J.H. Green, D.J. Walland, N. Nandakumar
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.
Cold water pollution occurs downstream of many Australian dams when water is released from well below the surface layer of a stratified reservoir during spring and summer. Water temperature can be depressed by 8 °C or more and this may impact negatively upon the survival and growth of native Australian fishes.
After many years in the ‘too hard basket’, mitigation of cold water pollution below dams is receiving increasing attention in Australia. Hume Dam is a case in point. Hume Reservoir, one of the largest irrigation reservoirs in Australia, has a high throughput of water (short residence time) and receives unseasonably cold water from Dartmouth Dam on the Mitta Mitta River and the Snowy Mountains Hydro Scheme on the Murray River.
The maximum possible discharge temperature below Hume Dam may be constrained by geomorphic and climatic features beyond human control. Specifically, the relatively short residence time of water may limit the extent to which it can heat up in the reservoir prior to discharge downstream. Here I present a heat budget for Lake Hume and address the question, “How much can we improve the thermal regime below Hume Dam.”
State Water # as manager of Keepit Dam has established a comprehensive upgrade project.
A portfolio risk assessment by State Water of its major dams placed Keepit Dam as the highest priority for an upgrade.
While extreme flood and earthquake dam safety are the main drivesr for this upgrade, the opportunity has been taken to integrate other key dam management considerations into the process including environmental improvements, flood mitigation and sustainable regional development.
The dam, which is located on the Namoi River some 45km upstream of Gunnedah, is, in tandem with Split Rock Dam upstream, a vital irrigation water supply for the Namoi Valley region in northern New South Wales.
In considering the most appropriate way of addressing the critical flood safety issue, it became very apparent that the solutions were many and they significantly impacted on the local community. Other important issues such as water quality and flood mitigation, and overall sustainable development in the valley, particularly system water reliability, could influence dam safety solutions and so also needed to be considered as part of the process. As such it was considered imperative that the local community be actively involved in determining both interim and long-term upgrade solutions.
To achieve the best outcome for the region, State Water since mid 2001, has used the community consultation approach to guide the project.
Currently interim works have been completed and long-term options are being evaluated.
An Environmental Impact Statement on the preferred proposal will be undertaken during the later part of 2004 and if approved, all works will be completed by end of 2007.
This paper will highlight our experiences to date including:
• the proposition of an integrated consultative process;
• the background to the project;
• the need for and extent of upgrade;
• an integrated consultation and communication approach including innovative processes and the creation of a high profile Community Reference Panel (CRP) to guide the upgrade project;
• some dos and don’ts from a consultation perspective, for use in other upgrade projects; and
• where to from now.
There is a widespread perception among dam engineers that tree root invasion occasions a very serious threat to embankment dams by virtue of its potential to initiate piping failure, with appropriate action invariably recommended. Remedial works can, on occasions, be extensive.
While the principle is ostensibly plausible and scarcely challenged, there has never been, to the Author’s knowledge, a satisfactory investigation to establish any credible scientific basis for it. One case that has attracted some attention in literature (by virtue of the extent of the investigation undertaken), viz a piping accident at Yan Yean Dam, is critically reviewed to show that the accepted view on the role of tree roots in this incident is less than satisfactory. In the course of this review, two physical Laws of Piping are proposed, and applied both to this case and to another nearby Melbourne Water dam that also has a history of piping.
Whilst the consequences of piping in a major dam are such that risk from this source must be kept to a very low level, it is concluded here that piping risk arising from tree root invasion has been considerably overstated and that a more balanced assessment is necessary before determining what, if any, action is required.
Dam safety emergency plans (DSEPs) are typically produced for individual dams. For owners of a large portfolio of dams, this approach creates document control difficulties, requires excessive time and effort and can lead to confusion when a single emergency affects multiple dams having individual DSEPs. Hydro Tasmania has developed a single DSEP which is applicable to its portfolio of 54 referable dams. The DSEP contains generic emergency response procedures, is applicable to a whole range of generic dam safety incidents, uses a simple colour-coded flowchart-action list format, has a two-stage emergency response, retains all necessary dam-specific information and can be easily adapted to any organisational structure. This approach was found to have benefits in document control, flexibility in the management of the emergency response and short lead time in terms of having DSEPs which cover an entire portfolio of dams.
Assessment of dam safety requires estimates of extreme rainfall together with the temporal and spatial distributions of extreme rainfall. In order to satisfy dam safety requirements for dams in the west coast of Tasmania, the Bureau of Meteorology has developed the method of storm transposition and maximisation for application in this region.
Daily, as well as continuously recorded rainfall data for all Bureau of Meteorology and Hydro Tasmania sites in western Tasmania have been analysed and the most outstanding rainfall events over one, two and three-day durations in the region have been identified. Meteorological analysis of these events reveals that the most significant rainfall events in the west coast of Tasmania are caused by the passage of fronts, which are sometimes associated with an intense extratropical cyclone, with a westerly or southwesterly airstream.
A database of isohyetal analyses of the most significant rainfall events in western Tasmania has been established. These can be used either ‘in situ’ or transposed to estimate mean catchment rainfall. Storm dewpoint temperatures for the purpose of moisture maximisation have been determined.
Cumulative and incremental three-hourly temporal distributions for sites having continuous rainfall data or three-hourly meteorological observations have been constructed and design temporal distributions of extreme rainfall have been derived.
An objective method for adjusting for differences in the topography between the storm and target locations is proposed