2016 – Hedges Dam – Upstream Face Slip from Rapid Drawdown and Subsequent Remedial Works
David Laan, Kim Matsen
A slip on the upstream face of Hedges Dam was observed during an annual site inspection in late March 2016. At that stage the slip appeared to be largely contained within the right hand third of the embankment.
By early April, the slip area had developed into a head scarp across the entire central portion of the embankment. Multiple other head scarps were observed, indicating multiple or segmented slips. Several tension cracks were also visible on the face of the dam. The toe of the slips was indicated by a poorly defined bulge.
The most recent drawdown of the reservoir level was identified as a potential driver for the initiation of the slip failure. During the most recent drawdown the maximum drawdown rate was approximately 0.6 m/day whereas in the previous 17 years the maximum drawdown rate was approximately 0.2 m/day.
The remedial works proposed are to place a rockfill weighting zone on the upstream face to stabilise the embankment. The strength of the materials along the sheared surface was back calculated from the mechanics of the failure surface. This data was then used to calculate the shape of the weighting zone required to stabilise the slope.
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2016 Papers
2016 – Spillway gates, CFD analysis, Uplift forces, Hydrodynamic forces, Flow Modification, Gate Locking Mechanism.
Learn moreChris Kuenne
Paradise Dam is located on the Burnett River 20 km northwest of the town of Biggenden in Queensland. It is a gravity dam with a height of 37 metres and a total capacity of 300,000 ML. It was primarily constructed to service local agriculture.
The dam features a complex outlet works contained within a tightly constrained footprint. It provides for irrigation releases, fish passage and power generation. Additionally, the outlet is required to pass very high environmental flows of up to 270 m3/s.
The dam was subjected to major flooding in 2013 resulting in significant damage to the mechanical equipment associated with the outlet works, and severe scour downstream of the spillway.Since construction, the operating range for the environmental outlet has been restricted. A rough operating zone has been identified through which the gates are quickly moved through. It is believed to be caused by the dynamics of the gates and the upstream conduit arrangement. Failure of the downstream stainless steel liner associated with the conduit has also occurred. The environmental outlet lacks the ability to be isolated from the storage, complicating the maintenance / modification of the gates. At the time of design, it was agreed by the alliance partners that major maintenance of the gate would be planned for when the reservoir was low, being below the intake bellmouth.
The irrigation release valves suffer from high vibration levels during operation. Component failure and severe corrosion have also been experienced.
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This paper details:
Operational and maintenance experiences and restrictions since commencing operation including the impact of flooding;
Investigation and testing of environmental gate dynamics and the impact of these on the intake tower;
Failure of the environmental conduit liner, investigation and proposed rectification;
Proposed method to enable servicing of environmental gates without the use of a bulkhead and without draining the storage;
Proposed enhancements to irrigation valves to reduce vibration and extend service life. -
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2016 Papers
2016 – Design Considerations for the Stortemelk Hydropower, South Africa
Learn moreBJ Rochecouste Collet, PC Blersch, AL Olivier
The paper shows how multidisciplinary engineering can create future-proofed solutions for dam management and how innovation is bringing those advancements to life. It includes the retrofitting of a small hydropower station to an existing dam structure, enhancing the use of the dam. The paper also reveals design, technical and project finance considerations underpinning the multi-disciplinary services.
Water supply in South Africa’s economic hub of Gauteng journeys from the Lesotho Highlands Water Project (LHWP) through three river systems that converge in to the Vaal Dam from which the water is treated and pumped for domestic usage. One of those systems delivers water originating in Lesotho to discharge into the Ash River. The Ash River was, by origin, a small river with an environmental reserve flow of 50 l/s. However, with the LHWP significantly adding flow, the annual average increased to 24,500 l/s (24.5 m3/s) and is set to increase further with future phases of the scheme. To mitigate significant erosion caused by the greatly increased flow, several structures were erected along the river, including the Botterkloof Dam. Whilst the energy was dissipated in the dam’s spillway, a private developer studied if the water could be used for energy generation. The river also offered some rapids some 1.6 km downstream, which also showed potential for hydropower generation. An option to combine the two sites was also considered.
Aurecon conducted the feasibility study in 2010 for both sites, including the combined option, which concluded that there would be significant benefits in the implementation the projects in two separate schemes. This boasted many advantages including reduced capital investment, reduced social impact (canoeist), reduced geotechnical risks, and lesser land acquisition leading to a better return on investment. Aurecon are currently providing engineering, procurement and construction management (EPCM) services for the entire project. Construction of the 4.4 MW hydropower station commenced construction in September 2014 and was commissioned ahead of time and under budget.
