There are currently around four new flood detention reservoirs (retarding basins) built each year in UK, which although only being modest structures with median height of 4m and reservoir capacity of 300,000m3 pose a significant risk to the community as they are located immediately upstream of the community they are protecting. These communities range from around five to several thousand households.
The cost and therefore viability of these structures can vary depending on the number of defensive features built into the design, which raises interesting conflicting issues of public safety contrasted to vulnerability to property inundation in operational (say, 1 in 100 chance) floods.
The authors have designed and supervised over 30 flood detention reservoirs in the UK in the last 20 years. This paper describes the engineering decisions which need to be made regarding defensive measures and the resilience of these structures to withstand flood loading on demand. Examples of measures to include resilience are described, with discussion of when selection of the options to increase resilience against a particular failure mode should be mandatory, and when it may be more appropriate to consider it on a case by case risk-based approach. The paper will also discuss more strategic issues of how to balance making flood detention reservoirs affordable, while at the same time maintaining high standards of public safety and compares Australian and UK approaches.
Following the failure of Paloona Dam’s intake trashrack during the 2016 floods in northern Tasmania, a replacement trashrack and support structure was designed, manufactured and installed (by diver) within five months. This was a remarkable feat and hailed as a success at the time.
The euphoria, however, was short lived. A routine dive inspection in January 2018 revealed cracked
trashrack bars on one of the panels and this was after less than twelve months’ operation. This prompted a rigorous investigation where it was determined that the bars suffered fatigue due to flow induced vibration. Indeed it is possible that the bars cracked within a few weeks of returning to service.
The science of flow induced vibration is relatively mature, having been extensively researched over several decades. Its application to trashracks is well documented. However, this experience has shown that the common design approach overly simplifies the fluid-structure interaction. For Paloona, the result was a trashrack design which has proven to be inadequate, not having the resilience required for a dam outlet works component.
This paper revisits flow induced vibration theory as it pertains to trashracks, outlines the findings of vibration testing at Paloona, and suggests a design approach which will avoid similar issues. It is hoped that similar failures can be prevented and the design life expected of trashracks achieved.
The paper evaluates the stability of the reinforced rockfill at the downstream side of Waimea Dam, a new CFRD dam that is currently under construction in New Zealand. The reinforced rockfill is part of the overall diversion strategy for the dam during construction and has been designed to allow for safe overtopping to a depth of 2.9m, which corresponds to the 1 in 1,000 AEP flood.
Design of reinforced rockfill for overtopping allows for the safe passage of floods that exceed the capacity of the primary diversion works. This may be required for dam safety during construction, as is the requirement for Waimea Dam. It also serves to protect the works whist the dam is being built.
The focus of the paper is the stability assessment of the reinforced rockfill to prevent seepage induced instability during overtopping. As seepage forces have a considerable effect on the stability of the dam, a finite element seepage analysis was undertaken to estimate the seepage forces throughout the embankment, which was used in the design of the reinforcement system.
Details of the design process, including the seepage and stability analysis for a range of configurations are outlined, and recommendations for the design of similar future projects are provided.
Recent tailings dam failures have led to worldwide alarm that we are still getting an average of two
significant tailings dam incidents a year. This is despite the efforts of various industry organisations aroundthe world to raise the standards of tailings dam management. Clearly, a significant number of mining dams are not re silient enough to ensure the required level of safety for sustainable mining operations in a modern world in which there is increasing concern for the environment. This paper updates ANCOLD with international developments in attempting to address shortcomings in the mining industry that is allowing these failures to continue to occur.
In Australia, ANCOLD have released an addendum to the 2012 ANCOLD Guidelines on Tailings Dams, Planning, Design, Construction, Operation and Closure, to coincide with the new ANCOLD Guidelines for Design of Dams and Appurtenant Structures for Earthquake. This addendum also addresses issues of governance of tailings dams and provides additional guidance on the serious issue of static-liquefaction, a critical factor in recent failures.
On the international scene, ICOLD is progressing a Tailings Dam Safety Bulletin that is hoped will set
minimum standards for Tailings Dams for all member countries. In addition, the International Council of Mining and Metallurgy (ICMM) similarly wants to establish an international standard. It is likely that these international bodies will cooperate to ensure a consistent set of guidelines and that countries will accept and implement these.
This paper updates the ANCOLD position regarding guidelines and describes the state of various
international guidelines following the June ICOLD meeting in Ottawa.
The importance of building and maintaining safe, resilient tailings dams has become increasingly apparent with the rise in catastrophic failures in recent years. According to the World Mine Tailings Failures (WMTF) data base, 11 major failures have occurred over the past decade, often with devastating impacts to nearby communities in terms of loss of life and impact to the environment. With the occurrence of these types of events only expected to increase in coming years, there has been a corresponding increase in global calls to action to develop monitoring systems to better predict and wherever possible, prevent these failures from occurring.
With up to an estimated 20,000 tailings dams around the world, the development and implementation of a worldwide monitoring protocol is a daunting task, particularly as many of these structures are remote and difficult to access. This is where a technology like InSAR can make an immediate impact. InSAR is a remote sensing technique that uses radar satellite imagery to measure ground movement with up to millimetric precision. Radar systems are active, meaning they collect information from reflections of the radar signal off the ground and therefore do not require the installation of any equipment. As satellite images cover areas that extend thousands of square kilometres, they can provide information not only on the stability of dams, but also entire regions. Global archives already exist due to the Sentinel constellation of satellites, which provide coverage since 2014 over most parts of the world.
In an ideal world, tailings dams are safe and constructed to provide permanent containment of mining by- products. However, experience has shown that they can fail, often with dire consequences, especially if these failures occur without warning. The development of an internationally accepted standard for tailings dam monitoring is imperative to ensure the safety and resiliency of these structures is continuously tracked. This paper explores the role InSAR can play in the development of a global protocol for tailings dam monitoring.
The Waimea Community Dam will be the largest multipurpose concrete face rockfill dam (CFRD) to be constructed in New Zealand. This 53 m high CFRD will impound a reservoir of 13 Mm3 and is essential to securing the future water needs of the community and environment of the Waimea Plains and wider Tasman/Nelson region.
The design of this unique large dam in the New Zealand context was a long-term collaboration of local dam design expertise and international experience that took the ‘historic precedent based design approach’ for CFRD’s and supplemented this with modern embankment design techniques for the highly seismic environment at the dam site. Design of this High Potential Impact Category dam presented a range of technical challenges for the designers and wider project team, which required new and innovative design solutions and approaches.
The dam features a number of unique arrangements in the New Zealand context including:
The project had its origins in the early 2000’s. Detailed design commenced in 2010, and was externally peer reviewed. The detailed design stage was undertaken in an Early Contractor Involvement (ECI) process which was completed in February 2019.
This paper covers the important seismic design aspects for this large dam, including understanding and designing for the potential range of displacements and embankment deformations to inform the crest parapet wall and diversion culvert designs, and understand how differing rockfill properties might affect the dam performance. Quantifying the range of potential dam performance enabled a more resilient dam design.