Gregg A Scott
Abstract: The Bureau of Reclamation has been performing quantitative risk analysis as the primary dam safety decision making tool for well over a decade. This paper summarizes some of the key concepts and basic methodology currently used in the dam safety risk analysis process at Reclamation.
Keywords: dam safety, risk analysis, reliability analysis, event trees, subjective probability.
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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
Steven O’Brien, Brent Mefford, John Harris
Abstract: The Hinze Dam Stage 3 Upgrade required ‘Waterway Barrier Works Approval’, including fish-passage assessments. The dam, sited in the Nerang River catchment, has relatively undisturbed riverine habitat upstream which, together with the reservoir’s recreational fishery, was a key driver for inclusion of a fishway. Low outflows from the dam and impacts of development on the river downstream contrast starkly with upstream environments.
The Hinze Dam Alliance (HDA) assessed that upstream transfer of fish will have significant environmental benefits but downstream passage, beyond that achieved during spill events, could not be justified.
Constraints at the site for upstream passage include unvarying 7.25ML/d releases from the reservoir and a distance of nearly 300m between the reservoir and the downstream river pool. HDA has developed an innovative trap-and-haul system to collect upstream-migrating fish near the spillway and transport them by tanker to upstream release areas. This system is the first of its kind designed for Australia. It provides flexibility to accommodate varying fish biomass, the ability to operate over a range of flows up to 20-year ARI events, facilities for data collection and removal of pest species, and capacity to manage predation with small-fish refuges during trapping and transfer and by releasing fish at several protected sites in the reservoir.
Keywords: fishway, trap and haul System, fish passage
Matthew Pollard, John Vitkovsky, Richard Priman
Abstract: South East Queensland (SEQ) currently has severe (Target 140) restrictions imposed to help secure supplies during the current drought which is the worst on record. Additionally, a $9 billion water infrastructure program is being fast-tracked to increase the climate resilience of the region and provide for long term sustainable growth.
The Draft South East Queensland Water Strategy (SEQWS) released in March 2008 was prepared by the Queensland Water Commission to reduce the likelihood of ever experiencing such severe restrictions again and to ensure water security into the future. The Strategy includes a Water Supply Guarantee underpinned by advanced analytical techniques for estimating the system yield from surface water, groundwater and manufactured water supplies connected by the SEQ Water Grid. This approach builds on the Level of Service (LOS) Objectives methodology, originally promulgated by the Water Services Association of Australia in their June 2005 paper entitled “Framework for Urban Water Resource Planning”. The approach has led to a significant improvement in our understanding of water supply risks and the associated planning implications for assessed yields/water availability.
To assess yield using the LOS objective methodology and to determine the benefits of the new water infrastructure, a regional water balance model for the connected SEQ Water Grid simulating the water sharing arrangements of all of the SEQ water sources was established. The model uses a logic tree to allocate water using a “proportional storage rule” from multiple sources to meet competing demands. This approach enabled quantification of the increase in system yield resulting from the construction of the SEQ water grid which allows demands from previously disconnected areas to be met by more efficient allocation of water from supply sources. Stochastically generated dam inflow data was used to facilitate a more comprehensive assessment of climate variability and water supply risk than is possible with historic data alone.
The paper discusses the managed hydrologic risk approach adopted in developing the regional water balance model which implements the LOS Objectives approach and improves the understanding of the relationship between the level of service, supply yield and risks associated with climate variability. The result is a far more thorough approach to planning for future water service delivery and water infrastructure.
Keywords: level of service objectives, South East Queensland Water Strategy, SEQ Water Grid, yield, water security, regional water balance model, stochastic modelling, climate variability.
Chris Chamberlain, Don Macfarlane, Geoff Eades
Abstract: The Hinze Dam Stage 3 project is a significant upgrade of the existing 63.5 m high embankment dam, located on the Nerang River in the Hinterland of the Gold Coast in South East Queensland. Construction work commenced in early 2008 and is scheduled to be complete by the end of 2010.
Investigations at the dam have been undertaken in three stages since the early 1970’s for construction of the original dam and for each of the Stage 2 and Stage 3 raises. The Stage 3 investigation program began in November 2006 and was substantially complete by March 2008. The geology of the Hinze Dam foundation has been found to be more complex than had been previously determined, particularly with regard to the geological structure, variation in weathering profile and rock strength.
Development of the Stage 3 geological model of the foundation has been an iterative process utilising documented Stage 1 and Stage 2 data and new Stage 3 data. Much of the emphasis for the Stage 3 ground investigations has been placed on the right abutment ridge and saddle dam extension where most of the Stage 3 construction will take place. Here, the geological units comprise greywacke and deformed and variably silicified greenstone, phyllite and chert. Frequent review meetings were used to present each refinement of the geological model, challenge its geological credibility, identify uncertainties with the ground conditions and adjust the investigation program.
The paper describes the development of the Stage 3 geological model, provides an outline of the broader geology of the damsite and embankment foundation and also discusses how the complicated foundation geology has driven some of the engineering solutions that are currently being constructed at the dam.
Keywords: Hinze dam, geological model, foundation geology, design solution.
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.