Eric Lesleighter, Bronson McPherson, Karen Riddette, Jon Williams
Abstract: The paper describes in part the hydraulics investigations required for the major upgrade works for Lake Manchester Dam; investigations which utilised two modelling procedures. Following a brief outline of the dam rehabilitation program and works, the inadequacy of the former spillway for the revised hydrology is described. The urgency of the construction program led to the use of CFD modelling following preliminary desk studies of the hydraulics, in order to delineate certain features of the new spillway and the plunge pool area and allow the construction program to be fast-tracked. As part of the overall program to refine the spillway design, and due to the evident complexity of the flows over the spillway, physical hydraulic model studies were also carried out.
The investigations provided a very good opportunity to compare the results from two modelling approaches carried out within the same design activity. Specific results of aspects such as flow patterns, velocities, pressures, and wave action will be compared. A prominent purpose of the paper will be to acknowledge the advantages and limitations of both approaches, and seek to provide the guidelines and advice that designers and dam owners should follow and adopt to ensure the hydraulics requirements of projects are soundly engineered.
Keywords: spillways, physical modelling, numerical modelling, CFD.
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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.
Jerome Argue, Steven Slarke, Douglas Rudd
Abstract: The North Para River Flood Control Dam is an integral part of the works proposed to manage flood risk in the Gawler River, north of Adelaide. Undertaken by the Gawler River Floodplain Management Authority (GRFMA), an authority representing the six local Councils in the area, the flood storage is located on the North Para River, about 10 kms north of Gawler. Roller Compacted Concrete (RCC) was selected for the construction material, based on an assessment of the geotechnical and geological site conditions, together with advantages of reduced cost, time and structure required to pass design flood events. With an overall project cost of $16.1 million, the dam was constructed on time and well within pre-tender budget estimates.
Keywords: flood mitigation, roller compacted concrete, North Para River, construction.
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.
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
Brett J Heppermann
Abstract: As one of the main federal agencies with responsibility to build, operate, and maintain large dams in the United States, the US Army Corps of Engineers (USACE) is developing a risk based framework to better manage their portfolio of 600+ dams in terms of risk management and prioritization of funding. A key element to this effort is the development of risk-based analytical tools to evaluate primary features for applicable failure modes. These are used in conjunction with loading and consequence modules to assess the overall risk associated with the dam in terms of lives and economic damages. The focus of this paper is on the analytical tool being created for evaluating the probability of an uncontrolled reservoir release due to scour of concrete lined stilling basins and spillways.
This analysis module is broken down by potential failure modes that could initiate events that could lead to an uncontrolled release. The failure modes that are considered are Cavitation, Slab Uplift and Foundation Erosion, Ball Milling, Tunnel/Conduit Failure, Plunge Pool Erosion, Fuse Plug and Fuse Gate Failure, Hydraulic Loading and Profile, and Headcutting. Each failure mode analysis takes into account how the inlet structure, conveyance, and terminal structure is affected, if at all.
This paper gives and overview of the main characteristics of each failure mode and the methods used to quantify the risk associated with each.
Keywords: risk based analytical tools, concrete scour, spillway, stilling basin, risk assessment, U.S. Army Corps of Engineers.