David Brett, Robert Longey, Jiri Herza
The independent expert review panel for the Mount Polley Tailings Storage Facility failure came out strongly recommending changes to the technology of tailings dams in British Columbia (and by inference, world-wide). The Panel had examined the historical risk profile of tailings dams in British Columbia and recommended, amongst other things, that best available technology (BAT) be adopted for tailings disposal. Examples of BAT, described by the panel, included “dry-stacking” of filtered, unsaturated, compacted tailings and reduction in the use of water covers in a closure setting. The recommended technologies would require a major shift in current practice and raises many questions, such as:
– Are these recommendations appropriate in Australia?
– Does this signal the end of the tailings dams as we know them?
– Do the current Australian National Committee on Large Dams Guidelines (ANCOLD) apply to these new BAT technologies?
– If not, is there a role for ANCOLD in setting standards for the future?
This paper discusses the Mt Polley tailings dam failure and searches for answers to these questions. In particular, this paper reviews the background to “dry-stacking’, to explore the implications for the Australian mining industry.
Keywords: Tailings Dam, Dry Stacking, Best Available Technology
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Paul Southcott, Tim Griggs & Jamie Campbell
Suma Park Dam is the principal water supply dam for the City of Orange in central NSW. The 30m high single curvature concrete arch dam has a High A consequence category and required an upgrade due to an inadequate spillway capacity. To maximise the benefits of this major capital works, the Council also sought to increase the storage capacity and modernise the outlet works to help supply the rapidly growing population of the city.
Challenges that needed to be overcome to develop an affordable and safe solution included: very high flood inflows; limited freeboard; a highly stressed arch with a narrow crest width; poor access to the toe and right abutment; and a saddle dam located on a deeply weathered foundation.
Innovations incorporated into the design of the works included: Monte-Carlo based modelling of the flood hydrology that better estimated the design inflows resulting in a significant reduction in flood upgrade requirements; precast parapet crest units that incorporated crest widening to improve constructability; an anchored toe block to ensure the toe of the arch is stable; an upgrade to the stilling basin; and an auxiliary spillway incorporating Fusegates at the saddle location designed only to operate in floods in excess of the 1;1,000 AEP event with minimal loss of storage.
Construction of the works is now well underway. A number of challenges have been overcome during the construction stage including a re-design of the auxiliary spillway to use Fusegates and discovery of Naturally Occurring Asbestos (NOA) on site. Construction of the upgrade works is expected to be completed by the end of 2015.
Keywords: Concrete arch dam, flood upgrade, pre-cast, fuse gates, anchoring.
A. Scuero, G. Vaschetti, J. Cowland, B. Cai , L. Xuan
Nam Ou VI rockfill dam is part of the Nam Ou VI Hydropower Project under construction in Laos. The scheme includes an 88 metres high rockfill dam, designed as a Geomembrane Face Rockfill Dam (GFRD), which when completed will be the highest GFRD in Laos. The only element providing watertightness to the dam is an exposed composite PVC geomembrane, installed according to an innovative design now being increasingly adopted to construct safe rockfill dams at lower costs. The same system will shortly be installed on a water retaining embankment for a coal mine in NSW, Australia, and has been approved for a tailings dam in Queensland, Australia. At Nam Ou VI the geomembrane system is being installed in three separate stages, following construction of the dam. The first two stages have been completed, and the last stage will start in November 2015. The paper, after a brief discussion of the adopted system’s concept, advantages and precedents, focuses on the construction aspects.
Keywords: GFRD, PVC geomembrane, waterproofing, rockfill dam.
Steven E Pells, Philip J N Pells, William L. Peirson; Kurt Douglas and Robin Fell
The method of Annandale (1995) is widely used by Australian practitioners for the assessment of erosion in unlined spillways. This method is based on comparison to various case studies, where the geology at each site is characterised using the Kirsten index (a rock mass index previously developed to assess the rippability of rock), and the hydraulic conditions are characterised using the unit stream power dissipation. In this paper, the historical development of this comparative design technique is traced and is critically reviewed against the original geotechnical and hydraulic data, and against a new, independent, dataset gained from unlined spillways in fractured rock in Australia, South Africa and the USA. It is shown that, while erosion can be usefully correlated against rock-mass indices and hydraulic indices, this ‘comparative’ design technique has been promoted beyond its reach – the data do not support the inference of an erosion ‘threshold’ as presented by Annandale (1995). It is argued that this type of analysis should be used only as an initial ‘first indication of erosion potential’, as originally proposed by van Schalkwyk (1994b).
Keywords: scour; erosion; spillways.
Sarah McComber, Peyman Bozorgmehr
Boondooma Dam is a concrete-faced rockfill dam with an unlined, uncontrolled spillway chute. Construction was scheduled for completion in 1983; however a spill event occurred during the last stage.of construction Following this spill event an Erosion Control Structure (ECS) was built across the spillway chute to help mitigate any future scouring.
The spillway performed as expected during minor spill events in the 1990s and early 2000s. During the significant rainfall event of 2010/11, significant scour occurred to the spillway chute and downstream of the ECS, as a result of the spillway operation.
Following the 2010/11 flood, emergency repairs were made and long term repair solutions were investigated. However, during Tropical Cyclone Oswald in January 2013, the dam experienced the flood of record, and further scour occurred in the spillway chute.
The long term repair solution was reviewed in light of the 2013 damage. A solution is required that would satisfy the engineering problem and prevent further damage, while satisfying the commercial considerations faced by dam owners, insurers, customers and downstream stakeholders.
Keywords: Boondooma Dam, flood damage, scour damage, commercial engineering solutions.
Bronson L McPherson, Eric J Lesleighter, David C Scriven, Erik F R Bollaert
A number of medium to major floods in Queensland caused substantial scour around spillway structures. This included the Paradise Dam primary spillway which experienced significant scour of the rock body below the spillway during flooding in January 2013. The occurrence has led to a series of evaluations of the geology, and the prevailing hydraulics behaviour as part of a process to determine the scour mechanism, and to determine the response of the spillway and areas downstream to future floods of larger magnitude. Part of the process has been to utilise a large-scale physical model to obtain transient data which together with the detailed geologic assessment would be incorporated into the comprehensive scour modelling procedures developed by Dr Erik Bollaert, AquaVision Engineering, Switzerland.
The paper will describe the design and construction of the physical model with special features to obtain pressure transients from more than 60 transducers, and velocity transients in more than 40 locations using Acoustic Doppler Velocimeter (ADV) instrumentation. The features of the rock scour will be discussed and the geology of the area below the spillway apron will be described. The range of discharges, and the model’s results including the pressure and velocity characteristics will be described in detail to illustrate the violent nature of the turbulence in the energy dissipation zone. The paper will go on to describe the computational scour modelling procedures of calibration and application, demonstrating a “system” approach to spillway scour analysis for plunge pools and similar situations with energy dissipation on natural materials.
Keywords: Spillways, flood hydraulics, hydraulic modelling, rock scour, transients, numerical analysis, energy dissipation.