David Ho, Karen Boyes, Shane Donohoo and Brian Cooper
Many dam structures in Australia were designed and built in the 1950s and 60s with limited hydrological information. As a result existing spillway structures are under-sized for today’s revised probable maximum floods (PMF). Potential problems such as the generation of excessive negative pressure over spillway crest under increased flood condition could be encountered. This may cause instability or cavitation damage to the spillway. The raised flow profile may also have adverse impacts on crest bridges and gate structures.
Historically, physical models have been constructed in hydraulic laboratories to study these behaviours, but they are expensive, time-consuming and there are many difficulties associated with scaling effects. Today, with the use of high-performance computers and more efficient computational fluid dynamics (CFD) codes, the behaviour of hydraulic structures can be investigated numerically in reasonable time and expense.
This paper describes the two- and three-dimensional CFD modelling of spillway behaviour under rising flood levels. The results have been validated against published data and good agreement was obtained. The technique has been applied to investigate several spillway structures in Australia.
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In September 2000, pressures being monitored in a geological fracture beneath Arapuni Dam were found to be rising significantly, indicating that a deteriorating condition was developing in the foundation. Two boreholes drilled in 1995 had intersected high water pressures within the fracture in an area close to the downstream face of the dam, posing a risk of major leakage developing from where the fracture day-lighted downstream of the dam. Lumps of clay, bitumen and lake biota, including snails and small fish, were identified discharging from the boreholes, indicating that a significant leakage path had developed. Detailed investigations, the subject of this paper, were carried out from September 2000 to confirm the extent and nature of the deterioration. A range of groundwater investigation techniques and tools were used, while the reservoir remained full, to identify the source of the leak and confirm the path it took. The investigations culminated in development of a groundwater model that described the seepage behaviour in the dam foundation. Based on the investigation information gathered, the foundation fracture bearing the high water pressure was successfully grouted in December 2001 without lowering the reservoir.
The paper highlights the fundamental importance of correct data selection and storage for the quality of Asset Management demanded for today’s water industry infrastructure.
In developing this theme, the concept of Risk driven maintenance is introduced to focus attention on those issues that not only the identify the appropriate data to be collected and stored, but also, by way of illustrated examples, the direct relevance and application of reliability engineering principles in Risk Analysis.
The author’s principle objective is to demonstrate that the historical data on reliability, condition and performance must be supported with detailed costing information if any worthwhile outcomes are to be forthcoming from analysis.
Yarrawonga Weir was constructed in the 1930’s and is located on the Victoria / New South Wales border, between the towns of Yarrawonga and Mulwala. Dam safety investigations revealed that the main embankment was founded on a very loose layer of sand that would be vulnerable to liquefaction even under the operating basis earthquake
This paper details the statutory approvals and community consultation processes that were employed and the benefits that they provided to the $13 million remedial works project.
Local communities can assist, be neutral or obstruct a project. By engaging the community in a positive manner it is possible to deliver excellent results without increasing costs.
M. Barker, B. Vivian, J. Matthews and P. Oliver
This paper discusses reliability issues of the fourteen 3.85m high by 7.89m wide radial gates at Glenmaggie Dam in Victoria and the twin 3.6m high by 16.5m wide drum gates at Little Nerang Dam in Queensland. The Glenmaggie dam radial gates are manually controlled using electrically driven (mains and diesel generator power supply) hoist motors with a petrol driven hydraulic pack for use in the event of complete electrical power supply failure. A detailed fault tree analysis was developed for the spillway gate reliability of the Glenmaggie Dam gates as part of the risk assessment for the dam, which was being completed at the time of publishing the paper. Each of the identified components of the spillway gates, including human error in operation was used to evaluate the probability of failure of a single gate or multiple gates for inclusion in the event tree to estimate the risk and assist the evaluation of the requirement for remedial works. The Little Nerang drum gates are fully automatic hydraulically operated gates with independent operating mechanics and a common override system in the event of automatic system failure. Drum gates are uncommon on dams and the system operation is discussed together with an assessment of the reliability and measures taken for handling operating risks during floods for the dam, which has some stability concerns.
Chi-fai WAN, Robin FELL
This paper presents the findings of experimental investigation of internal erosion by the process of suffusion within embankment dams and their foundations.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces. Soils susceptible to suffusion are usually described as internally unstable. Understanding of the suffusion process is important to the assessment of the risk of internal erosion in an embankment dam and its foundation. Suffusion results in a coarser soil structure, leading to increased seepage, progressive deterioration of the dam or its foundation, and a higher risk of toe instability. Suffusion within the protective filter of a dam may result in a coarser filter, rendering it ineffective in protecting the core materials from erosion.
Two types of suffusion tests, namely the downflow test and the upflow test, have been conducted at the University of New South Wales. The downflow test aims at identifying the types of soils that are susceptible to suffusion, whereas the upflow test aims at identifying the hydraulic gradient at which suffusion is initiated. This paper presents the initial findings of the downflow test. Eighteen downflow tests have been carried out on fourteen clay-silt-sand-gravel soils. The Kenney and Lau (1985, 86) method, which is commonly used for assessing the internal stability of coarse-grained soils, appears to be too conservative when used to predict the internal stability of silt-sand-gravel or clay-silt-sand- gravel soils, whereas the Burenkova (1993) method appears to provide better predictions. Further testing is required to define more accurate criteria for determining the internal stability of broadly-graded clay-silt-sand-gravel soils.