Peter A Ballantine, Christopher V Seddon
Massingir Dam, constructed in the late 1970’s on the Olifants River in Mozambique, is a 48 m high zoned earthfill dam. Due to various safety concerns, the dam was operated at a reduced full supply level of 110 masl, compared to the design full supply level of 125 masl. Between 2004 and 2006 remedial works were undertaken, including the construction of a berm on the downstream face of the dam, grouting and drainage of the foundations and installation of the spillway crest gates. From December 2005 the storage level of the dam was allowed to increase.
On 22 May 2008, with the reservoir storage level at 122.43 masl and the gates on the outlet conduits closed, the reinforced concrete conduits failed at the downstream end, releasing an estimated 1,000 m3 /s of water into the Olifants River.
A 2-D finite element analysis was undertaken in order to establish the safe load bearing capacity of the as-constructed conduits. On the basis of the analysis, it was concluded that the original design did not take proper account of the pressure that would develop within the thick concrete sections of the conduit. In view of assumptions regarding the load paths, the reinforcement was not placed in the most appropriate positions.
This paper describes the events leading up to the failure of the conduit, presents the findings of the investigation into the failure and makes recommendations on the basis of the findings.
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Kristen Sih, Peter Hill, Susan Ryan, Siraj Perera
Although ANCOLD provides guidance on good dam safety practices, in Australia it is the State and Territory Governments’ role to protect the public from dam safety incidents and in many cases these jurisdictions have legally binding regulations in place that dam owners must adhere to. This paper presents a comparative analysis of the dam safety regulations currently in place for Australian states, as well as selected international jurisdictions. The limit of applicability of the regulations, number of dams regulated, content of the regulations and powers and responsibilities of the regulator are all compared. It was found that there is a large range within each of these categories with regulatory approaches varying from light-handed and objective based, to highly prescriptive. The extent to which risk management principles are used in the regulations for each jurisdiction has also been investigated. It was found that in jurisdictions where higher hazard category dams account for a higher proportion of dams being regulated, risk analysis is included in the regulations. Finally, the ANCOLD societal risk criteria and ALARP considerations have been compared and contrasted with those from international jurisdictions and other hazardous industries.
Rick W. Schultz P.E.
The Corps of Engineers Risk Management Center is undergoing a nationwide assessment of its navigation and flood control projects. Development of the methodology and tools used to determine probability of failure of mechanical and electrical systems for dams is being presented in this document. Development of the Weibull formulas for specific use in dam will be addressed along with use of fault tree analysis to determine system reliability.
Keywords: Dormant-Weibull Formula, Fault Tree, Characteristic Life of Components, Beta Shape Parameters, Inspection intervals.
Shao Kwan Ng
Asset management aims to ensure that assets, such as dams, are sustainable. In order to achieve this, management decisions need to be defensible and the long-term impacts of short-term decisions need to be clearly demonstrated, such that an asset operates and is maintained in an appropriate fashion and in a satisfactory condition. Expert rule systems are becoming widely recognised as powerful and elegant tools suitable for engineering and management decision-making. They are powerful, transparent and flexible tools that mimic how people make decisions, and hence provide a natural way of thinking for decision-making. This paper reviews the current usage of expert systems in asset management, and illustrates the potential of these tools, in conjunction with the available (ANCOLD) guidelines, to assist dam owners in decision-making, such as in condition evaluation and dam hazard assessment applications.
Keywords: Decision-making, expert rule systems.
Richard Herweynen, Colleen Stratford
Assessing the potential for erosion of foundation rock downstream of a spillway is a problem faced on many dams, whether new or existing. The problem is made particularly difficult not only due to the uncertainty in determining the erosion potential of the rock, but also due to the variable hydrologic characteristics of flood events.
The selected spillway option for Wyaralong Dam comprises a centrally located primary spillway with a secondary spillway located on the left abutment. A stilling basin energy dissipater is provided at the toe of the primary spillway. Downstream of the secondary spillway, an apron channel will direct flows back to the stilling basin. However, for flood events larger than the 1 in 2000 AEP event, the capacity of the secondary spillway apron is exceeded and flows spill out across the left abutment of the dam towards the river channel. Erosion of this left abutment was viewed to be a potential dam safety issue, and as such, careful consideration was required during the design stage to determine the acceptability of this spillway arrangement.
In order to provide structure to a problem which often relies solely on engineering judgment, a decision process was developed, taking into consideration some of the more definable aspects of the problem. These aspects included the geological characteristics, the initial hydraulic characteristics, the flood duration, the nature of erosion should it occur and the stability of the dam. This paper describes the decision process and methodology used at Wyaralong Dam to
determine the acceptability of erosion. This paper will present the process in a way that it can be used by others in future dam projects, both new and upgrades.
A Unique and Holistic Approach to the Erodibility Assessment of Dam Foundations
After a period of drought for many years, inflows during May and June 2009 resulted in releases from North Pine Dam. These releases resulted in deaths of fish downstream of the dam wall including lungfish. The Australian Lungfish is a protected species under the Environmental Protection and Biodiversity Conservation Act 1999 (Australian Government). The events of 2009 have shown, however, that a proactive response supported by sound knowledge is required to minimise lungfish losses from flood events and other dam operations activities. A framework has been developed for the management of lungfish populations in Seqwater storages. The framework centres on a Seqwater Fish Management Policy, and four broad strategies that are considered necessary for addressing fish management in Seqwater storages: Fish Management, Storage Operations, Communication, and Research. These strategies are being used as a basis for identifying, planning and managing a range of actions designed to ensure that impacts to lungfish are minimised. Seqwater intends to develop the framework further to include long term management initiatives such as implementing viable technologies for preventing lungfish strandings, habitat protection and restoration activities that support viable lungfish populations, as well as establishing priorities for managing risks to other aquatic vertebrates in Seqwater storages, including other protected species, recreationally and commercially important species; e.g. turtles , carp, mullet, etc.