2008 – Estimating construction flood risk
Peter Hill, Phillip Jordan, Rory Nathan, Emily Payne
Abstract: There are a number of issues that need to be considered when deriving estimates of floods used to assess construction flood risk. This paper outlines the derivation of seasonal flood frequency curves and highlights the important differences in seasonality across Australia and the variation with the exceedance probability. Examples are provided as to how these seasonal frequency curves are used to estimate the construction flood risk during a particular construction activity in a safety upgrade for an existing dam or construction of a new dam. The paper also touches on the issues associated with estimating consequences for assessing construction flood risk.
Keywords: construction flood, risk, seasonal hydrology, hydrologic loading
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Papers 2008
2008 – Dams on Karst foundations risk assessment and remediation
Learn moreJeffrey A. Schaefer
Abstract: The U.S. Army Corps of Engineers (USACE) has numerous dams built on limestone foundations that are susceptible to solutioning (karst). Significant dam safety issues related to the karst foundation have developed in many of these projects. Screening risk assessments of the USACE portfolio of dams has shown that defects related to karst foundations is one of the largest contributors to our risk. To better evaluate this risk, a method to estimate the probability of failure from piping into karst foundations has been developed by team of experts from the University of New South Wales, URS, the US Bureau of Reclamation, the US Army Corps of Engineers. This paper summarizes the major failure modes associated with dams on karst foundations and the methodology developed to perform risk analysis. A summary of USACE case histories with karst foundation issues and recent projects to remediate the foundations are also included.
KEYWORDS: Dam, Seepage, Karst, Solution Feature, Risk
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Papers 2008
2008 – Geotechnical risk assessment and management for hydropower development sites in the Indian Himalaya
Learn moreRobert Goldsmith
Abstract: Selection of suitable dam and associated structure sites for run-of-river hydropower projects in the Himalayan region requires a thorough assessment of the hazards and the consequences of their occurrence. Typical projects in this environment of deep ridge and ravine terrain comprise concrete gravity dams with steel flood or sluice gates. The steep abutment slopes generally require spillways to be over the dam crest and not as separate structures.
Hazards to be considered for such structures are linked mainly to geological and hydrological factors and include landslides both at the dam and in the reservoir, outwash floods from a variety of initiating causes (landslide dams or glacial melt), floods from storm events, weak or unstable foundations, seepage and pore pressures and earthquakes. Each hazard is related to a risk scenario and a matrix of events is evaluated for potential cause, consequence and impact. A measure of consequence to the project at various stages of design, construction and operation is obtained and provides a basis for preparing risk mitigation measures. Risk management plans can then be prepared using a forum process with stakeholders to achieve a satisfactory outcome.
Examples are provided from specific studies carried out for hydropower projects in the Indian Himalaya. They include a risk assessment of a floodwave overtopping a dam from debris torrent after breaching of a landslide formed dam, terrain studies and geomorphological assessment to locate landslides in dam abutments and design and construction issues relating to a project site astride a major tectonic thrust zone.
Keywords: geotechnical risk, hazard and risk matrix, risk management plan, landslides, landslide dams, hydropower dams and tunnel, Himalayas
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Papers 2008
2008 – Innovative Spillway Developments in the ACT – Technical Memoire Paper
Learn moreGeoff Henkel
ABSTRACT
The author has been involved in the design of a number of dams and spillways and specifically the design of several ACT lakes and ponds working either for the development authorities or as a consultant to them. This paper, a memoire, describes seven innovative spillways utilised in the ACT, five of which he was directly involved with. The use of dual spillways has been a common feature of the designs and this has been a very economic approach. As well as some use of fuseplug spillways, a labyrinth spillway and the safety of embankment overtopping, this paper may provide some useful ideas for new developments.
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Papers 2008
2008 – Risk assessment in spillway remedial works design and construction and monitoring at Googong Dam
Learn moreJames Willey, Malcolm Barker, Javad Tabatabaei
Abstract: During successive flood events from the end of construction of Googong Dam in 1978 through to the late 1980s, erosion of in excess of 5,000 m³ of rock occurred in the partially unlined section at the downstream end of the spillway channel. Remedial works were undertaken in stages during the 1980s to stabilise the eroded chute and limit further erosion. A project is currently underway as part of the Bulk Water Alliance to construct remedial works in the spillway to repair erosion damage and increase the spillway capacity to safely pass the current estimate of the Probable Maximum Flood. The design was undertaken by GHD Pty Ltd as part of a separate engagement prior to the formation of the Alliance.
The recent work involved a review of the historical performance and prediction of future performance of the structure. A process involving the development and comparison of options and ultimately the detailed design of the preferred arrangement followed, including refinement and validation using a physical hydraulic model study.
This paper presents risk assessment techniques used throughout the project on a range of tasks including prediction of future spillway erosion damage and comparison of spillway remedial works options, assessment of construction flood risk and definition of instrumentation requirements for the dam and associated structures.
Keywords: risk assessment, remedial works, spillway erosion, rock erosion, construction risk, instrumentation.
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Papers 2008
2008 – Probabilistic Description of Scour Hole Downstream of Flip Bucket Spillway of Large Dams
Learn moreG. Shams Ghahfarokhi, PHAJM van Gelder, JK Vrijling
Abstract: Risk and reliability analysis is presently being performed in almost all fields of engineering depending upon the specific field and its particular area. Probabilistic risk analysis (PRA), also called quantitative risk analysis (QRA) is a central feature of hydraulic engineering structural design.
Actually, probabilistic methods, which consider resistance and load parameters as random variables, are more suitable than conventional deterministic methods to determine the safety level of a hydraulic structure. In fact, hydraulic variables involved in plunge pools, such as discharge, flow depth, and velocity, are stochastic in nature, which may be represented by relevant probability distributions. Therefore, the optimal design of a plunge pool needs to be modelled by probabilistic methods.
The main topic of this paper is concerned with the reliability-based assessment of the geometry of the plunge pool downstream of a ski jump bucket. Experimental data obtained from a model of a flip bucket spillway has been used to develop a number of equations for the prediction of scour geometry downstream from a flip bucket spillway of a large dam structure. The accuracy of the developed equations was examined both through statistical and experimental procedures with satisfactory results. In addition, reliability computations have been carried out using the Monte Carlo technique.
The main conclusions are that structural reliability analysis can be used as a tool in the dam safety risk management process and that the most important factors for further analysis are erosion, friction coefficient, uplift and self-weight.
Keywords: risk analysis, reliability, plunge pool, Monte Carlo simulation, flip bucket, large dams
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