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.
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In recent times two dimensional (2D) hydraulic modelling has become the most common type of modelling for undertaking dambreak assessments. Direct map outputs such as depth and depth-velocity product are very useful in assessing risk across a floodplain. The temporal output from 2D models also enables the tracking of flow across a floodplain, helping practitioners and dam owners alike make informed decisions on warning time and evacuation routes. These outputs form essential input to packages such as HEC-LifeSim an agent-based simulation model for estimating life loss by simulating population redistribution during an evacuation.
A number of investigations have shown the hydraulic model, TUFLOW, is able to simulate the hydraulic conditions expected in a dambreak flood wave, giving confidence in the model’s ability to correctly capture the flood wave propagation. Notwithstanding this ability, there remains uncertainty over the best methodology to adopt when assigning a breach hydrograph to the model and in turn the impact this choice has on assessing downstream populations at risk.
A commonplace method of assigning dam breach hydrographs is to model the reservoir and dam structure with a 1D model or spreadsheet, where the storage is represented with a stage storage relationship and outflow through a time-varying breach is calculated using level-pool routing. The resulting hydrograph is then applied directly to a 2D model immediately downstream of the dam to model the propagation of flow downstream.
An alternative approach consists of representing the entire reservoir, dam and downstream floodplain in the 2D model. This allows for the dynamic effects of bathymetric constrictions in the reservoir to be accounted for which could greatly impact on the timing and shape of the dam breach hydrograph. However, this comes at a cost, as representing the reservoir in 2D requires bathymetry data which can be expensive to capture and also may require a major extension of the model domain.
In this paper the ‘Fully 2D’ and ‘Stage storage relationship 1D/Spreadsheet’ approaches are compared for a number of case studies.
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 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.
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.
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.