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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Andrew Richardson, Stephen Farrelly and Phil Farnik
In 2012 an update to the Portfolio Risk Analysis (PRA) was undertaken by State Water Corporation for its 18 major dams in New South Wales. The updated portfolio level risk analysis of all the dams has taken account of the completion of major components of the 2006 dam safety upgrade program, while also incorporating continued engineering research into dam safety performance. This paper will provide an overview of the approach, the challenges faced in the process and it will highlight the innovative advances made representing industry best practice. Some future implications and directions will also be discussed.
The three main components of the PRA update in 2012 have included a significant amount of dam break hydraulic modelling including revised hydrology and flood inundation mapping delivered in-house by State Water with consultant support. The Consequence Assessment was developed with a spatial link to natural flooding and dam failure consequences by Sinclair Knight Mertz (SKM), while the third element in producing the event trees, risk analysis and PRA reports was undertaken by consultants GHD. Peer review of the PRA process and reports and additional technical review of the failure modes and event trees by a panel of industry experts provided the necessary independent input and oversight required by the NSW Dams Safety Committee.
State Water’s PRA update builds on the large body of work undertaken for and since the last PRA in 2002. The update process has applied a systematic and quantitative approach across the Portfolio that provides a robust basis for managing dam safety risk. The results of the PRA have identified further work required to investigate and assess the need for dam safety upgrade options for non-compliant dams. State Water’s investment in the PRA has produced a risk-based position on each dam in the portfolio that can be used to identify a range of measures in a revised dam safety upgrade program for the future.
Two techniques were used to calculate seismic hazard at a number of locations in southeast Australia. To simplify matters only Peak Ground Accelerations were compared.
The first technique used a seismological model of areal source zones that was based on the recorded seismicity as well as geological and tectonic inputs. Each zone was assigned a rate of earthquake activity that had been calculated from the recorded seismicity and a magnitude completeness function. Known geological faults that are also part of the model had to be excluded to allow a direct comparison with the second technique. A standard probabilistic seismic hazard analysis then gave PGA values versus return periods. This is the approach that has been used for the current Australian earthquake loading code (AS1170.4).
The second technique used a simple historical approach whereby recorded earthquakes were combined with an attenuation function to directly give the estimated return periods. This approach takes no account of tectonics, geological terranes or faulting – it simply uses the known, recorded earthquake catalogue. This is the technique used in the original Australian earthquake loading code (AS 2121).
The same ground motion attenuation function was used in both techniques but for a direct comparison the aleatory variability was set to zero in the probabilistic case because the historical approach did not include this effect.
In the historical approach the variability in completeness of the recorded catalogue was not considered. It was simply assumed that all earthquakes producing accelerations greater than a given value would be recorded over the last 100 years.
The comparisons were made for minimum considered magnitudes of 4 and 5.
There was general agreement between the two approaches especially at shorter return periods (lower PGA amplitudes). At longer return periods (higher PGA amplitudes) where there were higher uncertainties, the results at some sites diverged.
This simple comparison of two approaches to the same problem of estimating earthquake hazard is shown to be of value in ensuring that the AUS5 model used by SRC is producing results that are consistent with the historically recorded data.
Sam J. F. Knight and David C. Froehlich
Breaching of embankment dams can result in significant flood hazards, placing people and property downstream of the dam at risk. The consequences associated with the failure of a dam need to be assessed to determine appropriate design parameters, operational procedures, and maintenance requirements necessary to reduce the risks that the dam poses to an acceptable level. Adequacy of dam failure impact assessments can be affected significantly by the accuracy of the predicted breach hydrograph used when modelling dam failures.
This paper compares various methods for estimating parameters of a commonly used embankment dam breach model that considers the breach to form in the shape of a trapezoidal opening in the dam. Model parameters include measures of the breach shape and formation time. Parameter estimation methods are evaluated in terms of the reliability of their results for a range of dam heights and volumes.
The comparison includes the method proposed by MacDonald and Langridge-Monopolis (1984), which has been adopted in regulatory guidelines in the USA, the approach proposed by Von Thun and Gillette (1990), the method proposed by Allen (1994), which has been adopted in Queensland’s regulatory dam failure impact assessment guideline, and the method developed by Froehlich (2008), which has also been adopted in regulatory guidelines in the USA. The reliability of the different methods has been evaluated based on their accuracy in reproducing dam breach parameters for actual dam failures where well documented measurements are available. An example is given that demonstrates how the predicted breach hydrograph could vary with the use of the different breach parameter estimation methods, and with how the breach is assumed to develop.
Marius Jonker and Dr Radin Espandar
This paper provides a summary of the current state of practice for arch dam design criteria that have been adopted by some international dam organizations, and where relevant, compares that with the criteria provided in the updated ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams, with the view to provide a basis for consistent and unified design criteria for arch dams in Australia.
The paper draws on the authors’ experience with arch dams, including recent experience with a number of arch dam safety reviews in Australia, their past experience with arch dams over 200 m height, as well as their involvement with the development of the mentioned updated ANCOLD Guidelines.
Since the last arch dam was constructed in Australia, a number of international publications have been released on arch dam design practices, providing general information and guidance for the design of new dams and evaluation of the safety and structural integrity of existing arch dams. This paper compares these publications and proposes criteria that are aligned with the ANCOLD gravity dam guidelines.
Peter F Foster and Peter K Silvester
Clyde Dam, the largest concrete gravity dam in New Zealand, was constructed in the 1980’s on the Clutha River in New Zealand. Lake Dunstan, which is the reservoir formed by the dam, reached its full operating level in 1993, some 21 years ago.
This paper summarises the performance of the dam over this period, the changes in operations that have been undertaken and looks to future challenges. The performance and management of the landslides around Lake Dunstan that were remediated prior to lake filling is outlined. The large floods experienced in the Clutha River in the 1990’s highlighted aspects of the flood management procedures that needed amending to capture lessons learned and some modifications to appurtenant structures have been completed. Changes to the environmental management in moving from water rights to consent conditions under the Resource Management Act are addressed.
Over the last 21 years a sediment delta has progressed down Lake Dunstan, as expected, and a long term sediment management plan has been developed for both Lake Dunstan and Lake Roxburgh which is downstream of Clyde Dam. A summary of the plan is discussed. The seismic hazard at the dam site is currently under study to update the seismic assessment parameters for the dam.