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.
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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.
Chris Topham, Paul Southcott, Tim Cubit
Abstract: Dee Dam is a 15 m high and 270 m long central core earth and rockfill dam on the upper reaches of Hydro Tasmania’s Tungatinah Power Scheme. The dam is assessed to have a High A hazard category. Hydro Tasmania’s portfolio risk assessment found that a risk based upgrade was warranted to protect against both piping and flood overtopping failure modes.
A $4.2M modernisation project was implemented in 2008 comprising the installation of a full height downstream filter with rockfill buttress, repairs to cracking in the diversion conduit and raising of the core for improved flood capacity. Lowering of the Dee Lagoon to facilitate full height excavation of the downstream shoulder of the dam was impractical to Hydro Tasmania for production, environment, cost and stakeholder reasons. Hence, careful analysis and risk management was required to ensure the safety of the dam during the construction of the above works against a full storage.
This paper presents the risk objective for the upgrade work, modelling and analysis undertaken to assess dam safety during the works. A wide range of construction risk mitigation measures were employed prior to and during the upgrade works. The response to latent conditions and potential dam safety incidents that occurred during construction are described. The continuous adaptation of the construction methodology to suit site conditions encountered during the project is also presented. The project was successfully completed in June 2008. The approach of detailed investigations and design coupled with a strong risk based approach during the construction proved to be effective in managing the dam safety risks of construction work with a full reservoir.
Keywords: earth and rockfill dam, filters, construction risk mitigation, dam safety, dam safety incidents.
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.
Abstract: Recent studies in SE Queensland for existing and proposed dams and other flood studies have highlighted a number of issues with respect to the design event approach in deriving and applying design rainfalls to calibrated runoff routing models.
The estimation of design rainfall depths for frequent to large events is usually done by the intensity-frequent-durations (IFD) methods outlined in Australian Rainfall and Runoff (AR&R) or CRC-Forge.
The design temporal patterns applied to these rainfall depths are critical in the estimation of design floods as are the adopted loss rates.
This paper describes the methods used to derive the design rainfall and some of the issues which arose in their application in the design event approach to assessing design floods. It uses examples at several locations in studies undertaken by SunWater and refers to similar issues encountered in other studies.
Implications for flood studies are outlined.
Keywords: design rainfall, design floods, CRC-FORGE, IFD, temporal patterns, SunWater, Queensland.
Richard John Kenny
Abstract: Physical hydraulic models have traditionally provided the input to spillway design.
However, the application of Computational Fluid Dynamics (CFD) is increasing in popularity.
The reliability of CFD depends on several complex issues including the physics of the flow regime, the mathematical formulae describing the fluid flow and the solution method. CFD reliability should be confirmed for each particular application, including ogee spillways (common discharge structures on dams).
A literature review has been undertaken to determine validation techniques and the reliability of CFD related to ogee spillways. There is a great deal of information available about the aeronautical, marine, automotive and mining industries but relatively little available about ogee spillways. The degree of validation for ogee spillways was found to vary significantly and raises the question of whether better validation can be achieved. Better validation may improve confidence in CFD techniques.
A project has been undertaken through Queensland University of Technology to model a prototype scale notional 2D ogee spillway using FLUENT CFD software under steady state conditions. The project tests the sensitivity of the CFD results to various parameters. The sensitivity of the inlet mass flow rate was tested for several CFD parameters. The investigation considered:
1.The grid spacing required for geometric convergence;
2.The effect of domain extent on the computational results;
3.The influence of a boundary layer and wall roughness;
4.The importance of the choice of turbulence model; and
5.The impact of inlet turbulence assumptions.
The recommendations of the sensitivity investigations were used for validation against the United States Army Corp of Engineers (USACE) design data.
Excellent agreement was found between the CFD predictions and the USACE values.
Keywords: Computational fluid dynamics (CFD), dam, flood routing, fluent, Navier-Stokes equations, Numerical/computational modelling, Ogee spillway crest, physical model, turbulence.