The role of judgement in risk assessments as applied in dam safety management has been the source of considerable debate in recent years. With regard to risk analysis of dams, and while there is general agreement that judgement is an essential element of the process, essentially two schools of thought have emerged. One view holds that, in the assignment of probabilities, reliance can be based on collective engineering judgement that is anchored to a knowledge base. The second view holds that judgement should be based on the knowledge that is revealed by an appropriate amount of analysis. The paper, written from the perspective of the latter view, explores some of the underlying issues in this debate.
The role of judgement in risk evaluation, the process of judging the significance of risk, is considered to be equally important. However, the process of making value judgements and statements of principles is complex and often beyond the sphere of engineering. The third issue addressed in the paper concerns the search for answers to the question, “How good is the assessment?”
Buddhima Indraratna, Mark Locke and Gamini Adikari
The main objectives of the filter are to prevent erosion of the dam core, permit controlled passage of seepage flow through the dam and facilitate dissipation of excess pore pressures in the core. In most designs of dam filters, empirical methods based on particle size ratios have been used. These empirical rules are developed through extensive laboratory tests. Although the empirical rules benefit from directly or indirectly incorporating most factors affecting filtration, they cannot be extrapolated for distinctly different soils and do not describe the time dependent changes that occur within the filter medium.
Mathematical models can be formulated to explain the fundamental physics of particle interaction and migration, within a framework of well defined geohydraulic constraints. Considering the mass flow and momentum conservation principles; time dependent changes in particle size distributions, mass flow rates, retention capacity and base soil erosion rates can be simulated.
This paper reviews various empirical and mathematical models, based on the authors experience. A novel approach to large scale filtration is highlighted based on testing actual soil and filter materials from an Australian dam, in a new 500mm diameter apparatus.
Duane M. McClelland and David S. Bowles
There is a growing concern about the limitations of the approaches to life-loss estimation that are currently used in dam safety risk assessment. This paper summarises insights into factors that affect evacuation effectiveness, loss of life, and survival, based on a detailed review of historical dam breaks and other types of floods. The understanding and empirical characterisation of life loss dynamics being developed from these case histories are intended to provide the foundation for an improved practical life-loss estimation model.
Kumara Arachchi and Kelvin J Lambkin
Wetlands by their very nature act as storages of pollutants and nutrients in systems subject to environmental stresses. Wingecarribee Swamp acted in this manner and enhanced the quality of catchment runoff flowing into the Wingecarribee Reservoir until the structural failure of early August 1998 in which 6000 megalitres of peat and sedimentary material were moved into the Reservoir. Protection of the Swamp’s functions and values is directly related to Sydney Catchment Authority’s core objectives of protecting the environment and protecting public health by supplying drinking water of acceptable quality. Due to the catastrophic failure, water quality in the reservoir and the ecological integrity of the Swamp have been compromised. The incident has also resulted in significant dam safety issues.
This paper describes the dam safety, catchment management and water quality response to the failure of a major peatland which covered 8% of the catchment of Wingecarribee Reservoir in the Southern Highlands of New South Wales.
Douglas Gallacher, Richard Doake and Debbie Hay-Smith
Damage to the rip-rap protection on the upstream face of Megget Dam has occurred since first filling in 1983 and independent wind-wave investigations have demonstrated that waves exceeded anticipated wave heights. Value Planning Studies for alternative schemes to upgrade the rip-rap protection indicated that bituminous grouting was the preferred option and its satisfactory performance was proved by site trials during May 1997. The bituminous grouting works were carried out in two stages with a break over the winter season. The upper part of the face was completed over a 12 week period (September to early November 1997) and the grouting works for the remaining area was completed over a 24 week period (mid April to early October 1998).
Andrew Day, Rod Bridges and Corrado Fabbri
A joint venture between Astaldi SpA of Italy and Thiess Contractors Pty Ltd of Australia (ATJO) has just completed a 95m high roller compacted concrete (RCC) dam on the island of Sulawesi in Indonesia. The dam which includes 528,000m’ of RCC was completed in September 1999 and will provide hydro-electric power for a nearby nickel smelting operation.
One of the largest RCC dams built in the region in recent times, the construction presented a number of unique challenges in particular placing techniques to cope with the heavy rainfall in the area as well the logistics to this remote location. Other aspects which are addressed in the paper include production rates, RCC placing systems (Rotec), dam formwork systems, aggregate sources, RCC mixes and waterproofing (membrane).
After early problems with the river diversion, the works were accelerated and completed to a very tight program. To enable dam construction to commence prior to river diversion the wall was advanced as a series of separate monoliths which led to a number of RCC placing innovations.