It is inevitable that, sooner or later, most dams will fill with sediment. It is simply a matter of time.
When the sediment reaches the power intakes of a hydro dam, there is a risk of the turbines being destroyed and the power station being abandoned. If this happens the spillway will need to operate continuously and this may lead to spillway failure possibly followed by failure of the dam.
Spillways are likely to fail because they are not designed for continuously discharging large amounts of sediment. The concrete and fixed parts will soon be damaged and need to be repaired. Repair is possible only if the spillway is segregated into two or more chutes so that one chute can be isolated and the flow passed down the other chute(s).
Reservoir sedimentation is a serious long-term problem that threatens the long-term viability of storage hydropower schemes. In 2010 global storage capacity was estimated at 6,000,000 km³ but it is projected that 4,000,000 km³ will be lost to sedimentation by 2050.1 Storage loss occurs worldwide at a rate of about 0.8 percent per year, but the sedimentation rate in many regions such as Asia is much higher.
Many reservoirs will fill with sediment within the next 100 years or so but some will fill up in a much
shorter timeframe. The sediment builds up at the head of the lake and a wall of sediment moves slowly down the lake until it reaches the dam and, eventually, the power intakes.
This paper is intended to draw attention to the problem and to emphasise the need to mitigate or solve the problem by providing a scour intake beneath the turbine intakes.
The major problem is designing the upstream gate to operate reliably when finally needed after, possibly, many years with little or no maintenance. A solution is suggested but it is recognised that better ones may be found: the objective of this paper is to encourage designers and developers to consider a wide range of solutions and to examine the potential of modern materials to help solve this very serious problem.
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New Zealand’s economy is heavily dependent on export revenues generated by primary industries such as dairy, meat, agriculture, horticulture and viticulture. For these sectors, securing water for irrigation has been a key factor for growth. New Zealand has a temperate climate with generally wet winters and dry summers. The availability of water in the dry summer period is very important for these sectors to maximise production. A considerable amount of investment has already been made in the construction and operation of reservoirs for irrigation purposes. However, because climate change effects (more frequent occurrences of extreme events such as droughts and flash floods) have been observed around the world and the need for restrictions imposed on the use of water resources by regulators for environmental reasons, the need for developing water storage reservoirs has become more essential than ever. Climate change effects are already being factored into current practice. Drawing on the author’s experience, this paper discusses the potential impacts of climate change, with an emphasis on the effects of drought, on the design, construction and operation of water storage facilities with changes necessary to improve the resilience of new dams in
response to climate change. The paper also aims at raising awareness among the farming community so they can appreciate the associated risks and issues with climate change and be more cautious about planning and budgeting for their future investments in dam and irrigation projects.
Leslie Harrison Dam is located on Tingalpa Creek in the Redlands region, approximately 18 km southeast of Brisbane. It is classified as an extreme hazard category dam with a large population at risk only a short distance downstream.
The dam comprises a 25 m high zoned earthfill embankment, with a dry well concrete intake tower and an outlet conduit located at the base of the dam near the old river channel. The spillway has a 43 m wide concrete gravity ogee crest, with a concrete lined chute terminating in an energy dissipator structure.
Seqwater is undertaking a staged upgrade of Leslie Harrison Dam to address deficiencies identified during the Portfolio Risk Assessment (URS 2013) and Geotechnical Investigations (GHD 2016).
While the dam has met the water supply needs of the community for the past 50 years, the upgrade ensures local residents will be well served into the future. Additionally, the structure will meet the most up to date requirements of dam safety management and national industry standards.
Construction of the Stage 1 upgrade commenced in June 2018 and involved the removal and replacement of liquefiable material in the foundation, modernisation and extension of the outlet works, addition of a new downstream filter buttress to the embankment, and lastly, the installation of both active and passive anchors within the spillway ogee and lower chute floor.
As with any major project, the works involved a number of challenges that had to be addressed. This paper provides an insight into the key challenges encountered and how these were overcome by the design and construction teams using practical engineered solutions. The intent is to provide the reader with an account of the “lessons learned” during the construction phase, along with recommendations for future dam upgrades.
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.
Identification of people impacted by a hypothetical dam-break flood is required to understand the potential hazard a dam poses to downstream communities. The New Zealand Dam Safety Guidelines and the Australian Consequence Categories for Dams define these people collectively as the “Population at Risk” (PAR) and recommend that evaluation of PAR should include both permanent and temporary populations. However, there is limited guidance on specific methods to determine these populations. This paper provides an outline of an evidence-based, repeatable method to determine the PAR (both permanent and temporary) within a dam-break flood inundation zone. The method is intended to provide guidance for people tasked with estimating PAR in accordance with the New Zealand Dam Safety Guidelines. The methodology provides a current practice framework for users to apply and estimate the PAR in a clear and defendable manner.
Estimating the likely extent, depth and velocity of flooding should a dam fail – and planning to both prevent and respond to such a failure – are important parts of managing risk from dams and ensuring community resilience. This paper compares and contrasts current standards and practices for dambreak analyses and flood routing in New Zealand, Australia, the US, and the UK. Comparisons highlight consistent and evolving practices and consider how dambreak modelling supports robust dam safety decision making. In addition, the paper offers opinions regarding selected areas for future research, and insights into the benefits and limitations of increasing complexity in breach modelling.