The Bureau of Meteorology has recently revised the Probable Maximum Precipitation (PMP) estimates for the Generalised Tropical Storm Method (GTSM) region of Australia. The revision process has involved the application of the more technically rigorous Generalised Southeast Australia Method (GSAM) that was previously developed for the southern part of Australia to a much larger data set of severe tropical storms. This has generally lead to an increase in the total GTSM PMP depths with a resultant increase in the Probable Maximum Precipitation Design Flood (PMPDF) and the Probable Maximum Flood (PMF).
In addition, the revision process has produced significant modifications to the temporal and spatial patterns adopted when applying the PMP depths to a dam’s catchment and these changes have also generally lead to increases in the resultant floods.
This paper discusses the rationale behind the increases in PMP depths and changes in the associated temporal and spatial patterns and presents the justification for the adoption of these more scientifically defensible estimates.
The application of the revised PMP estimates to the Keepit Dam catchment in northern NSW is presented and a comparison between the PMPDF and PMF estimates based on the original GTSM and the revised GTSM (GTSMR) made for this specific case study.
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South East Queensland Water Corporation (SEQWater) as owner and operator is proceeding with an upgrade of the flood capacity of Wivenhoe Dam. SEQWater has formed an Alliance with Leighton Contractors, Coffey Geosciences, Montgomery Watson Harza (MWH) and the Department of Commerce-NSW (formerly DPWS, NSW) to upgrade Wivenhoe Dam. This paper presents feasibility level investigation and design activities for an upgrade option, comprising a large labyrinth auxiliary spillway at the right abutment of the dam, for supplementing the existing gated spillway in handling the Probable Maximum Flood (PMF) event. This right abutment auxiliary spillway option incorporates Hydroplus type concrete fuse gates. The investigation so far has proved the technical viability of this option, however, ranking along with the other three options against various criteria will lead to the selection of the preferred upgrade option.
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.
A survey of spillway gate systems and operations has recently been completed by dam organisations in Nth America, Australia and New Zealand. The survey sought to identify typical arrangements for spillway gate systems and common features pertaining to reliability such as system redundancy, actuation methods and back-up systems, gate and hoist types, remote and local operation, gate testing programmes, and human factors.
Sixteen organizations responded, covering sixty two dams and nearly four hundred gates. This Paper reports on the preliminary analysis of the data, providing an overview of the industries’ approach to spillway gate operation and control.
Chi-fai WAN, Robin FELL
Hydro Tasmania uses an electronic inclinometer to monitor the face deflections of nine of its CFRDs. The inclinometer is lowered down a steel pipe attached to the upstream face of each dam. The inclinometer was designed and constructed by the University of Tasmania and was first used on Cethana Dam when it was completed in 1972.
The success of its use on Cethana Dam lead to its use for the long term monitoring of eight subsequent CFRDs constructed by Hydro Tasmania.
After 25 years of successful operation some irregular readings of face deflection became apparent. This paper describes the investigation of the irregular readings that had been obtained, the assessment of other methods of observing concrete face deflection, and the refurbishment of the inclinometer using modern electronic components.