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S. Knight, B. Cooper and P. van Breda
Warragamba Dam was completed in 1960 and impounds Sydney’s main water supply storage. Hydrological studies in the 1980’s showed the existing spillway to be significantly undersized by modern standards. Considering the dam’s High Incremental Flood Hazard category, the current risk of dambreak is unacceptably high. This has resulted in a two-stage program to upgrade the dam to full Probable Maximum Flood (PMF) capability.
The interim (first stage) measures were completed in 1990 and involved a 5.1 metre raising of the dam crest and significant post-tensioning of the dam wall. Following many feasibility/option studies and detailed technical and environmental studies, a contract was let by Sydney Water Corporation (SWC) in late 1998 for the construction of an auxiliary spillway as the major (second stage) component of the flood security upgrading. The spillway will be a large capacity (about 18,000m*/s) concrete lined chute 700 metres long around the dam’s right abutment. In the upper curved section will be the largest fuse plug embankments in Australia (up to 14.5 metres high). The lower straight section will terminate with a flip bucket structure.
The NSW Department of Public Works and Services (DPWS) designed the earlier Interim Works, undertook the subsequent engineering option studies for the Major Works and carried out the concept design and technical specification for the new auxiliary spillway and associated dam modification works. This paper summarises the project, describes the main features of the concept design of the spillway and outlines the associated dam modifications.
The role of judgement in risk assessments as applied in dam safety management has been the source of considerable debate in recent years. With regard to risk analysis of dams, and while there is general agreement that judgement is an essential element of the process, essentially two schools of thought have emerged. One view holds that, in the assignment of probabilities, reliance can be based on collective engineering judgement that is anchored to a knowledge base. The second view holds that judgement should be based on the knowledge that is revealed by an appropriate amount of analysis. The paper, written from the perspective of the latter view, explores some of the underlying issues in this debate.
The role of judgement in risk evaluation, the process of judging the significance of risk, is considered to be equally important. However, the process of making value judgements and statements of principles is complex and often beyond the sphere of engineering. The third issue addressed in the paper concerns the search for answers to the question, “How good is the assessment?”
The Bundaberg Irrigation Area (BIA) is served by a reticulation system of channels, pipelines, pump stations and balancing storages drawing water from a major dam (Fred Haigh on the Kolan River), augmented by a number of weirs and tidal barrages. The scheme as originally proposed in the late 1960’s included a major dam on the Burnett River that has never been built. Accordingly, the reliability of the system was lower than desired, a situation exacerbated by prolonged drought during the 1990’s.
In the 1980s, alternative (cheaper) sources of water supply were investigated and a weir site on the Burnett River (Walla) was selected as the most promising. In 1993, the Commonwealth and Queensland Governments agreed to the Sugar Industry Infrastructure Package (SHP). Walla Weir was included in the Package, subject to environmental and economic assessment.
Detailed impact assessment studies were carried out and submitted to both State and Commonwealth Environment departments. In the light of strong opposition from environmental groups (whose major concern was the Queensland Lungfish), the Federal Minister for the Environment commissioned an independent review of the IAS before granting approval.
Approval was conditional on the implementation of an Environmental Management Plan and a River Operation Plan as well as a commitment to undertake extensive baseline studies before any new development is proposed in the area. This paper will discuss the investigation and approval process and describe the additional monitoring/studies being carried out.
David S. Bowles, Loren R. Anderson, Joseph B. Evelyn, Terry F. Glover and David M. Van Dorpe
A demonstration risk assessment was conducted on the 283-foot high rolled-earthfill Alamo Dam as part of a U.S. Army Corps of Engineers (USACE) Research and Development program. The existing dam and 19 structural risk reduction alternatives were evaluated for flood, earthquake and normal operating conditions. The paper summarizes the risk assessment process, results, findings and recommendations. It also provides an evaluation of the risk assessment process and recommendations for better positioning the USACE to use risk assessment for dam safety evaluation and decision support.
Buddhima Indraratna, Mark Locke and Gamini Adikari
The main objectives of the filter are to prevent erosion of the dam core, permit controlled passage of seepage flow through the dam and facilitate dissipation of excess pore pressures in the core. In most designs of dam filters, empirical methods based on particle size ratios have been used. These empirical rules are developed through extensive laboratory tests. Although the empirical rules benefit from directly or indirectly incorporating most factors affecting filtration, they cannot be extrapolated for distinctly different soils and do not describe the time dependent changes that occur within the filter medium.
Mathematical models can be formulated to explain the fundamental physics of particle interaction and migration, within a framework of well defined geohydraulic constraints. Considering the mass flow and momentum conservation principles; time dependent changes in particle size distributions, mass flow rates, retention capacity and base soil erosion rates can be simulated.
This paper reviews various empirical and mathematical models, based on the authors experience. A novel approach to large scale filtration is highlighted based on testing actual soil and filter materials from an Australian dam, in a new 500mm diameter apparatus.