P Amos, N Logan, and J Walker
There are a number of geological faults in close proximity to Aviemore Power Station in the South Island of New Zealand, including a fault in the foundation of the 48m high earth dam component of the power station. Possible movement of the Waitangi Fault in the earth dam foundation is of particular concern for dam safety, and the effects on the dam of a fault rupture has been the subject of detailed investigation by the dam’s owner Meridian Energy Ltd. These investigations have concluded that the dam will withstand the anticipated fault displacement related to the Safety Evaluation Earthquake without catastrophic release of the reservoir.
The identification of damage to the dam following an earthquake and monitoring of the dam to identify the development of potential failure mechanisms are important for determining the post-earthquake safety of the power station. The first stage of the post-earthquake response plan is the quick identification of any foundation fault rupture and damage to the dam to enable immediate post-earthquake mitigation measures to be initiated, such as reservoir drawdown. Following initial response, the next stage of the post-earthquake monitoring programme for the embankment dam is longer term monitoring to identify a changing seepage condition due to damage to the dam that might lead to a piping incident. Such an incident may not occur immediately after an earthquake, and it can be some time before the piping process becomes evident.
This paper presents some key instrumentation installed at Aviemore Dam and included in the emergency response plan for the post-earthquake monitoring of the embankment dam.
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Janice H. Green and Jeanette Meighen
The Probable Maximum Precipitation (PMP) is defined as ‘the theoretical greatest depth of
precipitation that is physically possible over a particular catchment’. The PMP depths provided by
the Bureau of Meteorology are described as ‘operational estimates of the PMP’ as they represent the best estimate of the PMP depth that can be made, based on the relatively small number of large events that have been observed and our limited knowledge of the causative mechanisms of extreme rainfalls.
Nevertheless, the magnitudes of the PMP depths provided by the Bureau are often met with scepticism concerning their accuracy when compared to large rainfall events which have been observed within catchments and which are, typically, only 20% to 25% of the PMP estimates. The recent increases in the PMP depths, resulting from the revision of the Generalised Tropical Storm Method (GTSMR), have served only to entrench this cynicism.
However, analyses of the magnitudes of the storms in the databases adopted for deriving PMP depths show that these observed storms constituted up to 76% of the corresponding GTSMR PMP depths and up to 80% of the Generalised Southeast Australia Method PMPs for the storm location. Further, comparisons of the PMP depths to large storms observed in similar climatic regions around the world indicate that the PMPs are not outliers.
The results of these analyses are presented for a range of catchment locations and sizes and storm durations and demonstrate that the PMP estimates provided by the Bureau of Meteorology are reasonable and are not unduly large.
G.L. Sills, N.D. Vroman, J.B. Dunbar, R.E. Wahl
In August 2005, Hurricane Katrina made landfall just east of New Orleans and inflicted widespread damage on the Hurricane Protection System (HPS) for southeast Louisiana. Subsequent flooding was a major catastrophe for the region and the Nation.
The response to this disaster by the U.S. Army Corps of Engineers included forming an Interagency Performance Evaluation Taskforce (IPET) to study the response of the system and, among many lines of inquiry, to identify causes of failure of levees and floodwalls. Beginning in September 2005, the IPET gathered geotechnical forensic data from failed portions of levees and floodwalls. Major clues discovered at the 17th Street break, including clay wedges dividing a formerly continuous layer of peat, led to an explanation of the failures. Field data from the failure sites were interpreted within the regional geologic setting of the New Orleans area to identify geologic and geotechnical factors that contributed to the catastrophe. The data gathered provided a method that resulted in the “IPET Strength Model.” This strength was used in analyses of the I-walls and levees using limit equilibrium stability analyses, physical modeling using a powerful centrifuge, and finite-element analyses. The results of all three types of studies revealed a consistent mode of failure that included deformation of the I-walls and foundation instability. The IPET also studied non-failed I-walls at Orleans and Michoud Canals, to identify geotechnical, structural, and geologic distinctions between failed and non-failed reaches.
Performance of the HPS during Hurricane Katrina offered many lessons to be learned. These lessons learned include: the lack of resiliency in the HPS; the need for risk-based planning and design approach; the need for the examination of system-wide functionality; and knowledge, technology, and expertise deficiencies in the HPS arena. In addition, understanding of the failure mechanisms and related causes of the levee and floodwall breaches provides a new direction for future designs of hurricane protection systems.
David S. Bowles
Portfolio Risk Management is a risk-informed approach for improved management of dam safety for a portfolio of dams in the context of the owner’s business. It can be used to identify ways to strengthen technical and organisational aspects of a dam safety program, and to provide valuable inputs to various business processes. Portfolio Risk Assessment is a decision-support tool, which is incorporated in Portfolio Risk Management. It can combine engineering standards and risk assessment approaches to provide a systematic means for identifying, estimating and evaluating dam safety risks, including comparisons with other industries. It should be periodically updated to provide a basis for managing prioritised queues of investigations and risk-reduction measures to achieve more rapid and cost-effective reduction of both knowledge uncertainty and risk.
Portfolio Risk Assessment is a standard of practice in Australia and is being applied by the US Army Corps of Engineers and others. When properly conducted and used within its limitations, the Portfolio Risk Assessment process is generally considered to be robust, adaptive, defensible for corporate governance, and to justify its cost through such benefits as increased dam safety funding, identification of failure modes that were not previously recognised, identification of opportunities for improved risk management, and more rapid “knowledge uncertainty” and risk reduction.
Legal and moral requirements necessitate an “equivalent to industry standard” approach to dam management by all dam owners. As an urban authority Central Highlands Water has a portfolio of dams with a broad range of classification and risk. ANCOLD Guidelines form the basis of our approach to dam management. Thus any guidelines developed can have significant affect on our budget and operation. Guidelines with requirements targeted at extreme and high hazard dams managed by large authorities with “deep pockets” may not be reasonable to impose upon low risk structures managed by lesser authorities. This does not mean smaller authorities want to do it on the “cheap” but budgets for such infrastructure can be hard to sustain. Consequently when guidelines are considered so too should the flow on affect to those who must implement them.
B Simmons, N Mudge
In 2004 the NSW Government released its Metropolitan Water Plan (MWP). This plan detailed the government’s initiatives to secure Sydney’s water needs during the current drought and into the future. The MWP outlined a range of both demand and supply side measures. These included modification to Warragamba and Nepean dams so that the water at the bottom of the dams that is currently unavailable for water supply can be accessed.
Accessing this deep water will increase the available water supply by an additional six months in the immediate drought and will provide, on average, an additional 40GL/annum to our long term available water supply.
The Warragamba Dam Deep Water Access Project involves accessing and transferring water from deep in Warragamba Dam to the existing water supply system.
Phase One of the project saw an abandoned underground pumping station 1.5km downstream of the dam wall, being enlarged and upgraded to pump water from the low level pipeline into the existing water transfer pipelines.
Phase Two of the project involved making a penetration low on the dam wall, some ninety metres below full storage level to access the deep water. This enabled the water to flow into the new pumping station, through an existing underground pipeline.
This project and in particular Phase Two was extremely unique due to the saturation diving systems and specialist tooling systems needed to create the penetration in the dam wall. The project provides a reference point for the water industry for future similar works.
This paper describes the project that was initiated at Warragamba Dam to access the deep water and is focused on the extremely difficult and unique works associated with creating the low level penetration in the dam wall.