Kim Robinson, Andrew Pattle and Thomas Shurvell
Rowallan Dam is a 43m high clay core rock fill dam located in Northern Tasmania. The dam impounds 121GL used for hydro power generation and has a High A consequence category.
Over the summer of 2014/15 major reconstruction works were carried out on the dam to repair a piping incident from 1968. The work entailed reconstructing two sections of the dam down to foundation level and the upper 7m of the 568m dam crest. During the work, the dam was temporarily exposed to a significantly increased flood overtopping risk.
A range of measures were taken to manage the overtopping risk; such as increasing the dewatering capacity of the dam, lake draw down, installation of a sheetpile wall, development of emergency backfill procedures and a flood forecasting system.
The focus of this paper is on the flood forecasting system and how this was integrated into the overall management of overtopping risk during construction. The forecast models were run automatically on a 2 hour schedule using the latest BoM forecast, telemetered lake levels and rainfall from 7 gauges surrounding the catchment. The system provided a continuous 7 day lake level forecast which guided the site team on when to release water to manage the storage.
In the event that the lake level forecast reached a predetermined trigger level, the dam safety team would have been automatically notified and various emergency procedures would have been triggered in response to the flood warning.
This paper discusses the measures that were taken to manage the flood risk, how it worked in practice and conclusions which are applicable more generally to managing overtopping risk during dam works.
Keywords: dam construction flood risk, flood forecasting
George Bolliger and Clare Bales
Traditionally, the dams engineering profession has been a career path for engineers of civil/structural or geotechnical persuasion. As dams are constructed there is understandably a predominate focus on the civil requirements. Beyond the first few years of the dam’s life, effective operation and maintenance becomes increasingly important. A number of mechanical/electrical components and plant items form part of the critical infrastructure of the dam. A good maintenance routine is an essential requirement of the dam safety management program.
State Water Corporation, as the owner of 20 large dams and over 280 weir and regulator structures, runs a dam safety management program that is in line with the Australian National Committee on Large Dams Guidelines and NSW Dams Safety Committee requirements. The maintenance procedures and outcomes are audited through an internal maintenance audit program.
The maintenance audits form an integral part of the total asset management plan as well as the dam safety program. They are used to identify areas of strength as well as common errors or defects. Using State Water’s internal maintenance audits as case studies, the paper elaborates the role of maintenance audit program in enabling a cultural change to further include mechanical/electrical aspects and thereby enhance the longevity and safety of the assets.
Cultural Change – A Mechanical Perspective on Dam Safety Management
Craig Messer, Francisco Lopez, and Manoj Laxman
The Enlarged Cotter Dam is a new 80m high Roller Compacted Concrete Dam being constructed to augment the water supply for the Canberra region. Due to the size of the main dam and the extreme climatic variations in the ACT, where temperatures range from sub zero in winter to in excess of forty degrees in summer, it is expected that significant stresses will be generated during the cooling of the structure. For this reason it is essential that an understanding of the magnitude of these stresses is developed through the initial strength development period and at critical periods such as the first and second winter when the temperature differential between ambient conditions and the core of the structure may be greatest. The development of thermal stress within the structure has critical impacts on both the RCC mix design and the dam construction equipment and methodology.
For the Enlarged Cotter Dam, thermal stresses were investigated using both two and three dimensional finite element transient heat transfer analyses, making use of the thermal properties derived from laboratory testing including instrumented thermal blocks, as well as established literature. Modelling of the thermal stresses in the dam required the development of time dependent concrete properties, such as strength, stiffness and heat generation, with the latter based on test results and calibrated to actual measured values. Additionally, site dependent conditions for ambient temperature, external conduction, convection and radiation factors, dam foundation temperatures and restraint, dam construction sequence, formwork, joint spacing, insulation and timing of reservoir filling were also modelled.
Initial thermal modelling of the dam demonstrated that significant tensile stresses and potential cracking could develop within the structure, at both early and mature concrete ages. Subsequent analyses were developed to investigate methods of reducing these stresses to within acceptable limits. This paper presents the results of the thermal analyses, including the methods to be employed during and after construction to minimise cracking without impacting construction costs and even optimising the speed of construction.
Finite Element Transient Thermal Analysis of the Enlarged Cotter Dam
Richard R. Davidson, Joergen Pilzand Bruce Brown
Recent earthquakes in Chile, New Zealand and Japan have created a new focus on the safe design of tailings dams in seismic regions of the world. Building sand and rockfill embankments to sustain large ground motions and provide crucial drainage of excess pore pressures remain daunting challenges at each site. Are conventional hydraulic deposition practices still viable? What new technologies can be considered? Addressing seismic stability of existing upstream method tailings dams whether currently in operation or closed is stretching our seismic geotechnical engineering profession to its limits of understanding of behaviour. Creating a safe, secure environmental storage must also be integrated with the geotechnical and hydrologic concerns. Is there a viable risk context to consider these competing issues? This paper will raise these issues within the international context and suggest a prudent path forward.
2011 – The Challenges of Building Tailings Dams in Seismic Regions
David R Jeffery
In 2004 the Victorian Government announced the decision to proceed with Australia’s largest dam decommissioning project, the return of the 365,000ML capacity Lake Mokoan to a wetland.
The project has been completed and has resulted in significant river health benefits through liberating environmental flows in the Broken, Goulburn, Murray and Snowy Rivers. Decommissioning has allowed the recovery of water savings for return as environmental flow to the River Murray (30,000 ML/year) and Snowy River (21,000 ML/year).
With decommissioning complete, development of a significant wetland complex across the 8100 hectare site has commenced.
This project has been undertaken at a time when the Broken River basin was exposed to its worst drought conditions in over 100 years and within 11 years of the worst flooding experienced in the nearby Rural City of Benalla. These extremes of climatic conditions and their impacts on the local and irrigation communities have ensured considerable community and stakeholder interest in the decision to proceed with decommissioning and in the subsequent delivery of each of the project elements.
This paper provides an explanation of the drivers for the project, describes the process followed and some of the challenges experienced over the projects seven year life and presents some of the lessons learned along the way.
2011 – MOKOAN – RETURN TO WETLAND PROJECT