Aida Baharestani, Dominic Kerr
North East Water (NEW) manages two reservoirs in series on Bakers Gully Creek, approximately 1.5km south of Bright in north-east Victoria. Both dams were constructed more than 100 years ago and taken out of service in the 1970s.
The Bakers Gully dams had an unacceptable risk profile according to ANCOLD’s Limit of Tolerability.
As the dams are out of service and have no operational benefit, NEW made the decision to partially decommission the dams.
The objective of the work was to lower the consequence categories of the dams from “High C” to “Low” and increase the spillway capacities according to ANCOLD Guidelines and ultimately reduce the dam safety risks to an acceptable level.
This paper describes the different stages of the project ranging from concept design, community engagement, environmental assessment and detailed design. In particular the paper explores the complexities of balancing in cost and public safety with community and ecological values.
Keywords: Dam decommissioning, Community engagement, Severity of damage and loss
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
Kirsty Carroll, Kelly Maslin, Richard Rodd
Melbourne Water manages over 210 retarding basins across Greater Melbourne ranging in size from 4ML to 4700 ML with embankment heights from 0.3m to 10m. Over the years the basins have been designed and constructed by a range of different owners and authorities. Varying design and construction standards with the majority of retarding basins generally being located in highly urbanised areas, has resulted in Melbourne Water having a large portfolio of assets that have potential to pose a significant risk to the downstream communities they are designed to protect.
High level hazard category assessments completed over the last10 years identified that approximately 90 structures were either High or Extreme hazard categories based on the ANCOLD Guidelines on Assessment of the Consequences of Dam Failure.
In an attempt to identify retarding basins requiring priority consideration for remedial works Melbourne Water embarked on a process of completing a dam safety risk assessment for five of the retarding basins in accordance with the ANCOLD Guidelines on Risk Assessment. The objective of the risk assessment was to develop an understanding of the key risk issues that might affect retarding basins as distinct from water supply storages, identify potential remedial works and develop a prioritised risk management strategy for the five basins considered. In completing the risk assessment there was also significant discussion about ways to streamline the process to allow assessment of the remaining basins.
This paper details the results obtained from the risk assessment, investigates the application of the base safety condition and implementation of a risk management strategy. It also looks at similarities between sites to enable common upgrades to be implemented across the range of retarding basins. This paper also discusses the need for guidelines specific to retarding basins to be developed.
How do you solve a problem like retarding basins? An asset owner’s perspective
Monique de Moel, Mark Arnold, Gamini Adikari
Monbulk Saddle Dam, built in 1929, is one of two saddle dams located at the southern end of Silvan Reservoir, near the township of Monbulk, Victoria. The saddle dam is a 5.3m high earthfill embankment with a 230mm wide, centrally located, concrete core wall. The reservoir retained is located in the valley of Stonyford Creek, and impounds approximately 40,500 ML of water at FSL.
Excessive seepage at the right abutment of Monbulk Saddle Dam has been an issue since the early 1970’s. The reservoir has been operating with a level restriction since then to reduce the seepage flows. However; this restriction limits the operational flexibility of the storage. Early investigations concluded that the most likely mechanism for these excessive seepage flows was a defect in the concrete core wall.
Melbourne Water Corporation, (the owner and the operator of the reservoir), undertook a risk assessment for Silvan Reservoir as part of a review of its dams asset portfolio. Based on the information then available, the risk assessment was undertaken using the criteria and guidelines developed by ANCOLD. The result was that the piping risks associated with the seepage from the west abutment at Monbulk Saddle Dam was unacceptable. The risk assessment Panel also cast doubt on the likelihood of the seepage being caused by a defect in the concrete core wall. Melbourne Water therefore engaged SMEC Australia to investigate the likely causes and mechanisms for this seepage and to develop suitable remedial measures for the dam.
The investigations have included a desktop review of historical information, test pit investigations, Sonic borehole drilling, dynamic cone penetration tests, an infrared thermal imaging investigation and an electromagnetic groundwater seepage flow mapping investigation.
These investigations have shown that the most likely cause of the seepage is the presence of permeable foundation layers located beneath and around the existing core wall as the core wall does not extend over the full length of the embankment and becomes shallower towards the abutments.
To satisfy the ALARP principle; risk reduction remedial works Concept Designs are being developed and reviewed.
2011 – Investigating the Piping Risk Associated with Seepage at Monbulk Saddle Dam of Silvan Reservoir, Victoria
Lesa Delaere, Ivor Stuart, Thomas Ewing, David Marsh
As part of Wide Bay Water’s commitment to minimising environmental impacts of its water supply weirs, a “Nature Like” Fishway is under development for the Burrum No 1 Weir. This project is a fishway offset provision for the raising of Lenthalls Dam in the upper reaches of the Burrum River in Hervey Bay. The Burrum No 1 weir forms the primary pumping pool for the Hervey Bay water supply and is located at the tidal limit of the Burrum River. Understanding fish biology and behaviour is critical to the effectiveness of the design of a fishway as much as the balance between the goals of maximising fish passage versus cost, construction and operational difficulties that a fish passage solution may present.
This paper presents the aquatic ecology of the project and the inter-relationship of fish biology and river flow frequency. It discusses the fish species of the Burrum River, their behaviour, seasonal migration and criteria for successful passage. It presents the analysis of river flows with respect to frequency and headwater/tailwater relationships to weir drownout, which was complicated by the tidal flow regimes downstream of the weir. These aspects were also applied in consideration of river behaviour; low flow characteristics for fishway operation during dry seasons and drought, and high flow characteristics during the wet season and floods.
The biological needs for successful fish passage for two very different river flow characteristics were analysed. This allowed targeted design criteria and fishway solution to be developed to provide maximum benefit without causing undue cost to the project.
Burrum Weir Fishway – Fish Biology and River Flows: Two Faces