As New Zealand’s largest dam owner, ECNZ has actively managed dam safety since its inception in 1987. During this time it has managed several major dam safety issues and enhanced its dam safety management practices. This has occurred in an environment of organisational change and increasingly competitive commercial pressures.
The change in emphasis from a primarily technical emphasis to dam safety towards a commercial focus is described together with details of highly rated dam surveillance system, some continuous improvement initiatives, and recent enhancements to the dam safety programme. The position of responsible ownership in regard to risk and legal requirements is also discussed.
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.
Raymond A. Stewart
On I7 June 1996 while investigating a small pothole on the crest 183 m high Bennett Dam an unexpected crest collapse occurred resulting in a large sinkhole. Following this incident the safety status of the dam was uncertain. The reservoir was lowered by 2 m over a six week period by spilling up to 5,000 m 3 over the spillway and through the turbines.
An unprecedented dam investigation commenced immediately and was completed December 1996. During drilling a second sinkhole was discovered at another location on the dam.
A sophisticated compaction grouting technique was developed to remediate the sinkholes to the depth of 5 m and the work was successfully completed by 1997. -The reservoir was returned to service in time to collect the freshet in spring 1997. This event was the most dam safety concern in the history of BC Hydro operations.
This paper describes how B.C. Hydro managed the crisis, and the subsequent safety assessment.
A safety review of the Corin dam has identified several deficiencies including an inadequate spillway capacity. A hydraulic model test, included in the review indicated that the construction of a 1.3m wave wall along the top of the dam was required to prevent overtopping during the flood of 10,000 years.
The original post tensioning anchors installed along the spillway crest were also identified as unreliable due to inadequate corrosion protection measures.
This paper presents safety assessment and aspects of the construction of the remedial works for Corin Dam. As part of the safety review, the condition of the dam was reviewed against the risks of piping, slope instability, flood and seismic forces. The paper also discusses the long term effects of the acidic leakage on the grout curtain and on the integrity of the core.
The risk associated with the flooding during anchor installation and the discovery of a gap formation between the clay core and the concrete spillway wall are also considered.
D. C. Green
The disaggregation of public water supply bodies in recent years has seen the functions of ownership, design and operation transferred to separate bodies. Consequently , issues of risk management associated with legal liability which previously could be ignored because all risks were absorbed in -house must now be faced and addressed in a more formal way.
This paper looks firstly at the general principles of legal liability for dam performance associated with construction and design, ownership of an existing dam and monitoring of its performance. Liability under several different areas of the law is discussed. Special issues associated with “design and construct” contracts are then highlighted, and warnings are given for project sponsors who control the letting of contracts and the briefing of consultants.
One of the most important issues during design and construction of an earthfill dam is how to secure a dam against unwanted events which may occur as a result of water flow (uncontrolled seepage, leakage & piping) through the dam.
Although earthfill dams are the largest by volume compared with other types of dams and they are designed to cope with seepage, their integrity is most sensitive to the effects which may be caused by it. The reason being that the earthfill materials are generally extremely heterogeneous and only one “unwanted” pocket is enough to create problems.
Another critical area is the foundation. In many situations it is not possible to avoid the complex geology which includes faults and joints as part of the foundation. An additional complication may be the presence of dispersive clay in the foundation.
In the area of tailings dams, the problems with seepage are slightly reduced as in most cases, tailings provide a degree of sealing. Tailings dams are very often designed as leaky dams. However, there is a hidden danger in approaching the design this way as at any stage of their lives they can retain water.
This paper presents two case histories of repairs carried out to tailings dams suffering leakage. One case describes leakage through the embankment wall while the other describes seepage through the foundation which contains dispersive soil.