The founding conditions under the proposed hydropower station location, comprising interbedded sandstone and mudstone was fairly poor with the mudstone effectively decomposing in less than two days. The Botterkloof dam was build on a thick layer of sandstone which dips quite steeply towards the right bank. The right bank on the other hand comprises an old paleo channel. The Boston A dam, located on the left bank immediately adjacent to the Botterkloof Dam, is founded on the weather mudstone and the spillway is grass lined. The power station construction was constructed in the narrow space in between the two existing dams – the Botterkloof Dam (owned by the Department of Water and Sanitation (DWS), Government of South Africa) and the adjacent privately-owned Boston A Dam. Permission had to be obtained from the respective owners and all regulatory permits approved before the project could be submitted to the South African Renewable Energy Independent Power Producer Programme (REIPPP) implemented by the Department of Energy (DoE) of the South African Government.
Another significant challenge in the construction itself included the need for deep excavations through the left embankment of the Botterkloof Dam and adjacent to the spillway stilling basin whilst such construction needed to be done without affecting operations and stability of either of the two dams.
The solution was a shallow intake, followed by a cut and cover concrete penstock leading to a compact hydropower station housing a single 4.4 MW vertical “Compact Axial Turbine” Kaplan turbine ending in the tailrace, which was rotated at 90 degrees.
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2016 Papers
2016 – Towards Improved Efficiency of Dambreak Modelling and Consequence Assessment Projects
Learn moreD Stephens and P Hill
Dambreak modelling and consequence assessment is a key component of many dam safety related studies. The outputs from these assessments can be used to inform the consequence category, dam safety emergency planning, risk-based surveillance and dam safety risk assessment. These studies are complex, intensive and expensive to complete, and all too often there is a need to manipulate or extrapolate the results of these assessments to fit a purpose other than what they were intended for. This issue is particularly prevalent for risk assessment, where the likelihood calculations are directly tied to analysis of the key failure modes, but consequences may be taken from previous studies which were not informed by failure mode selection. The result of this mismatch may lead to inefficiencies and uncertainties in preparing the risk estimates. Subtle changes to the timing or scope of the original dambreak modelling and consequence assessments, at relatively small incremental cost, may help to prevent these issues arising for future studies. Advice is provided on specific issues such as the determination of the downstream extent of the dambreak modelling, selection of the dambreak modelling scenarios and reconciliation of the consequence assessment results with flood and seismic loading partitions for risk assessment. It is hoped that the advice provided will lead to an overall increase in the efficiency and value for money of these studies.
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2016 Papers
2016 – Upcoming New National Seismic Hazard Assessment of Australia – Improvements & Novel ideas
Learn moreVicki-Ann Dimas
The National Seismic Hazard Assessment 2018 (NSHA 18) project intends to revise the existing seismic hazard map (AS1170.4 2007) for Australia. Geoscience Australia (GA) are leading the project along with a consortium of seismologists, geologists and earthquake engineers.
The NSHA 18, due to be released in 2018 is of great importance to dam owners and operators. The project intends to incorporate a comprehensive approach to seismic hazard, particularly in modelling uncertainty and variability.The Global Earthquake Model (GEM) is an international consortium of scientists, engineers and policy makers. One of the primary aims of GEM is to provide a uniform set of tools for analysis in seismic hazard and risk. GEM was established to provide a framework for global standards in comparing risk analysis, awareness and actions in an effort to increase resilience to vulnerable communities.
The NSHA 18 will use the GEM framework in order to meet its own objectives for the new upcoming hazard map. The Seismology Research Centre will contribute to the NSHA 18 in three areas. Firstly, to produce a unified earthquake catalogue where GA will homogenise magnitudes to a uniform scale. Secondly, to produce a number of applicable alternate seismotectonic models, and thirdly, through the contribution of ground motion data collected over the last forty years within Australia.
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2016 Papers
2016 – Roads Can Drive Your Risk! A Look at the Importance of Itinerants on Roads in your Consequence Assessments
Learn morePeter Woodman, Andrew Northfield, Tim Kallady
Currently there is little guidance available on how itinerants on roads should be included in a consequence assessment. The methods available are often subjective which can lead to itinerants on roads either being ignored or insufficiently considered. A fact that can in turn lead to consequence categories being inappropriately assigned to the asset being assessed or risks being under or over estimated. Consideration of these itinerants is especially important for smaller dams or retarding basins in urban areas where often the Potential Loss of Life (PLL) in buildings is small but there are major roads carrying a large Population At Risk (PAR) through the inundation extent, which experience flooding of sufficient severity to pose a threat to life.
This paper looks at how the method used to assess itinerants on roads can affect the consequence category assigned to an asset and/or the risk of the dam or retarding basin. It will draw on a number of recent assessments undertaken for retarding basins within Melbourne and make comment on a possible approach to consider itinerants on roads in the future.
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