Phillip Jordan, Alan Seed, Rory Nathan, Peter Hill, Eva Kordomenidi, Clive Pierce, Michael Leonard
This paper discusses the stochastic framework that was used to generate the 5449 sets of inflow hydrographs, to develop and stress test a dam operations model. The stochastic simulations were driven by 600 different space-time patterns of rainfall generated using a stochastic space-time multiplicative cascade model. Eight significant storms were identified in the radar archive to identify parameter sets for the stochastic generation algorithm and 600 replicates of space-time rainfall were generated. The statistical properties of spatial patterns of 48-hour rainfall bursts on eight major subcatchments of the Brisbane River catchment from the 600 stochastic replicates were verified against the same statistics derived from 38 major flood causing rainfall events observed in the catchment. The hydrographs were generated using an URBS rainfall runoff routing model of the Brisbane River catchment, which was calibrated to 38 historical flood events (between 1955 and 2013) and tested on a further 10 historical flood events (between 1887 and 1947).
The stochastically simulated sets of inflow hydrographs were then used to assess the impact of variations in flood operation rules for Wivenhoe and Somerset dams. The stochastically generated events exhibit substantial variability in runoff hydrographs but with variability that is statistically consistent with observed events. The stochastically generated hydrographs provide a considerably more realistic basis for testing the outcomes for different flood operations strategies than the single design event approaches that have previously been adopted.
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Dennis C. Green
Current good practice for risk management as represented in ANCOLD guidelines emphasises risk reduction beyond tolerable risk levels to As Low As Reasonably Practicable (ALARP). Risk reduction reflected in key design parameters such as the spillway design flood is monitored on a quantitative basis, while the guidelines also draw attention to a number of non-quantifiable measures.
Recent work health and safety legislation in Australia does not at first appear to relate to dam safety, but it mandates elimination of risk, and, if that is not possible, then it mandates reduction of risk So Far As Is Reasonably Practicable (SFAIRP). It is tempting to believe that this is equivalent to ANCOLD’s approach to ALARP, but the devil is in the detail of the legislation. This paper argues for a change to a more systematic presentation of recording of decisions on dam safety risk management, lest the legislation expose dam owners unwittingly to liability when they thought they were following good practice. In particular, the re-focussing of ANCOLD Guidelines to align more recognisably with the new legal paradigm, including preparation and adoption of a Safety Case, is recommended.
JN Rossouw, AHM Görgens and PC Blersch
Shallow lakes or reservoirs generally exist in either of two stable states; a clear water state dominated by rooted water plants, or a turbid state dominated by free floating algae. A dramatic event can switch a shallow reservoir from one state to another. Voëlvlei Dam, a relatively shallow off-channel storage reservoir in the Berg River catchment, South Africa, switched from a stable, clear water system to a turbid, algal dominated system when it was severely drawn down during a drought in the mid-2000s.
It appears that there is tipping point beyond which a shallow reservoir can switch from one stable state to another and that there are buffers that maintain it in a specific state. Voëlvlei Dam is a good example of what such a switch might be (low water levels and high wind mixing) and what buffers (change to bottom-feeding fish species) may maintain it in the new state. It is only by understanding the hydrodynamic behaviour of a shallow reservoir that one can predict what these switches and buffers could be. Complex hydrodynamic modelling and comprehensive fish monitoring will facilitate more informed decision making and better management of reservoirs.
This paper describes the mechanisms that lead to the switch and how it can be prevented by developing an understanding of the hydrodynamic behaviour of shallow reservoirs through hydrodynamic water quality modelling.
Lyndon Johnson and Jamie Campbell
Data presentation is an important and much discussed aspect of Dam and asset safety worldwide. We rely on drawings and graphs of instrumentation data to tell us things about our assets that are hidden from the eye and to monitor changes linked to failure modes. It’s common that we look at data gaps for our assets, data quality and data processing but how often do we rethink the fundamentals of data presentation?
Engineers and data analysts, as humans, have evolved in a 3D world with our senses to match match. According to Keller GB, et al (2012) almost 20% of the human brain is dedicated to processing vision with up to 60% involved when locating, scaling and referencing objects in 3D space. As a result, 3D is an extremely efficient platform from which to display and disseminate information.
This paper discusses methods to efficiently transfer asset information into 3D and how to present animated surveillance data against asset models. The paper discusses how these methods work, benefits and limitations in the context of modern dam asset portfolio management and presents some key case studies of where and how these methods have assisted with asset diagnoses.
Francisco Lopez and Michael McKay
At 36 m high and completed in 1902, Barossa Dam is one of the first true concrete arch dams in the world. During the 1954 Darlington Earthquake the dam sustained some damage, in the form of several vertical cracks on both dam’s abutments. In 2013, GHD conducted a nonlinear time-history seismic assessment of Barossa Dam. The analyses, carried out using finite element techniques, included ground motion loading corresponding to Maximum Design Earthquakes (MDEs) with 1 in 10,000 Annual Exceedance Probability (AEP).
This paper will explain the purpose of the study, the material investigation phase, the methodology, model results, the anticipated seismic behaviour of the dam wall, as well as the predicted level of damage under the MDEs. The paper examines the dam construction practices of the beginning of the 20th century, and how such practices affected the material properties and the structural performance of Barossa Dam.
George Samios and Steve Gough
Following an extreme flood event late on Friday 22nd February 2013, water overtopped the dam crest leading to total collapse of the dam’s right embankment and a span of the nearby intake tower access bridge as well as the cutting of power to the dam gates. The dam’s left embankment was also severely damaged. Options considered for remediation of the dam include decommissioning or reinstatement and upgrading to NSW Dams Safety Committee requirements.