N. Vitharana, G. McNally, C. Johnson, A. Thomas, K. Dart and P. Russell
Millbrook Reservoir is an offline storage with an earthen embankment dam containing a puddle clay core and a moderately sized upstream catchment. The dam is 31m high and has a capacity of 16.5 GL when the storage water level is at the Full SupplyLevel (FSL). The reservoir is 25km NE of Adelaide on Chain of Ponds Creek, a tributary of the River Torrens. The dam was constructed during the years 1914-1918. Earthworks were carried out only during summer as the five winters during the construction period were very wet.
Dam safety reviews and geotechnical investigations, undertaken between 2001 and 2004 by SKM, showed that these winter recesses would have created weak layers, increasing the potential for piping due to the lack of a filter. This was highlighted by the large deformations which occurred at the end of construction in 1918. The spillway was assessed as able to pass a flood event with AEP of 1:1,300,000. Given the location of the dam, ANCOLD(2000b) Guidelines suggest the dam should be able to safely pass the PMF flood event. Accordingly, the dam required upgrading to modern guidelines.
The 2005 detailed design of the upgrade included the construction of a 70m wide unlined spillway, construction of filters on the downstream face of the dam with a stabilisation (weighting) fill, installation of instrumentation and seismic protection of the outlet tower. The construction of these works is currently underway.
Janice H. Green and Jeanette Meighen
The Probable Maximum Precipitation (PMP) is defined as ‘the theoretical greatest depth of
precipitation that is physically possible over a particular catchment’. The PMP depths provided by
the Bureau of Meteorology are described as ‘operational estimates of the PMP’ as they represent the best estimate of the PMP depth that can be made, based on the relatively small number of large events that have been observed and our limited knowledge of the causative mechanisms of extreme rainfalls.
Nevertheless, the magnitudes of the PMP depths provided by the Bureau are often met with scepticism concerning their accuracy when compared to large rainfall events which have been observed within catchments and which are, typically, only 20% to 25% of the PMP estimates. The recent increases in the PMP depths, resulting from the revision of the Generalised Tropical Storm Method (GTSMR), have served only to entrench this cynicism.
However, analyses of the magnitudes of the storms in the databases adopted for deriving PMP depths show that these observed storms constituted up to 76% of the corresponding GTSMR PMP depths and up to 80% of the Generalised Southeast Australia Method PMPs for the storm location. Further, comparisons of the PMP depths to large storms observed in similar climatic regions around the world indicate that the PMPs are not outliers.
The results of these analyses are presented for a range of catchment locations and sizes and storm durations and demonstrate that the PMP estimates provided by the Bureau of Meteorology are reasonable and are not unduly large.
This paper reviews the general principles of duty of care which assist in the understanding of responsibilities that may exist for surveillance of dam safety, including the inter-play of the common law and statutory law. Only when there is a foundation in the general principles can obligations upon dam owners/operators with respect to surveillance and instrumentation be interpreted. Some legal issues around the development and use of industry guidelines are also explored.
Dr. J. M. Rüeger
After a brief review of the origin and early days of the technique, the present role of geodetic deformation measurements is discussed. The design of geodetic measurement schemes is then considered, followed by a review of geodetic measurement, analysis and reporting techniques. An overview of the important discussions, that need to take place between engineers and surveyors in the design phase, follows. This covers the definition of the engineering needs and the resolution of surveying issues.
Peter Hill, Rory Nathan, Phillip Jordan, Mark Pearse
This paper outlines the development and application of the Risk Analysis Prioritisation Tool (RAPT) which has been developed as an interactive tool to aid dam safety risk management. RAPT allows the risk profile and prioritisation of upgrades to be incrementally updated as inputs are refined. The paper outlines some of the requirements of a risk management tool and the resulting functionality of RAPT and the lessons learnt from its application to more than 75 dams.
Issues covered include:
B Simmons, N Mudge
In 2004 the NSW Government released its Metropolitan Water Plan (MWP). This plan detailed the government’s initiatives to secure Sydney’s water needs during the current drought and into the future. The MWP outlined a range of both demand and supply side measures. These included modification to Warragamba and Nepean dams so that the water at the bottom of the dams that is currently unavailable for water supply can be accessed.
Accessing this deep water will increase the available water supply by an additional six months in the immediate drought and will provide, on average, an additional 40GL/annum to our long term available water supply.
The Warragamba Dam Deep Water Access Project involves accessing and transferring water from deep in Warragamba Dam to the existing water supply system.
Phase One of the project saw an abandoned underground pumping station 1.5km downstream of the dam wall, being enlarged and upgraded to pump water from the low level pipeline into the existing water transfer pipelines.
Phase Two of the project involved making a penetration low on the dam wall, some ninety metres below full storage level to access the deep water. This enabled the water to flow into the new pumping station, through an existing underground pipeline.
This project and in particular Phase Two was extremely unique due to the saturation diving systems and specialist tooling systems needed to create the penetration in the dam wall. The project provides a reference point for the water industry for future similar works.
This paper describes the project that was initiated at Warragamba Dam to access the deep water and is focused on the extremely difficult and unique works associated with creating the low level penetration in the dam wall.