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.
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James 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.
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.
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
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.
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.