Karen Riddette, David Ho, Mike Phillips
Abstract: Stepped spillways have seen a resurgence in popularity in recent years as new research has better described flow conditions and quantified the energy dissipation capacity of this type of spillway.
While carrying out hydraulic modelling for the Hinze Dam Stage 3 upgrade project using Computational Fluid Dynamics (CFD) analysis, extensive numerical testing and validation of the CFD model for the proposed 0.8H:1V stepped spillway design was undertaken. It was found that for the expected Probable Maximum Flood (PMF) discharge, the high flow depth would result in a non-aerated, non-uniform flow, typically seen in the developing flow region just beyond the spillway crest, occurring over the entire length of the stepped spillway. To date, only limited laboratory studies have been published concerning energy losses in this particular flow regime.
This paper examines the ability of CFD modelling to compute energy dissipation and air entrainment effects in stepped spillways under extreme flow conditions. Where possible, the computed results were validated against limited published data. Some new data obtained by the CFD model is presented for energy loss in the developing flow region that has not been established before.
Keywords: stepped spillway, CFD, energy loss, developing flow.
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Marius Jonker, Mike Taylor and Glen Hobbs
Abstract: One of the activities authorized by organizations such as ANCOLD is the development of guidelines to enhance the ability of organizations to assure that adequate dam safety programs and practices are in place.
However, due to the absence of a single recognized guideline covering the various design aspects of dam outlet works, there is currently great inconsistency in the underlying principles for design and review processes for these facilities. A single, nationally recognized “standard” would lead to greater consistency between similar project designs, facilitate more effective and consistent review of proposed designs, and result in increased potential for safer more reliable facilities.
The need for a design standard is further underscored by the fact that poorly designed and/or constructed outlet works have been identified as a significant contributor to the occurrence of embankment dam failures. The failure of critical components in the outlet works of dams, not leading to an uncontrolled release of water, is generally considered less important due to apparent lesser consequences compared with a dam failure. However, the drought experienced over the past decade in Australia has highlighted the value of water and securing it for the future. The potential impact caused by the inability of a primary storage to supply a town or city, due to a failure in the outlet works, potentially affecting thousands of people and industries, could be devastating.
This paper summarises the current state of practice with regard to outlet works for dams concerning design, construction, inspection and evaluation, as well as maintenance and renovation. It also underlines some commonly occurring deficiencies encountered at existing dams. It provides a basis for further discussions of the state of practice for these topics in order to work towards consistent and unified outlet works design guidelines for dams.
Keywords: outlet works, state of practice, design guidelines
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
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.
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
David M Schaaf, Jeffrey A Schaefer, Rick W Schultz, Jason T Needham
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 engineering analysis modules used to generate fragility curves for dam features.
The analysis modules are broken into three main categories by engineering discipline: geotechnical, structural, and mechanical/electrical. The risk based assessment tools associated with geotechnical failure modes include Seepage & Piping, Embankment Stability, Seismic Performance, and Erosion of Unlined Spillways. The structural assessment tools include Concrete Monolith Stability, Spillway Gates, Scour of Concrete Lined Spillways, Spillway Training Wall Stability, Performance of Pipes through Dams, Hydropower Superstructures and Intake Towers. The mechanical and electrical are primarily focused on the performance of machinery used to operate dam gates.
This paper gives a broad overview of the main characteristics and methods used for each of these analysis tools. Some of the modules use historical performance to establish failure rates, while others are more analytically based. The context of each within the framework of the overall risk assessment effort of USACE dams is covered.
Keywords: risk based analytical tools, fragility curves, risk assessment, US. Army Corps of Engineers, portfolio, dams.