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
Michael Bassett-Foss , David Bouma , Dewi Knappstein
The Wairarapa Water Use Project (WWUP) in the southern North Island, New Zealand, is investigating new water storage schemes involving large dams that will allow the community to make use of the water resources that are currently available, but not necessarily available at the time they are needed. It is estimated that the 12,000 hectares currently irrigated in the Wairarapa could be increased to about 42,000 hectares depending on actual demand. The WWUP provides for a range of possible needs, such as supply of new areas of irrigation, increased reliability for existing irrigation and frost fighting, environmental augmentation of low summer river flows, environmental flushing flows, stock drinking water, power generation, municipal water supply, and recreational use.
WWUP objectives include early engagement of stakeholders, early integration of financial, social, cultural and environmental factors in decision-making, management of uncertainty associated with the preliminary level of investigation and evolving regulatory framework, development of an equitable framework for efficiently comparing options, and balancing long and short-term considerations.
A large number of dam options were identified, storing 3 to 80 million m3 of water, and progressively narrowed to a shortlist of 2 sites through a complex process of concept development, desktop studies, site visits, hydrological analyses, cost estimates and multi-criteria analyses.
The WWUP demonstrates how sustainable new major water storage schemes can be promoted in a highly regulated environment of a developed nation.
Keywords: Dams, water storage, stakeholder engagement, environment, water allocation, multi-criteria analysis
Richard Herweynen, Tim Griggs, Alan White
The Ministry of Public Utilities, Sarawak, Malaysia used an independent dam safety consultant to advise them on whether the Murum Dam was ready for impoundment. They were looking for a holistic assessment of the dam from a dam safety perspective. As a result, a risk framework was adopted to identify the key issues that needed to be addressed prior to impoundment of the Murum Dam. The process adopted which is presented in this paper, was transparent and defensible; and provided a reasoned approach for which items must be completed prior to the commencement of impoundment. As a result effort was focused on the key activities required prior to impoundment – whether this was the completion of specific works, the availability of key instrumentation to monitor the dams performance, the availability and operation of key dam safety systems, or the appropriate emergency preparedness should a dam safety incident occur during first filling. This systematic process based on a risk based approach, was a useful method of determining the dam’s readiness for impoundment, and provided an excellent way of communicating the importance of activities to the key stakeholders. The authors believe that this method is transferable to other dam projects, for an assessment of a dam’s readiness for impoundment.
Keywords: Dam safety, risk, impoundment, reservoir filling.
Kim Robinson, Andrew Pattle and Thomas Shurvell
Rowallan Dam is a 43m high clay core rock fill dam located in Northern Tasmania. The dam impounds 121GL used for hydro power generation and has a High A consequence category.
Over the summer of 2014/15 major reconstruction works were carried out on the dam to repair a piping incident from 1968. The work entailed reconstructing two sections of the dam down to foundation level and the upper 7m of the 568m dam crest. During the work, the dam was temporarily exposed to a significantly increased flood overtopping risk.
A range of measures were taken to manage the overtopping risk; such as increasing the dewatering capacity of the dam, lake draw down, installation of a sheetpile wall, development of emergency backfill procedures and a flood forecasting system.
The focus of this paper is on the flood forecasting system and how this was integrated into the overall management of overtopping risk during construction. The forecast models were run automatically on a 2 hour schedule using the latest BoM forecast, telemetered lake levels and rainfall from 7 gauges surrounding the catchment. The system provided a continuous 7 day lake level forecast which guided the site team on when to release water to manage the storage.
In the event that the lake level forecast reached a predetermined trigger level, the dam safety team would have been automatically notified and various emergency procedures would have been triggered in response to the flood warning.
This paper discusses the measures that were taken to manage the flood risk, how it worked in practice and conclusions which are applicable more generally to managing overtopping risk during dam works.
Keywords: dam construction flood risk, flood forecasting
Vicki-Ann Dimas, Wayne Peck, Gary Gibson and Russell Cuthbertson
Globally, reservoir triggered seismicity (RTS) is a phenomenon sometimes observed in newly constructed large dams worldwide, for over 50 years now. Over 95 sites have been identified to have caused RTS by the infilling of water reservoirs upon completion of their constructions worldwide. In Australia, there are seven confirmed sites with observed RTS phenomenon that are summarized by temporal and spatial means.
With almost 40 years of seismic monitoring, primarily within eastern Australia, several of Australia’s largest dams have monitored and recorded many RTS events. At present, twelve dams are 100 metres and above in height as possible candidates, with seven of these actually causing RTS and a disputed possible eighth dam.
Important factors of RTS are reservoir characteristics (depth of the water column and reservoir volume), geological and tectonic features (how active nearby faults are and how close to the next cycle of stress release they are temporally) and ground water pore pressure (decrease in pore volume under compaction of weight of reservoir and diffusion of reservoir water through porous rock beneath). RTS is an adjustment process often delayed for several years after infilling of reservoir before eventually subsiding within 10 to 30 years, when seismic activity then returns to its prior state of stress.
Generally there are two type of RTS events, either a major fault near the reservoir most likely leading to an earthquake exceeding magnitude 5.0 to 6.0, or more commonly, a series of small shallow earthquakes.
Seismic monitoring of all dams (except for Ord River) are presented with spatial and temporal series of maps and cross sections, showing the largest earthquake, build-up and decay of RTS events.
Keywords: Seismic monitoring, reservoir triggered seismicity (RTS), earthquake cycle
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.