Auckland Council (Council) is developing a dam safety management system with an overall objective to protect people, property, infrastructure, and the environment, from the harmful effects of a dam failure.
Council has responsibilities as an owner and operator of approximately 600 stormwater ponds and wetlands, many associated with dams. Council also has wider responsibilities for safety in the Auckland region, which may be affected by dams owned by others and even by inadvertent dams, such as road or rail embankments across streams that have the unintended but potential function of diverting, storing or holding back water. Three categories of dams have been distinguished, associated with Council’s different types of responsibility. Each category of dam is managed differently in the dam safety management system.
Given the large number of structures, which are not always obviously dams, a key activity has been the initial identification of dams across the Auckland region. Prioritisation has also been a necessary tool to direct resources and programme. Once dams have been identified, the consequences and risk of dam failure have been assessed, and commensurate measures have been established to manage those risks. There is limited guidance for some of these activities, and new procedures and tools have been developed.
This paper describes the process and the challenges encountered, for consideration by other councils when developing their own systems, and for consideration by the wider dams’ community.
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Vertical gates and their operating plant are an essential part of dam safety at many dam sites. Apportioning appropriate levels of resilience during the design phase requires a thorough understanding of a gate system as a whole, not only of a single component in isolation.
This paper offers a designer’s perspective on modern engineering design features, materials and practices which can improve gate resilience during onerous operating conditions. This is of particular relevance to gates that are seldom used. Design aspects relating to the capability and limitations of the gate, hoist type, power supply arrangement and control system equipment to work together as a complete system are paramount design considerations in ensuring overall system resilience.
A discussion of the role and duty a hydraulic gate has in a dam safety context is presented. Supporting commentary is offered on appropriate levels of reliability, redundancy and diversity, including a comparison of different gate, bearing and hoist types. The authors draw on their own experience regarding gate design, fabrication and operation from completed and ongoing projects both locally and internationally.
International emergency agencies such as the Federal Emergency Management Authority (FEMA) in the U.S. highlight a lack of public awareness of hazards relating to dams (FEMA, 2012). This is an issue faced by emergency management agencies around the world, including in Australia and New Zealand. Without hazard awareness, communities who live downstream of large dams are potentially more vulnerable to possible risks, and are likely to be less resilient when hazards arise. One way to address this knowledge gap is risk communication or the meaningful and purposeful exchange of information about risk among relevant parties (Covello, von Winterfeldt, & Slovic, 1984).
This study adopted a mental models approach (see Lazrus et al., 2016) to identify community members’ knowledge of dam failure by comparing their views with those of experts. Data were collected via depth interviews with dam safety experts (n=5) from across Australia, and community members (n=26) living downstream of dams in South East Queensland in Australia. Participants were asked to discuss knowledge about dam failure and to evaluate a dam safety message taken from a U.S. dam authority that was verbally read to them. Interviews were transcribed and analysed to identify the gaps between expert and community member knowledge.
Analysis showed some convergence on general dam operations but, less comprehensive community understanding of the causes of dam failure and dam safety management. Response to the U.S. dam safety message was mixed, with some participants believing it delivered the message appropriately, and others feeling it overstated risk or that its intended use was primarily to protect dam operators. Notably, these varied responses were often related to participants’ level of knowledge of dams. Combined, the findings highlight an opportunity to close the gap in knowledge. These findings will inform the strategies and materials for the South East Queensland bulk water authority Seqwater in engaging with communities downstream of their 26 dams. The research will guide the approach in conveying knowledge with an appropriate tone to support ongoing community engagement activities and increase resilience.
Otago Regional Council (ORC) own and operate the Lower Taieri, Lower Clutha, and Alexandra Flood Protection Schemes. Collectively the schemes comprise over 220 km of earthfill levees, together with numerous appurtenant structures, such as major spillways, flood gates and pumping stations. The schemes provide flood protection to significant and varied communities and infrastructure adjacent to the Clutha and Taieri rivers, for example Dunedin Airport, and towns such as Balclutha and Outram. The works were constructed at various times since the 19th century to a range of standards, and assets are at various lifecycle stages.
