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.
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.
Dams and levees within the U.S. Army Corps of Engineers (USACE) inventory were constructed for a variety of purposes including flood control, navigation, hydropower, recreation, and fish and wildlife conservation. USACE transitioned to using life safety risk as a key input to all dam and levee safety decisions in 2006. This was implemented for many reasons, paramount among them is forming a consistent basis to evaluate the safety of dams and levees and prioritize the implementation of risk reduction measures in a consistent manner across the agency to best utilize available resources. This requires knowledge of what constitutes unacceptable risks that would require risk reduction actions. The Tolerable Risk Guidelines (TRG) were developed for this purpose, and to form a common basis for dam and levee safety evaluations and decisions. Protection of life is paramount, and there are four TRG related to (1) understanding the risks surrounding dams and levees, (2) building risk awareness, (3) fulfilling daily responsibilities, and (4) continually considering actions to reduce risks. The USACE policies have evolved over time, but the fundamental principles that underpin the TRG have been fairly consistent for the past 10 years. The evolution of the TRG have come as a result of the experiences using these principles to support more than 2,500 safety decisions. This paper describes the rationale behind the selection of the TRG.
K.A. Crawford-Flett, J.J.Eldridge, E.T. Bowman, C. Gordon
This paper provides an interpretation of factors governing the manifestation of internal erosion in a New Zealand canal that was constructed during the 1970s. Liner and subgrade soils were sampled during de- watering of Tekapo Canal in 2013, following the surveillance of erosion events over the preceding decades. This paper focuses on the interpretation of erosion susceptibility of liner and subgrade soil gradations sampled at four locations. Of the four locations, Sites 2, 3, and 4 were associated with internal erosion defects. A single location (Site 1) was selected to provide benchmark “intact” (un-eroded) samples.
Interpretation of susceptibility of the widely-graded soils to internal erosion mechanisms was achieved through the application of established empirical techniques for internal stability, filter compatibility, and segregation. Analysis of gradations, which are believed representative of some – but likely not all – canal soils, showed that Sites associated with erosion defects had liner-subgrade interfaces that permitted “some erosion” (NE < D15F < EE), while the Site showing no sign of erosion possessed an interface that met modern filter retention criteria for No Erosion. Based on gradation analysis, internal instability is considered a possibility for subgrade materials in particular. It is possible that subgrade materials that fail No Erosion criteria for liner retention may not represent as-built material and may instead have lost finer fractions in situ due to seepage-induced instability, leaving a coarser-than-placed and filter-incompatible subgrade.
This case study demonstrates the use of gradation-based empirical methods as initial screening tools to assess the susceptibility of soils to internal instability, filter compatibility, and segregation. The relationship between the internal stability of a filter and the filter’s particle retention performance (compatibility) is emphasised. As well as gradation susceptibility, the assessment of other factors such as segregation and hydraulic loads must be considered in order to better-understand susceptibility to erosion mechanisms.
The use of simulation models to assess dam failure consequences has progressively advanced in Australia over the past few years. For example, it is now common for HEC-LifeSim to be used to estimate potential loss of life from the failure of large dams with large populations at risk downstream. Since its introduction to Australia, numerous presentations and papers have been provided by USACE and industry professionals that highlight the benefits of using HEC-LifeSim for a range of different case studies.
Whilst the majority of the literature published to date have focused on the benefits of simulation modelling, this paper identifies some of the technical challenges that can arise, particularly in the evacuation modelling component of HEC-LifeSim. The techniques that have been used to overcome these challenges are also discussed using three case studies.
The first case study demonstrates the sensitivity of the life loss to changes in cell size and the output interval of the gridded hydraulic data. This is done by comparing the differences in life loss between high-resolution and low-resolution models for three dambreak models. The second case study illustrates the importance of the road network representation in HEC-LifeSim because the resolution of the road network is important to achieve plausible estimates of the fatalities along roads, and logical animations of the mobilisation. The final case study demonstrates the implications of coincident flow modelling on the life loss, and therefore the importance of understanding the hydrology of the target and neighbouring catchments.
This paper provides a checklist that prompts practitioners to consider some of the lessons learnt over the last few years and is envisaged to be a working document that improves the defensibility and robustness of HEC-LifeSim estimates throughout the industry.
Vietnam has many embankment dams to supply water to the agricultural sector. Most of these dams were built between 1970 and 2000 but have degraded significantly since their construction due to a number of different reasons. Identifying the main potential failure modes for these dams aims to improve their dam safety management systems as well as help to target dam safety rehabilitation works. The research was conducted by analysing 207 Dam Safety Reports and Feasibility Studies published by the Vietnamese Ministry for Agricultural and Rural Development between 2017 and 2019. The priority level of rehabilitation required to these dams was assessed by analysing whether overtopping, seepage and slope stability related potential failure modes were likely to occur. The results revealed the main potential failure modes of embankment dams in Vietnam and the possible reasons for these are discussed. The approaches to rehabilitate the dams that are outlined in the Feasibility Studies were also analysed and are discussed in general terms. The results provide valuable insight into commonly encountered dam safety issues with embankment dams in Vietnam.