Dr Andy Hughes
Tailings dams continue to undergo failures at an unacceptable rate compared to water storage dams, including failures at operations owned by high profile mining companies.Tailings dams have often a different form and method of construction than water storage dams in that tailings dams continue to be raised over time as part of the mine operations and rise to considerable heights. These failures are often the result of a combination of design, construction and operations actions that are controlled by humans and must be better coordinated and managed in the future. The consequence of failure can be widespread flows of tailings and water over the landscape and water courses. This can have extreme consequences in terms of life loss, environmental damage, social license to operate, company value, and mining industry sustainability. Therefore,it is necessary that the mining industry strive for zero failures of tailings facilities. Any additional technology and information that enables an owner of a tailings dam to be more certain of its condition and thereby reduce the risk of failure is of tremendous value to reliable tailings and mine water management.The Willowstick method uses low voltage, low amperage, and alternating electrical current to directly energise the groundwater by way of electrodes placed in wells or in contact with seepage or leaks. This approach has been successfully used to identify water flow paths through, under and around tailings dam in plan and elevation.The Willowstick technology provides additional information to supplement the geological, geotechnical and hydrological, evaluations analyses and designs, and to further improve tailings dam safety by more robust designs if necessary. This paper, using several tailings dam case studies, illustrates the procedure, findings, and the benefits of the Willowstick methodology. The findings of many Willowstick surveys range from tailings dams where the methodology has confirmed the design evaluations, to tailings dams where new groundwater and leakage flow paths were identified. In the latter case, the dam designers were able to update the designs, based on the new information,to mitigate the identified risks and to improve the overall safety of the tailings dams in accordance with the goal of zero failure.
In recent years, there has been greater expectations of waterway barriers to have more consideration of environmental factors; in particular associated with facilitating biopassage across the site.
The scope of the Gympie Weir Detailed Design Project included facilitating biopassage to as many aquatic species as possible,including the protected Mary River Turtle,while maintaining the required gauging accuracy and public amenities.Very rarely has a turtle been observed successfully traversing a fishway, as the designed velocity and depth criteria required to optimise fish passage is unsuitable for turtle passage. Thus for Gympie Weir,a leading-edge innovative solution was designed.
The design incorporates a low flow trapezoidal fishway chute, high flow rock ramp and turtle ramp. The turtle passage is the first of its kind and includes a curved concrete ramp with a variety of textures, two resting pools with protective niches, and limited vertical drop heights. Construction of the weir is intended for the near future and ongoing monitoring will be critical to assessing and improving performance, as well as contributing to the scientific knowledge base for future designs.
Richard Herweynen, Suraj Neupane, Paul Southcott and Ashish B. Khanal
Kathmandu, the capital city of Nepal, is home to more than five million people. Three major rivers including the Bagmati run through the city of Kathmandu, providing the environmental and cultural lifelines for the civilisation and local people. High population growth in Kathmandu over the past 30years has put a serious environmental strain on the Bagmati River. Water is drawn from the Bagmati River for drinking, farming, industries and construction. Due to the lack of capacity in the current sewerage systems, untreated sewage is entering the river system, along with high quantities of rubbish. Although a holy river, the Bagmati River is highly degraded, with reduced flows, high pollution, and a fresh water ecosystem that is now destroyed.To revive the Bagmati River, the Government of Nepal with funding from the Asian Development Bank (ADB), is undertaking the Bagmati River Basin Improvement Project (BRBIP). One of the sub-projects is the construction of a dam on the Nagmati River to store water during the monsoon period for environmental release during dry season.Since November 2015, Entura have been involved in the investigation and detailed design of the Nagmati Dam. Through a simple storage model, it was determined that 8.2Mm 3 of live storage was required to meet the environmental flow objectives. To achieve this storage a 95m high dam was required at the Nagmati site, with a concrete faced rockfill dam (CFRD) determined to be the best option.This paper will present the development of this unique project, highlighting how a number of the challenges were addressed, leading to a sustainable project.
Although the total tailings dam failure frequency peaked in 1960s through 1980s, the failure rate of significant tailings dams has not dropped. The significant tailings dam failures the mining industry experienced in the recent history include: Merriespruit, South Africa, 1994; Los Frails, Spain, 1998; Kolontár, Hungry, 2010; Mount Polley, Canada, 2014; and Samarco, Brazil, 2015. The dam failures may be due to inadequate design, poor construction and inappropriate operations.This paper discusses the lessons learned and some recommendations and good practices to reduce the tailings dam failure risks. It addresses existing issues and provides some recommendations in risk based design, water management-integrity of facilities and water balance modelling, loading rates, tailings farming, adequate governance and roles and responsibilities of designers and nominated engineer.
Mark Pearse, Peter Hill
Risk assessments for large dams and the design of upgrades are often dependent on estimates of peak inflows and outflows well beyond those observed in the historic record. The flood frequencies are therefore simulated using rainfall-runoff models and design rainfalls. The recent update of Australian Rainfall and Runoff (ARR) has revised the design rainfalls used to model floods that are of interest to dam owners. This will change the best estimate of flood frequencies for some dams. However, for most dams the impact of revised design rainfalls on flood frequencies is small compared to other factors that can change (independent of dam upgrades). These include model re-calibrations to larger floods, changes to operating procedures that affect the drawdown distribution and improvements in how the joint probabilities of flood causing factors are simulated. In this paper, we look at how the design flood frequencies for some of Australia’s large dams have changed, the reasons for this and then identify five key questions for dam owners to ask to aid assessment of whether the hydrology for a dam should be reviewed
On Hampstead Heath in Central London, just 3 kilometres from the centre of the city, there are more than 20 dams and reservoirs set within the landscape setting of Hampstead Heath. A number of dams were built in the 16th century and formed the original water supply to the City of London. They are set in a landscape laid out by the world renowned Humphrey Repton.Three of the embankments which are laid out in two chains of reservoirs across the Heath are subject to safety legislation in the UK. As such they were identified as being deficient in spillway capacity and thus fairly significant works were required to be carried out in this sensitive setting.The Heath is protected by the Hampstead Heath Act of 1870 which seeks to prevent significant changes to the Heath and thus it was quite clear that there would be opposition to any works on the Heath, even though they were required by law to protect persons and property downstream. In fact a significant lobby group formed which challenged the need for the works and also the legislation of the UK via a judicial review. This paper will describe the process by which significant stakeholder consultation was undertaken (costing more than £2M), the judicial review that took place in the Royal Courts of Justice, the option study and the major engineered elements carried out on the Heath.