C.Johnson, D.Stephens, M.Arnold and N.Vitharana
As part of Melbourne Water’s dam safety upgrade program, emergency release capacity is being investigated at a number of dams. Recent work undertaken by the Water Resources Alliance (WRA) for Melbourne Water has highlighted the lack of current Australian guidelines for appropriate emergency release capacity. With no relevant ANCOLD Guidelines, current practice still references the 1990 USBR guidelines which relate the length of time to empty a reservoir to the hazard and risk associated with dam failure. As hazard category assessment criteria has been improved since and dam design and safety standards are more stringent, the applicability of the USBR criteria in today’s environment is under consideration.
With the prevailing climatic conditions requiring the augmentation of Melbourne’s water supplies, the Tarago Reservoir was recently brought back into service. However, the dam lacked adequate emergency and environmental release capacity, with this being critical to manage construction flood risk for a pending filter raising project. Through an analysis of recorded inflow data, it was evident the existing scour facility had insufficient capacity to handle the recorded inflows, and would not be able to maintain the reservoir at an appropriate level during the proposed works. The length of time to empty the reservoir for the existing scour facility and the preferred scour upgrade option were calculated and it was found that by providing a new 1200mm scour facility, USBR emptying times were met or exceeded. The enlarged outlet capacity was also required to meet the new environmental flow requirements for the dam.
The paper will review international guidelines, share the experience of several Australian water authorities in assigning emergency release capacity for their dams, and discuss the specific work undertaken to provide suitable emergency release capacity at Tarago Reservoir for Melbourne Water.
Jim Walker, Sergio Vallesi, Neil Sutherland, Peter Amos, Tim Mills
The Tekapo Canal is a 26km long hydropower canal owned by Meridian Energy Ltd in New Zealand. Completed in 1976, the canal is 40m wide, 7m deep and has a capacity of 120m3/s. The canal was constructed from compacted local glacial soils with a compacted silt lining sourced from till deposits.
During 2007 and 2008 the canal showed signs of leakage where it crossed over a twin barrel culvert structure. In October 2008 a diver inspection identified depressions and sinkholes on the invert of the canal above the culvert. Approximately 6m3 of silty gravel lining material had settled. Testing showed direct and rapid connections between lining defects and seepage outflows at the culvert outlet headwall. Subsequent ground penetrating radar survey confirmed the presence of voids above the culvert barrels. Diver placed filling of the defects with granular materials was immediately implemented, and a series of remedial actions over the next four months were required to arrest deterioration and enable the canal to remain operational.
The paper describes the initial response to this situation and the immediate measures taken to prevent failure. It also describes the medium term and ongoing measures implemented to maintain the safety of the canal while permanent remediation requirements are assessed. The lessons learned from this event, and their impacts on Meridian’s Dam Safety Assurance Programme (DSAP) are also discussed.
Immediate response measures included ongoing filling of lining defects with filter gravel, intensive land based and diver surveillance of the canal, planning and resourcing for emergency contingency actions in the event that a risk of breach developed. Medium term measures included arresting leakage by placing a low permeability blanket of silty gravel over the damaged area using a concrete pump, and constructing external buttresses capable of safely withstanding large discharges should deterioration of the canal structure occur.
These short and medium term remedial measures were completed with the canal full and in operation and continue to perform well 20 months later. Continuing risk mitigation measures include enhanced surveillance and monitoring (land based and using divers), localised treatment of defects, as well as ongoing monitoring and review of the Dam Safety management regime and sustained Emergency Management preparedness.
Justin Howes, Peter Amos
For many years Mighty River Power has operated an intensive Dam Safety Assurance Programme with respect to our nine large hydro assets, a unique run of river cascade system built between 1927 and 1972. From 2001 to 2007 the Arapuni Foundation Enhancement Project was a high profile activity, but there has also been much dam safety analysis and minor mitigation work that could be classified as “Business As Usual Dam Safety Activity” – this paper seeks to give a high level overview of the work carried out from 2000 to 2010. Items covered include; an overview of the hydraulic structures, their hydrological and geological setting, and the current dam safety regime. Examples of typical issues identified by the Programme are given on a structure by structure basis along the river. Seismic, Flooding, Emergency Planning, Documentation, Monitoring, Control, Electrical and Mechanical type issues are covered.
G.L. Vaschetti, C.A. Verani, J.W. Cowland
Geomembranes are an established technique for long-term waterproofing of hydraulic structures including all types of dams, canals, tunnels and reservoirs.
Three construction projects are presented that feature unique waterproofing solutions leading to faster construction programmes and granting safer and longer service life at lower costs: The 35 m high Paradise Dam (aka Burnett River), Australia’s largest volume Roller Compacted Concrete (RCC) dam waterproofed using a PolyVinylChloride (PVC) geomembrane sandwiched between prefabricated concrete panels and the RCC itself; The 50 m high multipurpose Meander Dam in Tasmania, designed as a RCC dam of the low cementitious content type whose imperviousness is provided by a PVC geomembrane installed in exposed position and mechanically anchored to the upstream face of the dam; And the Eidsvold Weir, a 115 m long 15.45 m high RCC structure used for water supply, waterproofed using an external PVC waterstop installed on the upstream face and able to accommodate the expected movements at the joints.
The paper will outline the technical details, installation and performance of the geomembranes.
Advantages gained from the use of a geomembrane waterproofing system on RCC dams – experiences from Australia
Ted Montoya, David Hughes, Orville Werner
The existing Hinze Dam was raised beginning in 2007 to increase water storage capacity, improve its ability to regulate floods, and raise the level of structural safety as compared to the current dam. As part of the 15 m raise of Hinze Dam, the existing 33 m high spillway structure was raised using mass concrete. This new composite structure was constructed as a downstream raise, placing mass concrete on the downstream and top of the existing spillway. The designers of the composite spillway structure developed a finite-element model to consider the early expansion and subsequent slow contraction of the new concrete against the existing concrete. The temperature rise of the new section of mass concrete had to be monitored and controlled to reduce the tensile strains along its interface with the existing spillway, and differential temperatures had to be limited to avoid cracking of the new mass section. Low-heat cement for a conventional mass concrete mix was not readily available so a mix was developed using local materials.
Typical mass concrete dams are monolithic structures constructed with lowheat cement. The Hinze Dam spillway design was predicated on the use of materials readily available. The paper presents the assumptions, methods, and criteria that were used in developing the mass concrete mix. It also presents the means and methods for tracking temperature gain during construction of the raised spillway, and how temperature was influenced by placement temperature, construction sequencing, and seasonal conditions. Lastly, the paper will compare the actual performance of the mix with the design analysis, laboratory testing, and finite element studies that were performed during the design.
Shao Kwan Ng
Asset management aims to ensure that assets, such as dams, are sustainable. In order to achieve this, management decisions need to be defensible and the long-term impacts of short-term decisions need to be clearly demonstrated, such that an asset operates and is maintained in an appropriate fashion and in a satisfactory condition. Expert rule systems are becoming widely recognised as powerful and elegant tools suitable for engineering and management decision-making. They are powerful, transparent and flexible tools that mimic how people make decisions, and hence provide a natural way of thinking for decision-making. This paper reviews the current usage of expert systems in asset management, and illustrates the potential of these tools, in conjunction with the available (ANCOLD) guidelines, to assist dam owners in decision-making, such as in condition evaluation and dam hazard assessment applications.
Keywords: Decision-making, expert rule systems.