David Ho, Chee Wei Tan and Glen Dominish
Upper Cordeaux Number 2 Dam is founded on an igneous intrusion rock mass which overlays sedimentary rock layers above the Wongawilli Coal seam. The coal mining company, BHP Billiton Illawarra Coal, planned to extract coal close to the dam. Although the dam is classified as a low hazard dam, its importance, both as part of the Sydney Catchment Authority’s water supply system and for its significant heritage value, mean that the proposed mining should not have undesirable impact on the structure. This paper describes how the mining impact on the dam was assessed using a nonlinear 3D finite element model. The model considered the pre-existing cracks in the dam wall, uplift water pressure along the dam/foundation interface and the hydrostatic pressure at full supply level. Mining-induced movement such as valley opening, closure and upsidence were applied to the model. Stability and strength assessments were made against a set of acceptance criteria developed for mining impact. The development of different stabilising mechanisms was examined. From the numerical investigation, WorleyParsons was able to provide technical advice to the mining company, the dam owner and the NSW Dam Safety Committee to facilitate the mining application and to satisfy dam safety requirements.
Keywords: Mining subsidence, Arch/gravity dam, Nonlinear numerical analysis, Safety assessment
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Mike Phillips and Karen Riddette
The use of Computational Fluid Dynamics (CFD) models in the dams industry has increased significantly in recent years and conversely the use of physical hydraulic models has decreased. Typical design approaches for an upgrade of similar magnitude to the Hinze Dam Stage 3 project would have allowed for considerable time to develop a preliminary spillway design before hydraulic modelling was introduced, potentially requiring only one type of model. So is there a need for both types of models?
Because of the complex hydraulics associated with the spillway required for the Hinze Dam Stage 3 raise and accelerated schedule, the utilisation of CFD and 1:50 Froude Scale physical hydraulic models was necessary. Both models were constructed independent of each other. Both models complemented each others strengths and weaknesses, and each provided critical information at the following different stages of design:
• Spillway selection and conceptual design stage – the CFD model results were highly valuable in steering the selection of spillway type and configuration, particularly with visual representations of the ranges of flow for each spillway option.
• Preliminary design – in a one week period, 90 to 95% of the final spillway layout was resolved with interactive modifications of the physical hydraulic model.
• Detailed design – both the physical hydraulic model and the CFD model were utilised to determine water pressures, velocities and water surfaces and evaluate cavitation potential as input to detailed design.
In the case of the Hinze Dam Stage 3 project, it was highly advantageous to utilise a CFD and physical hydraulic model to achieve the design outcomes at each phase of the design. The dual-model study approach also provided advantages for project management of the design and stakeholder involvements.
Keywords: Computational fluid dynamics, CFD, physical hydraulic model, spillway, hydraulics
The Resource Management Act 1991 provides regional councils with responsibility for the control of taking, use, damming and diversion of water for the purpose of promoting sustainable resource management. The Act enables councils to develop plans, including objectives, policies and rules, to assist it carry out its functions. Otago Regional Council has an operative plan, Regional Plan: Water for Otago, which contains various provisions relevant to controlling damming and storage of water.
In Otago, where irrigation is significant, most surface water is over-allocated. Water for irrigation is largely allocated through deemed resource consents (issued by the Wardens Court under the Mining Acts of 1898 and 1926) which now have an imposed expiry date of 1 October 2021 under the Resource Management Act. Deemed consents have priority access allocations and are largely excluded from the provisions of the regional plan. Water resources are not efficiently utilised under the current regime, with water from dry areas transported long distances to areas with abundant water, and surface water taken when ground water is a more appropriate resource.
Otago Regional Council is undertaking a plan change program to allow smooth transition from deemed consents that ensures water resources are efficiently allocated and water is used efficiently after 2021. The paper describes the results of consultation undertaken with irrigators and discusses the role of irrigation infrastructure raised at those meetings.
Efficient water resource management requires Council to develop a policy regime that promotes water resource and use efficiency as a priority and encourages community based water management for efficient on farm use. Also, irrigators need to develop new storage and distribution infrastructure managed and operated at an inter farm – community level.
