recommended that less experienced dam engineers using these guidelines do so under the advice and guidance of an experienced dam engineer. It is the dam engineer who is responsible for the design.
On behalf of ANCOLD I would like to thank the members of the working group and all the other contributors that persevered in developing and bringing these Guidelines to fruition. They will be of great assistance to dam engineering professionals both within Australia and internationally.Ian Landon-Jones Chairman, ANCOLD Learn more
The Watching Brief Guidelines for Dam Instrumentation and Monitoring Systems (1983), established following the Dam Instrumentation Workshop in Sydney, continues to scan for information and topical developments related to the monitoring of dams using a range of instrumentation techniques. A position paper outlining the current status for the Executive is planned for 2013 and will provide a mechanism to move forward on the need for an updated guideline.
Members wishing to contribute papers, views, or information to the watching brief should contact Chris Topham via the ANCOLD secretariat [email protected]ancold.org.auLearn more
In Australia we have a very diverse portfolio of dams, some regarded as being designed and constructed before there was a good understanding of the loading conditions that could apply. Sometimes there is limited knowledge of the actual details of construction and always there is a desire to ensure that the community’s resources are properly applied. The difficulty facing those who allocate funds for the benefit of the community measure the cost against the benefit achieved and to compare that with opportunities available.
Risk Assessment gives us the tools to estimate the likelihood of a circumstance occurring and of its consequences. It also gives us the tools to estimate what these will be after any improvement. These tools are diverse and can be imprecise. As time goes on our methods will improve but chance and circumstance will always be with us. But, if we use consistent methods, we will generally be able to rank the possible outcomes and compare them, in scale, with others.
ANCOLD is aware of the diversity of dams and the circumstance associated with them and offers this document merely as an indicator of the approach that may be taken. Nevertheless we hope that its adoption will lead to greater comparability between individual workers.Learn more