2006 – Ross River Dam Spillway Gate Reliability and the Impact on the Design
Malcolm Barker, Barry Vivian and David S. Bowles
Ross River Dam is located approximately 15 km upstream of Townsville and provides a dual role of water supply and flood mitigation. The dam comprises a 39.6m long concrete overflow spillway flanked by a central core rockfill embankment of 300 m in length with a 7,620 m long left bank earth fill embankment, which has inadequate internal filter zones for piping protection. Since completion, design rainfall predictions for the area have doubled, technical data has changed and so, too, have dam safety standards. Dam safety evaluations during 2000-2002 showed that the dam required upgrading in order to bring it up to international standards. As an interim measure, the spillway was cut down by 3.6m.
Upgrade design works were then completed using risk-based design criteria to validate the design, and construction is in progress. The upgrade works comprise spillway anchoring, installation of three radial gates on the spillway, stilling basin modifications, embankment filter protection, and dam crest raising.
This paper presents the options considered, the method of reliability analysis, and how the results influenced the spillway system design and overall risk evaluation for the upgrade design.
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2006 Papers
2006 – Some Lessons Derived from the Surveillance of the Dams of the Snowy Mountains Scheme
Learn moreSteve Donker
When undertaking a program of quantitative surveillance of dams the potential to make expensive decisions based on inaccurate and/or inappropriate data always exists. The implementation of a ‘quality’ based system of quantitative surveillance as identified in the ANCOLD Guidelines On Dam Safety Management 2003 can reduce the likelihood of making these inappropriate decisions.
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2006 Papers
2006 – Some Dam Monitoring Observations and Experiences from New Zealand
Learn moreM Gillon, T Logan, N Logan
The paper has been prepared to support the key questions selected for the ANCOLD Dam Instrumentation and Survey Seminar to be held in Sydney in November 2006 and to provide a New Zealand perspective. The paper is not a ‘state of dam monitoring practice in New Zealand’ dissertation but is rather a targeted summary of the authors’ experiences and observations from practicing in this area.
These experiences and observations on dam monitoring are grouped under the following headings, reflecting the key questions:
- Dam monitoring in New Zealand
- Monitoring in the dam life cycle
- How much monitoring is enough
- Instrumentation and survey issues
- Using and managing monitoring data
- Some early warning examples
- Monitoring as part of remediation
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2006 Papers
2006 – Contemporary Issues Concerning the Instrumentation/Survey of Dams from the Legal Perspective
Learn moreDermot Armstrong
This paper reviews the general principles of duty of care which assist in the understanding of responsibilities that may exist for surveillance of dam safety, including the inter-play of the common law and statutory law. Only when there is a foundation in the general principles can obligations upon dam owners/operators with respect to surveillance and instrumentation be interpreted. Some legal issues around the development and use of industry guidelines are also explored.
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2006 Papers
2006 – An Overview of the U.S. Army Corps of Engineers’ Screening for Portfolio Risk Assessment for Dam Safety Program
Learn moreDavid M. Schaaf, P.E., Jeff Schaefer, Ph.D., P.E., P.G
The United States Army Corps of Engineers (USACE) has an inventory of over 600 dams. The main purpose of many of these dams is for flood control, but there are a significant number of dams primarily used for navigation. Additional benefits at many of these projects are provided through hydropower generation, recreation, and irrigation for farmers. Many of the dams are quite old and represent an aging infrastructure across the inventory. In addition, leaner budgets relative to the need for repairs across the aging system require that USACE invest wisely in order to efficiently use available funds to reduce the greatest risks across the inventory. Previously, individual projects with perceived deficiencies were evaluated separately by the responsible district. This evaluation was not compared in any programmatic way to other USACE dams being evaluated for deficiencies.
In order to improve the process of making risk-based decisions across the entire spectrum of USACE dams, the Screening for Portfolio Risk Assessment (SPRA) for the USACE Dam Safety Program was initiated during the summer of 2005. This effort represents the first level of a multiple phased effort to bring full scale risk assessment to the decision-making regarding making investment decisions associated with dam safety by linking engineering reliability with economic and life loss impacts on a relative scale. The SPRA effort involved the development of a tool for evaluating the relative life and economic risk of dam failures for a variety of deficiencies across the inventory of USACE dams. This paper will focus on the basic aspects of the evaluation tool as well as the process by which the screening was completed.
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2006 Papers
2006 – The Ability of Monitoring to Detect Internal Erosion and Slope Instability in Embankment Dams
Learn moreRobin Fell
Internal erosion and piping within embankment dams may initiate in cracks caused by differential settlement or desiccation, in cracks caused by hydraulic fracture and in very poorly compacted layers of soil. It generally cannot occur unless one of these defects is present because backwards erosion, the other mechanism for internal erosion, will not occur in embankments under normal gradients and will not occur in cohesive soils unless gradients are exceptionally high.
As a result it is very unlikely that it will be possible to detect initiation of erosion with piezometers, and the most likely successful method is seepage observation and monitoring. However the time from the first detection of increased seepage to breach of the dam may be very short-a matter of hours in some situations.
Thoughtfully positioned and read piezometers are more likely to be successful in identifying the critical gradients which may lead to the onset of backwards erosion in cohesionless soils in the foundation of dams.
Piezometers are more useful in establishing the pore pressures for use in analysis of stability, but in most cases where stability is marginal undrained strength analysis is required and the pore pressures and effective strengths alone are not sufficient to assess stability. In a number of cases differential settlements, and acceleration of settlements have proven valuable in detecting the on-set of instability and the conditions in which internal erosion and piping to initiate. Once these conditions are recognised more detailed survey monitoring and borehole inclinometers can be valuable in better defining the geometry of instability.
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