Hamish Smith, Graeme Maher
In order to achieve environmental sustainability it has become standard engineering practice to include a fishway on all new or refurbished large dams in Australia.
As regulators expand their understanding of fishways, project approval conditions associated with these complex engineering structures are changing. Regulators now increasingly wish to participate in the development and selection of the final fishway to be adopted.
This paper describes the process developed and implemented at Queensland’s most recent dam under construction, the Wyaralong Dam, to ensure that the views and opinions of regulators and stakeholders were sought and considered during the fishway selection and design process.
With no written guidelines available on “how to select and design a suitable fishway”, all associated parties entered into the process without a full knowledge of how it would unfold and what the final outcome would be.
This paper demonstrates that in an increasingly regulated environment it is possible to have regulators, proponents and stakeholders work cooperatively together to achieve a result that provides for sustainable development and is acceptable to all parties.
This paper will provide a model that could be adopted for the development of new fishways or the refurbishment of existing fishways on large dams in Australasia.
Changing Regulatory Environment – Large Dams and Fishways
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Steven Slarke, Martin Mallen-Cooper, Andrew Evans, John Prentice
As part of the Murray-Darling Basin Authority ‘Sea to Hume Dam’ program to restore fish passage along the River Murray, an innovative Denil fishway is being retrofitted into Mildura Weir (Lock 11). Due for completion in the latter half of 2010, the fishway will allow the upstream and downstream passage of medium and large sized fish past Mildura Weir, which has a difference in water levels of 3.5 metres.
Constructed on the sloped concrete apron at the left abutment of the Dethridge weir, the Mildura Weir Denil fishway design is innovative in the River Murray. The Denil fishway is essentially separate from the existing weir, and its superstructure can be fully removed from the river during floods. The fishway can also be progressively removed during periods of rising floodwaters, maintaining operation during periods when fish migrate in particularly large numbers. The fishway represents a cost effective design, reflecting the decision to maintain the current weir structure for a further forty years, but still providing passage to a broad range of fish sizes and species. Innovative fish monitoring and carp separation facilities will be provided, shared with the other River Murray fishways. But, unique to the River Murray, viewing windows are provided to allow the public to observe fish negotiating the fishway, and to enable a better understanding of fish movement.
Peter A Ballantine, Christopher V Seddon
Massingir Dam, constructed in the late 1970’s on the Olifants River in Mozambique, is a 48 m high zoned earthfill dam. Due to various safety concerns, the dam was operated at a reduced full supply level of 110 masl, compared to the design full supply level of 125 masl. Between 2004 and 2006 remedial works were undertaken, including the construction of a berm on the downstream face of the dam, grouting and drainage of the foundations and installation of the spillway crest gates. From December 2005 the storage level of the dam was allowed to increase.
On 22 May 2008, with the reservoir storage level at 122.43 masl and the gates on the outlet conduits closed, the reinforced concrete conduits failed at the downstream end, releasing an estimated 1,000 m3 /s of water into the Olifants River.
A 2-D finite element analysis was undertaken in order to establish the safe load bearing capacity of the as-constructed conduits. On the basis of the analysis, it was concluded that the original design did not take proper account of the pressure that would develop within the thick concrete sections of the conduit. In view of assumptions regarding the load paths, the reinforcement was not placed in the most appropriate positions.
This paper describes the events leading up to the failure of the conduit, presents the findings of the investigation into the failure and makes recommendations on the basis of the findings.
Aric Torreyson, Krey Price, Bob Hall
In a 2004 feasibility study, the U.S. Army Corps of Engineers (Corps) and Ventura County Watershed Protection District (VCWPD) recommended decommissioning Matilija Dam, a concrete arch dam originally constructed to a 60-metre height in 1948. A decade after its completion, the United States Bureau of Reclamation (USBR) constructed the Ventura River Project, comprising additional facilities designed to meet the growing water demand of Ventura County. Robles Diversion Dam, a 7-metre high by 160-metre long diversion structure located downstream of Matilija Dam, was built under the Ventura River Project to feed Lake Casitas, a water supply reservoir that serves as an integral part of the overall project.
Due to extreme sedimentation, Matilija Dam no longer serves its intended water supply and flood control purposes. In addition to the loss of storage capacity, other issues surround the dam, including adverse environmental impacts from its continued operation, seismic considerations, and structural concerns. These concerns led to the decision to decommission the dam as an essential step in rehabilitating key ecosystems in the Ventura River Catchment and reducing future risks to public safety. According to current estimates, 5 million cubic metres of sediment has accumulated behind the dam and will need to be removed in conjunction with the dam decommissioning; minimising the associated downstream impacts has been the subject of additional government studies.
The USBR determined through detailed hydrologic, hydraulic, and sediment transport analyses, including numerical and physical modelling, that the existing Robles Diversion Dam was not capable of passing the increased sediment load expected to result from the removal of Matilija Dam. To increase the sediment transport capacity across its spillway, the existing diversion dam requires modification. Under contract with the Corps, Tetra Tech and its subcontractors are completing the design plans for the Robles Diversion Dam modifications.
This paper presents unique aspects of the Robles Diversion Dam modifications, including sediment management procedures guided by numerical and physical model results and issues associated with the design of a rock ramp spillway and high-flow fishway, expansion of the existing spillway gate structure, and raising of the dam embankment. The rehabilitation efforts reduce impacts to the migration of endangered fish species and allow for the eventual removal of Matilija Dam, which is the ultimate goal in the effort to balance engineered structures with a natural river setting. When completed, the project will provide fish passage to the upper catchment for the first time in over sixty years.
Cat McConkey, Zarmina Nasir, Rachel Caoil
The Enlarged Cotter Dam (ECD) is the first major project to be assessed and approved under the new planning regime in the Australian Capital Territory (ACT). ACTEW chose the ECD as its highest priority option in securing Canberra’s water supply for the future because of its relative economic benefit to the community, reliability of water supply, technical feasibility and comparatively low environmental impact.
The planning and construction of large dams has been reduced from a typical 10 plus years to four years in the ACT and surrounds for the ECD. Australian and International Dam design and construction has significantly developed from a time when dam approvals focused on engineering, economics and constructability to now include regulatory planning processes that seek to reconcile environmental, social and economic impacts.
This paper explores and contrasts the experience of securing approvals for the ECD in 2009 to past experiences of dam planning approvals and consultation processes.
Rick W. Schultz P.E.
The Corps of Engineers Risk Management Center is undergoing a nationwide assessment of its navigation and flood control projects. Development of the methodology and tools used to determine probability of failure of mechanical and electrical systems for dams is being presented in this document. Development of the Weibull formulas for specific use in dam will be addressed along with use of fault tree analysis to determine system reliability.
Keywords: Dormant-Weibull Formula, Fault Tree, Characteristic Life of Components, Beta Shape Parameters, Inspection intervals.