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.
Paul Somerville, Gary Gibson
Abstract: This paper describes current methods for seismic hazard analysis and their application at Hinze Dam. Although Southeastern Queensland has experienced significant earthquakes in historical time, none of them are known to have caused surface rupture, and no active faults that could be used to represent earthquake sources have been identified in the region that surrounds the site. Under these conditions, we must estimate the seismic potential of the region using historical seismicity. Two alternative approaches to modelling future earthquake occurrence based on historical seismicity have been used. The first approach is based on the AUS5 source model of ES&S (2005), which uses geological criteria to identify zones of uniform seismic potential, and then uses historical seismicity to characterize the seismic potential of each zone. The second approach, developed by Hall et al. (2007) at Risk Frontiers, is based on the spatial smoothing of historical seismicity without identifying discrete source zones. Previous work by ES&S has shown that the attenuation of strong ground motion in Southeastern Australia is fairly well represented by ground motion models developed using strong motion data from western North America. The recently developed NGA ground motion models based mainly on data from Western North America represent the local site conditions using Vs30, the shear wave velocity averaged over the top 30 metres at the site. This provides a significant advantage over previous models, which were for broad site categories such as rock or soil, and did not provide for the use of more site-specific information. The left abutment, lower tower and valley section foundation at Hinze Dam are characterized by hard unweathered rocks with shear wave velocity of 2.0 km/sec estimated from P wave velocity measurements. The right abutment of the main embankment and the saddle embankment foundation consist of extremely weathered rock, with shear wave velocity of 0.45 km/sec estimated from P wave velocity measurements. This causes the ground motion response spectra estimated for the right abutment and neighbouring foundation components to be significantly larger than for the left abutment and neighbouring foundation components, by factors of 1.4, 2.0 and 2.3 for periods of 0 (PGA), 0.5 sec and 1 sec respectively.
Keywords: seismic hazard analysis.
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.
Andrew Shields, Dr Mark Bailey, Graeme Hannan
Abstract: In recent years, water resources have declined markedly under the effects of long-term drought and climate change. Resource planning has turned from a process of routine allocation to the specialised management of a limited and highly valued commodity. Probabilistic outlooks of water availability based on historical inflows have proven an important component of the drought communication strategies used by Goulburn-Murray Water. In turn, Goulburn-Murray Water has examined alternative means of securing additional water for its customers, including revised operating criteria and pumping from the dead storage of its dams.
With the focus on meeting essential domestic water needs and boosting irrigation supplies, Goulburn-Murray Water has used the opportunity afforded by water levels to review the capacity of its storages. The regular, but unwelcome, outcome of reduced storage capacities from these studies, and the predictions of continuing drought, present Goulburn-Murray Water and its customers with an ongoing challenge.
Keywords: water resources, allocation outlook, capacity table, dead storage, resource position
Randy J James, Yuyi Zhang, Gabriela Lyvers, David Schaaf
Abstract: Following the flooding disaster in New Orleans, Louisiana, due to Hurricane Katrina in August 2005, the U. S. Army Corps of Engineers (COE) initiated a comprehensive program to survey, evaluate, and rank all dams and levees in the COE’s portfolio for risk of structural failure and associated economic consequences. One objective of this program is to improve safety and risk through efficient allocation of resources for rehabilitation efforts when needed. One area of great concern is internal stresses in aging concrete monoliths causing cracking. While some cracking in concrete monoliths is a common condition having little effect on the structural performance, extended cracking can lead to instability in parts of the monolith. Mass concrete monoliths generally are not reinforced, and cold joints at lift interfaces are a potential source of weak planes. Failure of concrete monoliths due to sliding instability along internal cracked planes can have serious consequences for loss of pool. This failure mode can occur quite suddenly, and detection of such cracking or the extent of such cracking is very difficult to establish from visual inspections or even core sampling. To help in this portfolio risk assessment, analysis methodology has been developed for establishing the structural risk due to cracking in mass concrete monoliths. Finite element modelling with automated mesh generation and employing advanced concrete constitutive relations for crack initiation, propagation, and arrest, are used to establish internal cracking. Monte Carlo based probabilistic analysis methods, directly coupled to the finite element analyses, are used to evaluate uncertainties and establish the probability of failure for increasing pool elevations and seismic hazards. The objective is to provide a probability of failure for possible pool elevations under current site conditions given that there is always some possibility of a range of seismic events that could occur at any given time.
Keywords: concrete dams, cracking, structural reliability, risk, and safety
Elizabeth Gardiner, Len Murray, David Gladwin
Abstract: The Mining Association of Canada has worked over the past 12 years to improve the management of tailings facilities. This effort began out of a realization that tailings present a major business risk to the mining industry, and that a series of major tailings failures around the world in the 1990’s were fundamentally indicative of need for improved care and management practices by tailings dam and facility owners and operators. MAC established and continues to sponsor the Tailings Working Group, which has assisted in development and publication of a three-volume set of guides to improve tailings management:
•“A Guide to the Management of Tailings Facilities”, initially published in 1998, updated edition published in 2008;
•“Developing an Operation, Maintenance and Surveillance Manual for Tailings and Water Management Facilities”, published in 2003; and
•“A Guide to Audit and Assessment of Tailings Facility Management”, published in 2008.
The three guides provide a strong message that the key to safe and environmentally responsible management of tailings is consistent application of sound engineering capability within an effective management framework.
The documents are almost directly applicable to all forms of dams and are widely used for water dams. Also, because the documents are available in Spanish and French they have found wide use in International projects.
A brief comparison with published ANCOLD guidelines is provided.
Keywords: Management of tailings facilities and water dams, Operation, Maintenance and Surveillance, Auditing, risk, safety, guides