M.A. Foster, R. Fell, R. Davidson, C.F. Wan
The probability of failure of embankment dams by internal erosion and piping can be estimated using historic performance, and event tree methods. Event tree methods are preferred for all except preliminary assessments, because they can better model the characteristics of the dam. This paper provides guidance on how to estimate the conditional probabilities within the event tree based on an understanding of the process involved, the historic performance of dams, and experience in recent risk analyses. This includes methods for representing the reservoir water level; assessing the likelihood that piping initiates; assessing the likelihood that erosion will be controlled by the filters or transition zones; and assessing the likelihood of development of a pipe and breach. The paper will be useful for those carrying out risk analyses and will also have relevance to those who are assessing piping of dams using traditional methods.
The report of the World Commission on Dams (WCD), issued in November 2000 was the result of a world-wide study of the development and effectiveness of dams. This paper describes a parallel but much smaller study, specifically relevant to the Asia-Pacific Region, carried out simultaneously for the Asian Development Bank (ADB) by a multi-disciplinary consortium of consulting organisations, whose work was reviewed by an Advisory Panel. This study comprised an analysis of performance compared with design expectations in case studies of four dams spread across the Region and constructed at different times between 1970 and 1999, supplemented by a literature survey. Although the case studies were conducted independently of WCD and to different Terms of Reference, they used similar methodologies, and the results were submitted to WCD to increase their knowledge base. The ADB study’ s recommendations take a somewhat different form to those of the WCD, but there is no conflict between the recommendations of the two studies.
G. A. Pickens, J. O. Grimston
The Opuha Dam Project is a multipurpose water resources development, for irrigation and other uses. The 50 m high irrigation dam incorporates a 7.3 MW hydro installation, enhances summer low flows downstream, increases potable water supply security, is a significant recreational facility and provides flood attenuation. Opuha Dam was the first large dam permitted under NZ’ s Resource Management Act, for which sustainability is the cornerstone. It was also built under a design-build contract arrangement. Although breached by a flood during construction, the dam was successfully completed and performance has met or exceeded expectations. Experiences of potential value to future developments are outlined including the positive features of design- build. Technical features which contributed to the cost-effectiveness and performance of the project, are described, including downstream reregulation to enable “on-off’ peak hydro operation, Obermeyer type spillway gates to maximise flow capture for hydro and a stepped service spillway.
Neil Gillespie, Chris Hansen
With the introduction of the Resource Management Act in 1991, two significant changes have meant a complete review has been necessary of the planning provisions for hydro generation facilities in plans prepared under the new legislation.
Firstly, the Resource Management Act 1991 introduced an ‘effects’ based approach to planning, as opposed to an ‘activity’ based approach that existed under the previous Town & Country Planning Act 1977. Secondly, the planning mechanisms available to hydro generators to provide for their facilities changed.
Contact Energy Ltd (Contact) is a significant hydropower generator based in Central Otago. The new Regional and District Plans prepared under the Resource Management Act 1991 now contain provisions controlling activities previously provided for by way of a designation. Contact has invested considerable time and effort into the plan preparation process to ensure their activities are not unduly restricted by the new plans.
This conference paper provides an overview of the changes to the planning associated with hydropower generation facilities, and Contact’ s experience in the plan preparation process.
P.I. Hill, D.S. Bowles, R.J. Nathan, R. Herweynen
With the growing emphasis on a risk-based approach to dam safety management, event tree models are increasingly being used as an analysis tool. The simple structure of event trees belies some of the more complex issues associated with their application to dam safety risk analysis. This paper outlines some of the basic principles of event tree analysis and then demonstrates how inappropriate construction of event trees, and particularly oversimplification, can result in a bias in the estimated risk and produce misleading results when used to assess the dam against various risk criteria. Issues considered include the partitioning of the loading event and the impact of conservative assumptions such as assuming the reservoir is initially at full supply level.
Allan Crichton, Jon Williams, Anthony Ford
Wivenhoe Dam was constructed in the early 1980’s and is the largest source of water to the southeast Queensland region. The dam also provides significant flood mitigation benefits to the large communities in the Brisbane valley including the cities of Ipswich and Brisbane. Changes in the methods of determining the probable maximum flood, which is the design flood for the dam, have meant that the dam was not capable of passing the significantly larger design flood event. The feasibility study undertaken to assess the most appropriate method of upgrading the flood passing capacity identified more than 240 options. These options were short-listed and the capital cost and consequences costs determined for each of the short listed options.
This paper describes the process used to identify the options to upgrade the flood passing capacity and the method used to assess the consequences costs, which are primarily the costs of flood damage resulting from each of the options. The consequences costs for each option are the costs associated with changing the flow conditions in the river downstream of the dam. For example the option to upgrade the dam to pass the design flood is a benefit to the community however if this benefit is achieved by installing large gates or a fuse plug that operates frequently the scheme may increase the costs to the community.
The preferred option initiates at the lowest probability of occurrence of all those analysed — average return period of 10,000 years. There will be opportunities during the Environmental Impact Assessment process to test acceptability of this initiation level — a more frequent occurrence would be a lower capital cost solution. The EIA process may require a solution with a higher initiating level. Informal talks with the regulator have indicated a preference for the less frequent initiation level.