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.
Ensuring compliance with the Regulator’s requirements is a cornerstone consideration for any water corporation in planning its risk minimisation strategies against dam failure. With the increased focus on due diligence and corporate governance however, there are emerging themes that are of equal importance for a water corporation in planning protections against its core risks to dam safety. These considerations include:
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.
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 postearthquake 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.
R. Dawson, J. Grimston, R. Cole, D. Bouma
The authors have been involved in the design and construction of several embankment dams in New Zealand over the past decade, and have considerable corporate knowledge from dams designed by the company in its 47-year history. This paper examines four dams which are relatively small to medium, ranging in height from 10 to 19 m with moderate storage volumes. Three of the dams service landfills and the fourth a wood processing mill. Such dams may provide the designer with considerable challenges due to their relatively low capital cost resulting in limited investment in geotechnical investigation at the front end of the project, with varying levels of change often required during construction due to unforeseen conditions as a result of the limited investigations.
The general arrangement and conceptual design principles for each of the dams is described followed by the field investigation and laboratory testing undertaken for each dam, together with the interpreted ground conditions.
The experiences from construction have helped to develop techniques for a balance between preliminary design, investigation, and evolution of the design and specification during construction. It is imperative to develop a sufficiently detailed preliminary design, on the basis of readily available information such as visual and geological assessment, to allow the investigation to be thoughtfully designed to allow the major assumptions to be verified. This needs to be followed by a skilfully executed geotechnical investigation with the designer advising on findings and changing direction as necessary through the investigation. An investigation trench along the full alignment of the cutoff trench (if envisaged in the design) is warranted. Earthworks specifications should be evolved early in the construction phase through compaction trials using specific plant for the site, and backed up by insitu and laboratory testing.
The paper describes the methodology, operative techniques and organizational aspects that are used for dam safety assessment procedures. Kelag owns 15 larger dams with wall heights up to 110 m. It is necessary to monitor the aging of the structures and to check all safety equipment regularly. The manned control centre is situated at the KELAG Headquarter in Klagenfurt, which is the capital of Austria’s southern-most Province, Carinthia. KELAG is the principal electricity supplier in Carinthia, and owns several reservoirs in the Austrian Alps. The whole hydropower system has a capacity of 434 MW with an annual production of 1000 GWh. During the last century KELAG employees designed, supervised and constructed most of the structures in cooperation with the authorities. Most of the rock-fill dams have a bituminous concrete sealing on the upstream face. KELAG owns one concrete arch dam with a height of 30 m. A pendulum monitors the movement of the dam crest. This information is transmitted to both the power house and the manned control centre in Klagenfurt. Seepage is monitored at all rock-fill dams. In case of an alarm a skilled engineer has to be informed by the staff of the manned control centre. This dam safety engineer starts to check the reasons on site and manages the emergency action plan. Data has been collected since 1998 and special software is used to handle this information, carry out interpretation and safety assessments. One aim of data collection is to develop a decision support system performing online evaluation, explanation and interpretation of dam behaviour. Normally, once a year geodetic measurements are carried out at all dams.
KELAG’s experience gained in the use of automatic monitoring and risk assessment of dams is covered in this paper. The monitoring systems show the state of the structures and those showing anomalous situations requiring human intervention can be identified as soon as possible. Although the repercussions of the free market system have led to substantial staff reductions, the quality of dam surveillance has had to remain unaffected. Dam safety is guaranteed by new types of instrumentation, data transmission and data assessment. A special software has to be updated constantly.