J.H. Green; C. Beesley; C. The and S. Podger
Rare design rainfalls for probabilities less frequent than 1% Annual Exceedance Probability (AEP) are an essential part of spillway adequacy assessment as they enable more accurate definition of the design rainfall and flood frequency curves between the 1% AEP and Probable Maximum events.
Estimates for rare design rainfalls were previously derived using the CRC-FORGE method which was developed in the 1990s. However, as the method was applied on a state-by-state basis, there are variations in the approach adopted for each region. Differences in the cut-off period for data, the amount of quality controlling of the data undertaken, the base used for the 2% AEP estimates, gridding settings and smoothing processes have created inconsistencies which are particularly apparent in overlapping state border areas.
The Bureau of Meteorology has derived new rare design rainfalls for the whole of Australia using the extensive, quality-controlled rainfall database established for the new Intensity-Frequency-Duration (IFD) design rainfalls. These data have been analysed using a regional LH-moments approach which is more consistent with the method used to derive the new IFDs and which overcomes the limitations of the spatial dependence model in the CRC-FORGE method. In particular, the selection and verification of homogenous regions and the identification of the most appropriate regional probability distribution to adopt relied heavily on the outcomes of the testing of methods undertaken for the new IFDs. However, to focus the analysis on the rarer rainfall events, only the largest events have been used to define the LH-moments.
Keywords: Rare design rainfalls; Intensity-Frequency-Duration (IFD); Annual Exceedance Probability
Conrad Ginther, Colleen Stratford
The Wyaralong Dam Alliance (WDA), a consortium of seven engineering and contracting companies, was contracted to design and construct the Wyaralong Dam, which impounds the Teviot Brook 14 km from Beaudesert in Queensland, Australia. The dam is an approximately 500 metre long, 48 metre high Roller Compacted Concrete (RCC) structure built on a foundation generally consisting of massive sandstone with intermittent conglomerate zones consisting of cemented gravels, mudclasts and sands. Geologic features of note with regard to dam stability and long term seepage at the site are dominated by downstream sloping bedding features and conglomerate zones. In addition to the bedding-related features, two predominant vertical to subvertical fracture sets exist. The condition of the vertical fractures ranges from tight and fresh at depth to highly weathered and filled with dispersive clay and gravels near the foundation surface. To provide a durable and effective long term seepage barrier for the dam, an extensive foundation cleaning and treatment operation was undertaken. This comprised drilling, blasting, and excavation of the majority of the highly weathered rock and dispersive materials supplemented by localized installation of small cut-offs and dental concrete and the construction of a double-line grout curtain installed using real time computer monitoring, the GIN methodology, and balanced, stable grout mixes.
Foundation Preparation and Seepage Barrier Installation at Wyaralong Dam Construction Project
Robert Keogh RPEQ, CE Civil (Hon), Mal Halwala, Peter Boettcher, Renee Butterfield
SunWater is a Government Owned Corporation (GOC), operating in a competitive market on an equal commercial footing with the private sector. SunWater owns 23 referable dams. Over the last fifty years there has been significant development of the methodologies used to estimate extreme rainfall events. These have resulted in substantial increases in probable maximum flood (PMF) estimates for most of SunWater’s dams.
SunWater has undertaken a Comprehensive Risk Assessment program across its portfolio. SunWater now has a good understanding of the deficiencies and available risk reduction options for each dam under all load conditions. The total cost to rectify all deficiencies is several hundred million dollars and well beyond the financial capacity of the organisation in the short term.
ANCOLD and Regulators have different published opinions on decision making criteria for dam safety upgrades. Once the conditions for the tolerability of Societal and Individual Risk are satisfied the onus remains with the dam owner to meet the ALARP principle. The decision making process is complicated by uncertainties in inputs to risk assessments. The authors have considered these uncertainties as well as the legal implications, differing ANCOLD and Regulator requirements, and business and economic loss, in formulating the decision making process. The methodology is simplified but effective. If the process is followed the dam owner’s investments will meet ANCOLD, Regulatory, legal and business requirements.
This Paper details a logical decision making process designed to allow a non technical Board to balance social, legal and financial objectives. The process considers overall risk, tolerability, the ALARP principle, and project prioritisation. The process is being used by SunWater to determine the Acceptable Flood Capacity of each dam, which dams will be upgraded, priorities and scheduling of each upgrade.
How SunWater, as a commercial dam owner makes investment decisions for dam safety upgrades
The enlargement of the Cotter Dam is being undertaken by ACTEW to provide a greater security of water supply to Canberra. The project involves constructing a larger, higher new dam wall immediately downstream of the existing Cotter Dam, to allow the present dam to continue functioning and supplying water while construction is underway. The project raised a number of environmental issues partly because the Cotter Dam currently supports a self-sustaining population of (endangered) Macquarie Perch, and because the Bendora Dam, upstream of Cotter Dam, contains a breeding population of (endangered) Trout Cod. Bendora Dam will not be physically affected by the works on Cotter Dam, but its operations may be altered. An ecological risk analysis was conducted to identify critical environmental risks that would need to be investigated and managed or ameliorated and management strategies were put in place to reduce risks. ACTEW have adopted an adaptive management approach to the project, but in order to implement that approach it is necessary to conduct effective monitoring of the fish populations of concern. These potentially include the two endangered species, as well as potential predators (such as cormorants) and competitors (such as trout). Power analysis has been used as a tool to evaluate whether it is feasible to monitor key populations sufficiently rigorously to be able to confidently detect a change (either an increase or decrease in a population). For Macquarie Perch and trout it should be possible to detect population changes statistically with a logistically feasible monitoring program.
2011 – Using risk analysis, power analysis and adaptive management to minimise ecological impacts of the Cotter Dam enlargement
George Bolliger and Clare Bales
Traditionally, the dams engineering profession has been a career path for engineers of civil/structural or geotechnical persuasion. As dams are constructed there is understandably a predominate focus on the civil requirements. Beyond the first few years of the dam’s life, effective operation and maintenance becomes increasingly important. A number of mechanical/electrical components and plant items form part of the critical infrastructure of the dam. A good maintenance routine is an essential requirement of the dam safety management program.
State Water Corporation, as the owner of 20 large dams and over 280 weir and regulator structures, runs a dam safety management program that is in line with the Australian National Committee on Large Dams Guidelines and NSW Dams Safety Committee requirements. The maintenance procedures and outcomes are audited through an internal maintenance audit program.
The maintenance audits form an integral part of the total asset management plan as well as the dam safety program. They are used to identify areas of strength as well as common errors or defects. Using State Water’s internal maintenance audits as case studies, the paper elaborates the role of maintenance audit program in enabling a cultural change to further include mechanical/electrical aspects and thereby enhance the longevity and safety of the assets.
Cultural Change – A Mechanical Perspective on Dam Safety Management