Kelly Maslin, Mark Foster, Len McDonald
A key requirement of assessing the tolerability of dam safety risks is the assessment of individual risk. The ANCOLD Guidelines on Risk Assessment provides guidance on acceptable levels of individual risk and some general guidance on the calculation of individual risk.
Individual risk is a key measure in the consideration of the tolerability of risk, ALARP and development of risk mitigation works. It is essential that there is consistency in the approach to estimating individual risk used across the dams industry.
This paper reviews the approaches taken to estimating individual risk across the dams industry both locally and internationally as well as the experience of other industries.
The paper includes a review of the various methods for estimating the vulnerability of individuals subjected to flood inundation based on historical fatality rates as well as identification of the individual most at risk
The paper then describes a method that has been developed based on the principles used for assessing individual risk due to other hazards, such as landslides. The method includes consideration of a range of factors such as warning time, temporal variation and vulnerability of the individuals most at risk. The method developed provides a transparent, defensible and pragmatic approach to estimating individual risk. Practical guidance and examples are also provided on the application of the method.
Keywords: individual, risk, exposure, fatality
David Hilyard, William Ziegler, Heather Middleton
New South Wales has a significant number of dams, including major water supply dams, located over or near mines. Mining near dams imposes dam safety risks including: mine subsidence, mine blast vibration, presence of mine personnel downstream, rapid changes in consequence during mining, and loss of stored waters. The NSW Dams Safety Committee(DSC) regulates mining near dams, using risk assessment to review applications to mine near dams. A structured approach allows rational, evidence-based decision making by stepping through a procedure involving: initial consultations, screening risk assessment, evaluation of technical arguments, risk assessment, and development of risk management strategies. The risk assessment for dam walls develops acceptance criteria, reviews 19 possible risks to dam walls, and site-specific hazards. For potential for loss of stored waters, four possible groups of flow paths from storage to underground mine are reviewed; flows are evaluated with Monte Carlo simulation in terms of tolerable loss. Risks are assessed from a dam engineering viewpoint, which may be more conservative than the perception of risk in the mining industry, considering both tolerable risks and operational time frames. Case studies include: a tailings dam 100 m upstream of an active open cut and underground portal was undermined by longwall mining, with about 1.5 m subsidence of parts of the embankment as each of four longwall panels was extracted; longwall mining beneath a major Sydney water reservoir, with no observed impact on the stored waters; and open cut mining immediately downstream of a mine water dam. Risk-based methodology has provided the DSC with increased confidence in reviewing applications to mine near dams.
Keywords: Mining, dams, risk assessment, New South Wales, Dam Safety Committee
Mike Phillips, Kelly Maslin
A spillway upgrade conceptual design and selection process was undertaken to identify options for upgrading the Dartmouth Dam to pass the Probable Maximum Flood (PMF). A number of upgrade options were investigated, including variations of dam raise heights and spillway modifications. One of the options, the piano key weir, was initially developed from the limited available publications on the weir design, and further developed with the use of a 1:60 scale model. The piano key weir, a variation of the labyrinth weir, is a passive spillway that utilises a total weir length several times that of the effective spillway width. For the Dartmouth Dam study, the piano key weir design that was developed consisted of a 7-cycle, 9 m high structure, with a total weir length of nearly 600 m, or more than 6 times the existing effective spillway width of 91 m. The spillway was designed to pass the routed PMF outflow of approximately11,500 m3/s with a head of approximately 11 m.
The piano key weir design was developed using the following analyses:
Initial 1:60 scale physical model of the piano key weir based on published papers on piano key weirs and design manuals for labyrinth weirs;
Structural analysis and weir member sizing using initial physical model results;
Computational Fluid Dynamics (CFD) modelling to improve the hydraulic efficiency of the weir for the range of flows;
Revised 1:60 scale physical model of the piano key weir; and
Confirmation of conceptual structure design.
This paper describes the process of developing the piano key weir option for the Dartmouth Dam spillway and lessons learned.
Keywords: Piano key weir, CFD, spillway, physical model
Shane McGrath, Andrew Reynolds, Garry Fyfe, Chris Kelly, Steven Fox
Goulburn-Murray Water is a rural water corporation located in Northern Victoria. It has responsibility for 12 State dams and is also the constructing authority for the Murray Darling Basin Authority’s Victorian assets.
Over the past 15 years G-MW has been engaged in a dam improvement program across its portfolio. To date 14 individual projects have been undertaken at 11 dams. The total expenditure is $125 million.
Starting from a base level of data at its inception in 1997, the program has encompassed all facets required for a dam improvement program. From early prioritisation to set the investigation program, through design reviews and risk assessments to develop the upgrading program and subsequent implementation. Some elements of the program were at the leading edge of practice at the time and a range of experiences along the way were character building as dam safety investment challenged other corporate priorities.
This paper sets out the lessons learned in developing the methodology and implementing the program of works, particularly relating to corporate adoption of the program, organisational capability, investigations, risk assessments, design and implementation.
Dr Andy Hughes
This paper will outline changes currently being implemented to the UK legislation via the Flood & Water Management Act 2010. This legislation has driven a change towards a risk based approach.
Significant consultation with the profession and owners has provided an interesting insight to the different and disparate views of owners, engineers and the public.
The guidance documents associated with the Act associated with floods and risk management are currently being rewritten and will be completed by the time of the conference in Perth and so progress with reformatting of those documents will be reported upon.
Keywords: Legislation, guidance, consultation
Chi-fai Wan, Jason Hascall, Andrew Richardson, John Sukkar
Oberon Dam is the major headwork of the Fish River Water Supply Scheme providing bulk water supply to Oberon Shire and Lithgow City Councils, Sydney Catchment Authority, and Delta Electricity. The dam is owned and operated by State Water Corporation (SWC).
Located on the Fish River 2km south of Oberon in New South Wales, Oberon Dam was completed in two stages in 1946 and 1957. In 1996 the dam was upgraded to pass the 1993 Probable Maximum Flood estimate by raising the dam 1.77m and constructing a 50m wide auxiliary spillway on the left abutment. The upgraded dam comprises a 232m long, 35.3m high concrete slab and buttress section and a 165m long earth embankment section.
A typical buttress dam has its inclined upstream face made up of relatively thin reinforced concrete slabs supported by but not integral with the buttresses, making a relatively flexible dam structure vulnerable to earthquake damage.
As buttress dams evolved from concrete gravity dams, their structural design follows the same principles as applied to gravity dams. However, many buttress dams were designed over 60 years ago using outdated methods that did not consider earthquake loads. Current overseas and local design guidelines do not provide sufficient guidance for checking the seismic stability of existing buttress dams. For instance, the simplified seismic analysis, proposed by Fenves and Chopra to investigate the seismic response of gravity dams to earthquake loads in the upstream-downstream direction, is not applicable to buttress dams which are also susceptible to damage by earthquake loads in the cross-valley direction.
SWC engaged Black & Veatch to carry out a three-dimensional finite element analysis of Oberon Dam to better understand the structural behaviour of the dam under earthquakes. The analysis used both the response spectrum and time history approaches. Due to the uncommon design of Oberon Dam and the limited discussion found in the literature on the dynamic behaviour of buttress dams, the Authors would like to share their experience in the assessment of the hazard, and on the use of modern finite element modelling techniques to investigate the dynamic response of this type of dam.
Keywords: Ambursen dams, Buttress dams, Risk assessment, Time history analysis, Finite element