Jamie Campbell, Gregg Barker, Paul Southcott and Michael Wallis
The assessment of consequences of dambreak is used as input to the design parameters of dams, dam safety requirements and dam risk assessments. For many low consequence category dams, the consequences of failure can be dominated by itinerants, in particular vehicles on roads within the dambreak inundation area. Estimating the population at risk (PAR) and potential loss of life (PLL) rigorously is mathematically complex, requires significant user judgment and can be very sensitive to input assumptions. This paper presents a simple, practical tool that has been developed to assist engineers and analysts in assessing the PLL of itinerant road users within a dambreak inundation zone. The tool allows for a logical and defensible analysis based on an event tree approach and provides guidance on appropriate factors to be used in calculating the overall fatality rate of people exposed to the dambreak hazard. This paper details the tool and how to apply it to typical dambreak problems, providing the reader with the information required to estimate the consequences on itinerant road users; the paper also details how the concepts discussed can be applied to other itinerants.
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Peter Buchanan, Damian Nott, Martin Weir and John Dymke
The Bulk Water Alliance (BWA) consisting of ACTEW and ACTEW-AGL, GHD, and John
Holland/Abigroup, was formed to deliver the Enlarged Cotter Dam project in Canberra,
ACT. This project consisted of the construction of an 87 m high RCC dam and two
saddle dams, 15 m and 20 m high, to provide additional capacity to the ACT‘s water
supply system. The project is scheduled to be completed in September 2013.
During construction, the dam site was subject to three significant flood events which
affected the construction program. The March 2012 flood, the largest of the three, also
indirectly caused the formation of longitudinal cracks at the top surface of the RCC, when
the dam had reached about 45 m in height.
This paper first looks at the consequence of the flooding on both the design and
construction of the dam; in particular the modification of the diversion strategy and the
impacts on the final dam arrangement. The risk mitigation strategies put in place,
including the construction of a significantly larger diversion conduit through the partially
completed dam, are also discussed. The paper then focusses on the formation of
longitudinal cracks in the dam; the cause of cracking, analysis of the likely extent of
cracking, and the treatment of the cracks to minimise the risk of any significant long-term
impacts on the safety of the dam.
Finally the paper will discuss lessons learned from constructing the Enlarged Cotter Dam
during a period of above average rainfall.
Kinchant Dam is a zoned earth and rockfill embankment situated on the north branch of Sandy Creek, approximately 30 km southwest of Mackay in central Queensland. Kinchant Dam was constructed in stages. The ‘Initial Development Stage’ which consisted of an embankment length of approximately 3.3 km and full supply level (FSL) of EL 49.21 m AHD was completed in 1977. Further development completed in 1986 (Stage I) increased the FSL to EL 57.21 m AHD with an embankment length of 5.5 km and a maximum embankment height of 22.3 m. The dam has a storage capacity of 62,800 Ml and a 60 m wide emergency spillway with a fixed crest level of EL 58.21 m AHD, one metre higher than the FSL.
A series of investigations have been carried out since its construction as a consequence of both regulatory safety reviews and observed excessive pore pressures within the foundation that have led to wet patches developing at the toe of the dam. In one area at the toe, pore pressures were such that artesian conditions developed. This paper outlines the history of various stages of construction of the dam, the foundation investigations since construction and the safety review and comprehensive risk assessment process that lead to the upgrade design and construction of remedial works. The remedial works include the extension of the downstream filter material adjacent to the clay core and the provision of additional pressure relief wells at the downstream toe of the dam.
Nanda Nandakumar and Stephen Farrelly
Fuseplug auxiliary spillways are used to increase the discharge capacity in dam upgrades for flood security. Hydrologic level-pool routings are used to determine the size and trigger levels for fuseplugs. In the level-pool routing, the water surface from the body of the storage to fuse bays is generally assumed to be horizontal and any drawdown effects on the water level are neglected. This paper assesses the validity of this assumption using the CFD model results for Keepit Dam. It is shown that equal spacing of trigger levels can result in premature activation, and the drawdown effects need to be taken into account in determining spacing of trigger levels. It was also shown that the design water levels for the intermediate AEPs are underestimated.
A comparison of inflow and outflow frequency curves showed that peak outflows can exceed the peak inflows due to fuseplug operations, but the downstream impact is expected to be negligible due the size of the flood in which the peak outflow will exceed the peak inflow.
Peter A. Ballantine
In view of the need for a safety upgrade for the Quipolly Dam and the plans of Liverpool Plains Shire Council for future growth in water supply, the Council took the opportunity to increase the storage capacity of the dam by raising the full supply level by 2.0 m. In 2009, the Council appointed GHD to design the upgrade in accordance with the ALARP principle.
The design of the dam included an innovative vertical crest wall, embedded into the embankment, a concrete-lined auxiliary spillway placed over the embankment adjacent to the existing spillway and the installation of Hydroplus Fusegates in the existing spillway channel.
This paper describes the design of the upgrade works.
Jeong Yeul, Lim
For various historical reasons and some technical reasons, the safety of dams has been evaluated using an engineering standards-based approach, which was developed over many years. It was used initially for the design of new dams, but increasingly has been applied over the past few decades to assess the safety of existing dams. Some countries have carried out risk assessments of existing dams that included both the structural and hydraulic safety of the dam and social risk. These methods developed by other countries could be adapted to assist in decision-making for dam safety management. Unfortunately, methods for risk assessment of dams were not established in Korea. This study outlines a beginning risk analysis for structural safety management. The first stage consisted of research on the present domestic dam safety guidelines and reviewing operations for management systems of dam safety abroad. Also, dam risk analysis requires reliable data on dam failure, past construction history and management records of existing dams. A suitable risk analysis method of dams for structural safety management in Korea is use of event tree, fault tree and conditioning indexes methods. A pilot risk assessment was carried out for two dams. The dam risk assessment process was thus established, and we learned the importance of risk assessment. The future includes additional research and risk analysis to develop the system.