Jeffrey 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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Francisco Lopez, John Bosler
Abstract: A study has been undertaken to determine the structural adequacy of the Intake Tower for the Dartmouth Dam Low Level Outlet Works under the Maximum Design Earthquake (MDE). The tower is a reinforced concrete frame structure. The study included the definition of appropriate seismic inputs and nonlinear time-history analyses for different levels of ground shaking.
The behaviour of the tower was assessed in terms of material strains. The seismic acceleration-time history was applied to the model and the nonlinear analysis identified the locations in the structure which reached the following performance thresholds: cracking of core concrete, yielding of reinforcing steel, fracture of reinforcing steel and crushing of core concrete.
The results of the study showed that the tower was able to resist, without collapse, the loadings imposed by three different MDE acceleration time-histories. The predicted level of damage consisted of generalised yielding of reinforcement, formation of numerous plastic hinges at different locations in the tower and generalised spalling of cover concrete.
Overall, the structural response of the tower under the MDE events was considered satisfactory and met the performance requirement that structural collapse should not occur.
Keywords: intake tower, nonlinear, inelastic, time-history, seismic analysis.
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
David S Bowles, Sanjay S Chauhan, Loren R Anderson, Terry F Glover
Abstract: A nested model is presented for considering variability and knowledge uncertainty in a dam safety risk assessment of an existing dam and interim risk-reduction alternatives (operating restrictions) during the staged implementation of a permanent structural risk reduction measure. The effects of some important aspects of natural variabilities on estimated risks are represented as cumulative distributions of probability of failure, annualised life loss, economic risk cost, and an F-N representation of life loss. Many cumulative distributions are generated to represent the effects of some important aspects of knowledge uncertainties.
An important aspect of the knowledge uncertainty is the current level of development of an already-initiated piping failure mode. Also, an approach to conditioning the system response probabilities (SRPs) for the piping failure mode on the duration of reservoir pool exceedance is included in the failure event tree risk model.
ANCOLD and Reclamation tolerable risk guidelines are evaluated at selected percentiles of variability and percentiles (levels of confidence) of knowledge uncertainty. The incremental cost-per-statistical-life saved and benefit-cost ratio for interim risk-reduction alternatives are estimated and evaluated to examine the case for more-severe levels of operating restriction than the least-severe operating restriction that is estimated to satisfy, at a selected percentile of variability and a desired level of confidence, the limit values in all of the tolerable risk guidelines that were considered.
Keywords: Dam safety, risk analysis, risk assessment, uncertainty analysis, aleatory uncertainty, epistemic uncertainty.
G. 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
Stuart Richardson, Peter Liepkalns, Rod Mauger
Abstract: Operations and maintenance of large dams can be complex and potentially high risk. Even on a relatively new dam like Dartmouth, Goulburn-Murray Water is regularly presented with maintenance tasks which are made very difficult because access for maintenance may not have been considered during the original design and construction of the dam.
Goulburn-Murray Water recently completed a change over of a 200mm cast iron gate valve at Dartmouth, not in itself a complex or high risk maintenance task. What made this task challenging was that the valve was subject to full reservoir head (up to 180m) with no means of isolation.