Chas Keys and Steve Opper
As the legislated ‘combat agency’ for dealing with floods, the NSW State Emergency Service has had considerable experience in planning for flooding on the state’s rivers and in developing arrangements to help keep people safe when floods occur. This experience has been put to use over the past decade in the particular context of managing floods caused or exacerbated by dam failure. Some of the complexities of the dam-failure planning problem are explored in this paper, specifically as they relate to warning and evacuation tasks and to the issue of preparing communities for the extreme flooding which dam failure can be expected to cause. The points are made that warning is not just about mechanical alerting devices, evacuation is not restricted to commanding people to move, and public education requires a sensitive comprehension of the problems of disseminating information about rare and difficult-to-believe events.
Tom Ryan, Charles Todd and Simon Nicol
The potential impacts of cold water releases on the downstream thermal regime include: reducing the seasonal temperature range (lowering of the maximum and raising the minimum); reducing the diurnal temperature variation; rapid temperature changes; and delaying the seasonal warming of stream temperatures. Recent investigations have identified at least 20 large dams within Victoria, that have the potential to release cold water from below the hypolimnium. A monitoring program is currently being implemented in Victoria to identify the occurrence of cold water releases and to estimate the extent of the downstream impacts.
Cold water releases have been shown to impact the biological processes within aquatic ecosystems and consequently reduce the natural productivity. The physiological development of native freshwater fish can be impacted in a number of ways. Growth and reproductive development of adult fish is impacted while the survival of eggs and larvae can also be retarded. As a result, the sustainability and viability of native fish populations are greatly compromised.
Using stream temperature data from the Mitta Mitta River downstream of Dartmouth Dam, the decline of the native fish populations, due to cold water releases, can be demonstrated under current operating conditions. The decline in population numbers can be further demonstrated with the use of a simple age-based population model for Murray Cod. The spawning opportunity and survival of egg and larvae can be improved for Murray Cod by increasing the overall spring release temperatures by 2, 4, 6 and 8 oC. The population model adjusted for these thermal improvements, results in increased survival prospects for the Murray Cod population.
David Brett, Anton van Velden and Phil Soden
The Main Creek Tailings Dam is a 60m high earth and rockfill dam constructed during the early 1980’s to store tailings from the Savage River Mine on Tasmania’s west coast. The dam served the mine well for nearly 20 years, storing around 32 million m3 of tailings, but has required raising due to the expanded mining plans of the current operators, Australian Bulk Minerals (ABM). ABM believe that the mine could require a further 60 million m3 of tailings storage over the next twenty years at increased production levels. This could be stored in the Main Creek Dam by raising it by around 35m. In the medium term this scale of raising would be feasible using waste rock product from ongoing mining but in the short term of several years an interim solution would be required. The feasibility of upstream construction on the tailings beach was reviewed and found feasible for
a maximum 12m in 4 lifts.
Of critical concern were
The paper discusses the investigation and design phases of the dam and describes the issues arising during construction recently completed over the period January to April 2002. The use of pore pressure, shear strength changes and tailings beach movement monitoring to control construction is discussed.
R.J. Nathan, P.E. Weinmann and P.I. Hill
Current practice for estimation of design floods is typically based on the “design event” approach, in which all parameters other than rainfall are input as fixed, single values. Considerable effort is made to ensure that the single values of the adopted parameters are “AEP-neutral”, that is, they are selected with the objective of ensuring that the resulting flood has the same annual exceedance probability as its causative rainfall. While this approach represents current best practice in Australia (and overseas), it does suffer from a number of limitations.
This paper describes the development and application of a Monte Carlo (or joint probability) framework which offer an alternative to the design event method. This technique recognises that any design flood characteristics (e.g. peakflow) could result from a variety of combinations of flood producing factors, rather than from a single combination. The approach mimics “mother nature” in that the influence of all probability distributed inputs are explicitly considered, thereby providing a more realistic representation of the flood generation processes.
The advantages of the technique are illustrated by application to a hypothetical dam located on a real catchment. The manner in which standard design inputs are incorporated are discussed, as is the relationship of the approach to current guidelines.
Water storage dams influence the lives of a large number of people. This influence may be through provision of essential water supply or risk of dam failure during sunny day or extreme flood scenarios. It is therefore imperative that these structures are managed in a responsible with a clear understanding of the associated uncertainty. In view of the large capital cost of the structures involved, this understanding is important to ensure that, where necessary,
practical and cost effective solutions are achieved. The NSW Dams Safety Committee largely regulates the management of dams in New South Wales, however, dam owners have the opportunity to display individual initiative in this process.
The Hunter Water Corporation (HWC) is a water authority based in Newcastle, New South Wales, responsible for the supply of water and wastewater services for over 470,000 people. HWC has realised, as a responsible dam owner, that safety improvements are a continuum over the life of the structure. Chichester Dam is an example of this on-going safety improvement process that is illustrated through the principle of ALARP in a risk assessment approach.
Richard R. Davidson, Shane McGrath, Adrian Bowden, Andrew Reynolds
Goulburn-Murray Water (G-MW) manages thirteen major dams for the State of Victoria. As part of its Dam Improvement Program (DIP), five priority dams were identified for detailed safety and performance evaluation. Over the last three years, the design reviews have been completed and a series of dam safety issues have been identified which pose societal and financial risk. Substantial financial resources will be required to be applied over a considerable period to bring these dams into compliance with established international and Australian standards. Which of these dam safety issues should be addressed first? In what sequence and with what urgency should the actions be implemented? Can cost-effective interim targets be set? How can the remaining eight
dams, which could also pose societal and financial risk, be prioritised for future detailed investigation? To answer these questions a quantitative risk assessment approach was used. The approach utilised expert engineering and consequence panels and included input to and review of the process and outcomes by a stakeholder reference panel reporting directly to the Board of G-MW. The implementation of a strategic risk management process has now begun to progressively and systematically reduce the dam safety risk across the entire dams portfolio. This process recognises that available funding for risk reduction measures is very limited, so the highest risks are reduced in an incremental fashion to achieve interim risk targets and eventually meet contemporary dam safety standards.