This paper relates to the conference sub-themes of Dam Safety Upgrades – Management of
Risk and Due Diligence and Dam Construction.
Specifically, it relates to the changing willingness of governments to fund risk reduction in
dams compared with risk reduction in other areas.
The cost of dam safety upgrades is just one of a portfolio of risk reduction strategies
affecting the general community that governments are required to underwrite.
This paper examines the variation in acceptable risk standards between dam safety and
other areas. This may be explained in terms of what is acceptable to the community and the
courts. For instance, a corporation is likely to attempt to minimise its liability (which may
differ to minimising risk for the community). We also examine:
• a portfolio approach to safety expenditure and the implicit cost-benefit relationship;
• the impact of the asymmetric relationship between expenditure and absolute size of
potential loss; and
• the importance of a whole-of-government approach and reviewing apparent
inconsistencies in approach to risk.
There is an increasingly well-established literature on the value of a human life and
increasingly systematic approaches to the evaluation of risk and the setting of risk
standards. Risk standards are particularly explicit in the area of dam safety – they set limits
of tolerable risks for large-scale loss of life (eg. for existing dams, a loss of life of more than
10 persons with a probability of more than one in a ten thousand per annum is regarded as
unacceptable under the Australian guidelines).
However, there are significant contrasts in what is tolerated as acceptable risk between
different areas of government activity. To date, there appears to be no systematic evaluation
of the portfolio of risks or a common view on what is acceptable levels.
SunWater has completed a portfolio risk assessment (PRA) on its 25 major dams and has identified a number of dams that do not currently satisfy the ANCOLD fallback position on spillway capacity. It has taken an initiative to target these dams for spillway upgrades to ultimately achieve the ANCOLD fallback standard and has prioritised these upgrades in a preliminary program for action in the short to medium term.
As background to this PRA, SunWater has developed and implemented a dam safety program which has successfully updated all necessary flood hydrology and dam break analyses and reassessed the consequences and hazards associated with dam failures. It has also completed within the last eight years, dam safety reviews on all its dams in preparation for a comprehensive risk assessment process which is now well in-hand. This process will identify and evaluate all other risks, in addition to floods, that should be addressed or at least considered in the planning and design of these spillway capacity upgrades.
This paper describes SunWater’s experience and approach to PRA and discusses the controlling factors considered in prioritisation. It shows the results and trends of a number of risk ranking methods, provides details of the current level of societal risks in respect of the ANCOLD tolerability limits and outlines SunWater’s current strategy for the timing and staging of spillway upgrades to achieve compliance and an optimum level of risk reduction.
Murray Gillon, Robin Fell, Harry Keys, M Foster
Volcanic eruptions at Mt Ruapehu in 1995-96 resulted in the deposition of about 7m of tephra over the rock rim overflow of Crater Lake. There is a long history of lahars (debris flows) associated with releases of water from Crater Lake. The 1995-96 eruptions emptied the lake and it has slowly been refilling from rainfall runoff and snow melt. When the lake level rises above the rock rim the tephra layer will act as a “barrier” or dam. Breaching of the barrier will release water and generate a lahar. The magnitude of the lahar flow will be a function of the lake level at the time of breaching
Extensive studies of the effects of the lahar that would be generated by the failure of the tephra barrier have been undertaken. The studies included a failure modes and likelihood analysis to provide information on the relative likelihood of failure as the lake level rises for the different failure modes applicable to this situation. The paper describes the failure modes considered and the results of the analysis.
The Wivenhoe Dam Spillway Augmentation Project involved the construction of an additional spillway on the right abutment of the main dam. The right abutment is located in massive sandstones and siltstones of Jurassic and Upper Triassic age.
Seismic refraction surveys and borehole drilling conducted at the design stage for the project
indicated that part of the spillway area was likely to be marginally rippable to unrippable using a
Caterpillar D9 bulldozer or equivalent. Further assessment and rock strength testing was conducted during the initial stages of excavation where D9 and D10 bulldozers were in operation. The results from this further work indicated that a section of the spillway extending from the proposed position of the ogee crest to approximately 100m further upstream were unlikely to be unrippable for a D9 dozer and marginally rippable for a D10.
Excavation options considered for this section included full scale blasting and load out, limited small scale ‘popping’ combined with ripping or the use of larger ripping equipment. Based on an
assessment of cost-benefit, and given the availability of larger ripping equipment, it was decided to
use a combination of D10 dozers and a Komatsu 475A bulldozer (D11 equivalent) equipped with
single tine ripping tools. The use of this equipment proved successful with better than anticipated
production rates being achieved. This resulted in significant cost and time savings for the project and reduced the likelihood of potential adverse impacts on the existing dam grout curtain, environment, travelling public and residents that may have occurred during blasting.
Stuart Macnish, Nikki Bennett
The $70 million upgrade of Wivenhoe Dam is being undertaken by the Wivenhoe Alliance, in close
proximity to the town of Fernvale, Queensland. As part of the Alliance’s commitment to delivering positive outcomes for the local community, it was decided part way through the project, to commit to delivering a ‘signature’ community legacy project. The team brainstormed a range of options and a decision-making matrix was used to choose the project that would best meet its objectives.
A partnership has been formed between the Alliance, Esk Shire Council and SEQWater to deliver a
master-planned project which incorporates elements such as a community information/service facility,upgrade of Fernvale Memorial Park, streetscape enhancements, improved parking and installation of shelters along the adjacent rail trail. These major partners, together with representatives of the local community, constitute the steering committee, which oversees planning of the project.
The project aims to encourage visitors to the area, to provide improved amenity and sense of pride for the region, and in turn encourage strong relationships for SEQWater in the area in which they operate. Due to tight time frames the partnership is managing the fund raising activities, community consultation and design processes in parallel.
This paper discusses the process by which the Alliance was able to deliver this remarkable project, within a short timeframe. It also discusses how the local community has been involved and the benefits, which have resulted.
Malcolm Barker, Jon Williams and Chi Fai Wan
The Ross River Dam, designed in the early seventies, does not meet current dam safety criteria for overtopping and piping within the embankment or the foundation. The dam comprises a 40m long concrete overflow spillway flanked by a central core rockfill embankment of 130 m on the right bank and 170m on the left bank with a 7620 m long left bank earth fill embankment, which has no internal filter zones for piping protection. The embankment was extensively assessed and treated for foundation deficiencies in 1982, and further assessed in 2000-2002 for appropriate upgrade options.
This paper describes the process of validation of the detailed design using Risk Based Design Criteria.This process included data mining for historical performance and original design intention,comparison of the original design against current and historical investigations and assessment of the upgrades using the large volume of data available from previous work. A design team comprising specialist hydrologists, hydrogeologists, geologists, geotechnical and dams engineers worked within a risk assessment framework at all stages of the design to ensure the design was validated using the design Validation Model. This process incorporated assessment of crest level based on flood risk and wave overtopping, review of 2D and 3D seepage models to assess piping and foundation erosion potential, assessment of fissured soils within the embankment foundation for structural stability and evaluation of spillway model testing for potential spillway failure modes.