Changes to the Building Act, giving regional councils responsibility for dam safety and building controls for dams, create opportunities for greater integration of dam construction and management with water resource management under the Resource Management Act. This paper explores an opportunity for major redesign of water infrastructure development and management for the future prosperity of Otago.
Keywords: Building Act, Resource Management Act, water, storage and distribution infrastructure, resource efficiency
Nigel Connell, Tim Logan and Tim Mills
Leakage from Tekapo Canal, between km 11 and 12, is investigated. A groundwater model is formulated based on construction records, detailed monitored seepage flow and groundwater levels in the canal embankment over the 30 year life of the canal, chemical analysis and flow history. Sieve analysis of embankment materials confirmed embankment fill was sourced from glacial outwash graves excavated from canal cut upstream. Anisotropic permeability of the fill embankment, inferred from the construction method using motor scrapers and vibratory rollers, contributed to explaining inflow to the model primarily from a source up to 500 m from the leakage outflows. Stability of the canal embankment is reassessed considering length of the seepage paths, which are long, hydraulic gradients, which are relatively flat, and resistance of the glacial outwash gravels to piping. The groundwater model that is developed indicated that stability of the canal embankments is not reduced significantly due to the seepage.
Keywords: Tekapo canal, groundwater model, canal leakage.
Bruce Walpole and Craig Scott
Monitoring and surveillance is crucial to managing the ongoing performance of dam structures.
The true value of appropriate monitoring, surveillance and review processes is only realised when
potential dam safety issues arise. TrustPower’s civil safety monitoring and surveillance program
includes nineteen hydro schemes throughout New Zealand and incorporates structures with
Potential Impact Classifications (PIC) ranging from Low to High.
TrustPower promotes a continual improvement policy on its management of safety issues and
conducts inspections on a regular basis. Routine and periodic independent inspections of the key
components within a scheme are paramount to the viability of the safety management system. The
importance and purpose of these inspections has recently been highlighted by the discovery of two
sinkholes on the face of the earth dam associated with the Cobb hydro electric power scheme.
This paper provides an example of the need for continual monitoring and surveillance, vigilance
of observations, good archiving systems and documentation. It discusses the broader issues
surrounding the subsequent response processes to potential dam safety deficiencies, and the
success (or otherwise) of investigative methods. It also highlights that an adequate dam safety
compliance system has commercial value as there is a measurable reduction in dam performance
uncertainty and hence greater efficiency in the speed at which accurate resolutions can be drawn.
Keywords: Dam safety, embankment, sinkholes, foundations, dam drainage, geophysical
Dörte Jakob, Robert Smalley, Jeanette Meighen, Brian Taylor and Karin Xuereb
Probable Maximum Precipitation (PMP) is one of the required inputs for estimating the PMP design flood. In estimating the PMP, currently no allowance is made for long-term climatic trends. A 2-year project funded jointly by the Australian Greenhouse Office and the Queensland Department for Natural Resources and Water, and with in-kind contributions by the Bureau of Meteorology began in May 2006. This study aims to assess how climate change might affect estimates of PMP. Preliminary results from this work will be presented.
Changes in factors used in PMP estimation, such as storm type and depth-duration-area curves, were assessed using a storm database covering the period 1893 to 2001 (Beesley et al. 2004). Based on the last 50 years, there is little evidence to support the notion that tropical cyclones (connected to major rainfall events) are penetrating further south or have become more frequent. A recent event that led to widespread flooding (Gold Coast, June 2005) was found to have very high storm efficiency. Changes in observed and projected moisture availability were assessed on the basis of a high-quality dataset of surface dewpoint temperatures and climate model output.
It is assumed that PMP received by a catchment is not uniformly distributed over a catchment but rather follows a typical spatial pattern. A pilot study to revise design rainfall estimates is currently under way at the Bureau of Meteorology. The methods developed in the pilot study were used to assess whether the spatial distribution of design rainfall estimates might be changing under a changing climate.
Keywords: Probable Maximum Precipitation, climate change, moisture availability, storm efficiency