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.
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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
Mike Marley, Greg Dryden, Geoff Eades, Edwin Brown, and Gary Huftile
Traveston Crossing Dam is proposed for construction at AMTD 207.6 km on the Mary River, about 25 km upstream of Gympie in South East Queensland. The Mary Valley at the dam site is located in a zone of complex geology resulting from formation in a tectonic accretionary wedge setting. This has been responsible for its complex geological structure, which has required a range of geological and geotechnical investigation and interpretation techniques to develop a model on which to base the dam’s preliminary design. This paper describes the tectonic history and the innovative techniques used in developing the geological model for the dam foundation.
The investigation involved aerial photograph interpretation, geological mapping; geotechnical drilling, including water pressure testing; seismic refraction profiling; downhole geophysical logging; excavation and geological mapping of large excavations; and hydrogeological investigation involving investigative drilling and pumping tests.
A Vulcan 3-D computerised geological model was constructed using borehole data, seismic refraction interpretation and downhole geophysics interpretation. The geological model has been used in the development of the preliminary design and confirms that the foundations are suitable for the proposed structure.
Keywords: Dam Foundation; Geophysics; Investigation Tectonics; Geological Strength Index Kinetic Analysis
Steven L. Barfuss and Blake P. Tullis
An important aspect of improving the safety of dams is selecting designs that are both hydraulically efficient and cost-effective. A powerful tool that can be used as part of the hydraulic structure design process is a physical model study. To obtain maximum benefit from the model, it should be implemented as a part of the design process rather than as a post-design verification phase. A model study included early in the conceptual design phase can also provide increased flexibility to the designers.
Hydraulic model studies can often provide cost-effective answers to difficult problems. Some of the issues that can be efficiently resolved using model studies include optimizing spillway head-discharge relationships to increase reservoir storage while minimizing upstream flooding potential, controlling downstream scour, quantifying hydraulic uplift forces and/or overturning moments of dam structures, evaluating alternatives for structure retrofit or repair, and optimizing control gate sequencing during floods. Model studies also allow the engineer to simulate prototype performance (e.g., three-dimensional flow patterns, velocities, pressures, scour potential) over the full range of expected discharges. Quick and easy changes to the model can be made at minimal cost when evaluating the performance, safety and economic impact of various design alternatives.This hands-on model study approach to dam safety represents a tool that in some cases is underutilized.
Brief discussions of several physical model studies conducted at the Utah Water Research Laboratory, Utah State University in Logan, Utah, USA, are presented to illustrate key points of the paper. The primary objective of each of these model studies was to provide and/or improve the safety of the dam and the spillway while minimizing construction costs. This paper discusses the cost-effectiveness and hydraulic improvements that can be achieved through physical model studies.
Keywords: Physical models studies, design, hydraulic efficiency, dam safety, construction costs
Ken Ho, Robert Davey and Jim Walker
The Aviemore Dam appurtenant structures were upgraded for seismic performance in 2006. A comprehensive dam safety review programme conducted by Meridian Energy evaluated the performance of the dam and appurtenant works under extreme ground movements and rupture displacements of the Waitangi Fault, which passes through the embankment dam foundation. The spillway and sluice gates are key elements of the dam safety critical plant for the passage of floods to prevent overtopping or emergency dewatering of the reservoir after a major seismic event if there are concerns about damage to the dam. This paper outlines the assessment undertaken for the spillway and sluice gates for seismic performance and the upgrade necessary to safeguard their integrity for operation after the event.
The spillway and sluice gates are large steel radial gates operated by electrically powered wire rope winches and hydraulic actuation, respectively. Combined hydrostatic and the Safety Evaluation Earthquake (SEE) induced hydrodynamic loads would be expected to stress the gate structures beyond their yield capacity. The yield would be downstream only due to the influence of the hydrostatic load under the earthquake response cycle. The resulting deformations were predicted to fracture connecting bolts in the spillway gate arms and cause severe leakages past the top leaf of the sluice gates. The solutions developed for the spillway gates to reduce connection bolt damage and the strengthening of the sluice gates will ensure their post-earthquake operation.
Keywords: Aviemore Dam, spillway, sluice, radial gate, seismic performance, post-earthquake operation.
A. Swindon, M. Gillon, D. Clark, P Somerville, R. Van Dissen and D. Rhoades
The 45 km long Lake Edgar Fault in south-west Tasmania passes through the right abutment of the Edgar Dam and into Lake Pedder, and within 30 km of three other large dams. In 2004 an independent seismotectonic study concluded that the fault had moved three times in the past 48–61,000 years, with the last movement around 18,000 years ago.
In order to better constrain the risk assessment for the nearby dams, the likelihood of a rupture recurrence along the fault was required. Two independent methods were investigated. The first was a comprehensive review of active faulting and deformation of stable continental region faults within Australia, and a comparison with similar faults worldwide with the well studied behaviour of the Lake Edgar Fault. The study results demonstrated the episodic nature of stable continental region fault activity, separated by much longer periods of quiescence, with a decreasing likelihood of rupture following each event within an active period. The time window of applicability of this paleoseismological study is thousands to tens of thousands of years.
The second study looked for evidence of precursory seismic activity in the vicinity of the fault which could indicate an increasing risk of rupture over the next decade or so. This method does not predict specific earthquakes, but does forecast whether the level of future earthquake activity in the short to intermediate term is relatively low, high or at an average level. Using a catalogue of seismic activity for south-eastern Australia, the study concluded that there is no evidence for precursory seismic activity in the area of the Lake Edgar Fault that would give rise to an elevated forecast rate of occurrence of moderate magnitude earthquakes either in the short to intermediate term. This precursory method has a window of applicability of a decade to perhaps several decades.
The combination of these two studies has advanced the understanding of the Lake Edgar Fault activity by both setting it in the long-term stable continental region fault context and investigating the presence of short-term behavioural activity. This has allowed the seismic hazard to be re-assessed as nearer to ambient levels than earlier postulated. This work has applicability for other fault scarps in Australia, both with regards to better defining the long-term hazard (103-105 years) posed by a fault, and potentially also giving advance (short-term 101 years) notification of increasing risk of fault rupture. Better long- and short-term hazard information allows more complete and thorough engineering decisions to be made.
Keywords: Earthquake, seismic, fault rupture, dam safety, risk assessment, Hydro Tasmania, Lake Edgar Fault.