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.
Now showing 1-12 of 53 3220:
Design Review Boards or Panels play an important role in supporting owners and designers in creating resilient design of water storage and tailings dams. Their essential roles are to constructively challenge the project team to deliver on the project objectives through a design which meets the 3R’s of resilience, robustness and reliability, and to provide assurance to potentially non-technical owner / project management. This can sometimes create an uncomfortable situation if one or more of the project team is not aligned with the agreed criteria. Time and cost pressures can often push a project or execution team to undertake insufficient analysis or to consider non-justifiable construction processes or shortcuts.
Regardless, the Review Board must remain steadfast in their advice and guidance with a strong focus on “data-supported decisions”. Finding and maintaining an effective board requires commitment at the highest levels. This paper will examine some of the challenges in addressing governance, membership and turnover, and conflict resolution.
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.
Many dams have low level outlets, most of them put in place as diversion structures during construction. However, once dam construction is complete and reservoir filling begins, the reality is that low level outlets are used very infrequently, and sometimes not at all.
This paper will discuss the merits of decommissioning low level outlets vs maintaining them as operational dam safety critical equipment. In this context the paper will examine the criticality of low level outlets in relation to the type of dam, the Potential Impact Category of the dam, the ratio between outlet capacity and mean reservoir inflows, possible resource consent issues and required maintenance and testing regimes.
Trustpower’s dam portfolio consists of a variety of dam types with multiple different types of low level outlets. Case studies from the portfolio will be used as arguments for maintaining or decommissioning the low level outlets in order to develop evaluation criteria for low level outlets and provide a basis for how to treat them from a wider dam safety perspective.
Computational Fluid Dynamics (CFD) is the science of predicting momentum, mass and heat transport, and can aid in design and safety issues for dam resilience in modern settings. Applications of CFD have historically been in the aerospace, automotive and chemical process industries with limited application in the hydraulic engineering field; possibly due to the associated computational intensity that is typically required. However, over the past two decades the cost of computing power has decreased substantially while the processing speed has increased exponentially. These developments have now made the application of CFD in the commercial environment feasible. CFD is particularly valuable in complex flow situations where the outputs required cannot be provided by a traditional hydraulic assessment approach and where there are stakeholder drivers such as service life, insurance cover and safety implications of infrastructure. The need for CFD when these drivers and complex flow situations arise, are demonstrated by means of a case study.
In the case study, CFD was used to investigate the flow patterns and the predicted performance of the outlet pipework from Massingir Dam in Mozambique. Three flow scenarios with appropriate pressure and flow boundary conditions were analysed for the outlet pipework, which included bifurcations for power generation from the main discharge conduits. Specific concerns addressed were, firstly, the possible excessive negative pressure in the region of the offtake for power generation and the potential for cavitation effects and, secondly, unacceptable velocity gradients in the power offtake pipework. Results showed that although some negative pressures were possible in one flow scenario, mitigation measures based on the CFD outputs could be considered and designed before construction.
The implementation of CFD in the above case study displays how risk in design can be reduced to ensure safety issues are addressed effectively.
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.