Regular and systematic condition and structural integrity assessment is a key aspect of operating flood protection schemes for resilient communities. This can be challenging due to the large spatial extent of multiple schemes. Efficient and effective on-the-ground visual inspection of the entire network is key. A field assessment methodology was developed which combined on-the-ground visual assessment with innovative use of GIS technology, for field data capture, recording, analysis and presentation.
The structural assessment methodology used LiDAR-derived digital elevation models (DEMs) integrated with the field data to screen the levee networks based on geometry condition, to identify critical locations for analysis. Levee susceptibility to hazards such as overtopping scour, piping, seismic performance and slope instability was assessed utilising a semi-quantitative multi criteria analysis. Subsequent efforts were focused on critical locations enabling analysis which would not be efficient on a scheme-wide scale. An outcome included a GIS database to enable rapid future review of asset information and condition.
The assessment coincided with the July 2017 Taieri River flood – the largest event in almost forty years, and a timely reminder of the importance of flood protection infrastructure for community resilience. This event also highlighted the importance of making use of such events to field-truth assessment results and test assumptions about scheme performance and vulnerable locations.
The majority of Australian tailings dams over the last 100 years have been successfully built using upstream construction. However, recent major tailings dam failures in some countries have led to a global industry wide review of the design and management of tailings storage facilities, with a focus on the upstream raise method as a common factor for some failures. As a reaction to the recent failures, there is the potential for regulations to become more restrictive and the potential for unjustified pressure on existing and new mines to rule out upstream raising due to possible safety and failure risks.
This paper looks at whether it is the upstream construction method or other more fundamental issues that have led to these failures and examines whether such issues are equally relevant in Australia. Does Australia have a specific advantage in being able to successfully use upstream tailings dam construction or are we fooling ourselves?
The topic of upstream tailings storage is a subject of broad and current interest and the lessons learned from historic failures are rightfully leading to improvements. Implementation of good practice starts with the overall management structure that guides how tailings dams are designed, constructed, operated and closed.
Critical design practice involves understanding the unique site conditions, properties of the tailings and management of tailings placement, as the tailings form part of the overall retaining structure. Good practice during operation of upstream tailings dams is key to reducing the risk of tailings dam failures and the success of safe and sustainable closure.
This paper presents key features of both good and bad practice for the upstream raising of tailings dams and discusses how the design and operation can be made more resilient to ensure the safety of the community and infrastructure. It concludes that upstream raising can be a safe and economical method of tailings disposal if designed, constructed and operated correctly.
Design floods for most dams and levees typically have an annual exceedance probability (AEP) of 1:100 (1E-2) or less frequent. In the U.S., high hazard dams are designed to pass the Probable Maximum Flood (PMF), which typically has an AEP of 1:10,000 (1E-4) or less frequent. In order to reduce epistemic uncertainties in the estimated AEP for extreme floods, such as the PMF, it is important to incorporate as much hydrologic information into the frequency analysis as reasonably possible. This paper presents a Bayesian analysis framework, originally profiled by Viglione et al. (2013), for combining at-site flood data with temporal information on historic and paleofloods, spatial information on precipitation-frequency, and causal information on the flood processes. This framework is used to evaluate the flood hazard for Lookout Point Dam, which is a high priority dam located in the Willamette River Basin, upstream of Portland, Oregon. Flood frequency results are compared with those from the Expected Moments Algorithm (EMA). Both analysis methods produce similar results for typical censored data, such as historical floods; however, unlike the Bayesian analysis framework, EMA is not capable of incorporating the causal rainfall-runoff information in a formal, probabilistic manner. Consequently, the Bayesian method considered herein provides higher confidence in the fitted flood frequency curves and resulting reservoir stage-frequency curves to be used in dam and levee safety risk assessments.