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.
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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.
This paper presents the methods used to apply a Flood Operation Simulation Model, and the methods used to present results of thousands of flood simulations in a way that different operational options could be compared. The approach was found to be valuable to understand the capacity of the dams to mitigate floods. The study identified shortcomings for the conventional design event approach to flood estimation. A broader range of stochastic floods was an advantage to assess flood mitigation performance and extreme floods of interest to dam safety.
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.
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.
Kathryn Whalley and Bob Clark
When Seqwater was established in 2008 it inherited from local governments 51 weirs of varying ages, sizes, design standards and condition. In order to better understand these structures, in 2012 Seqwater engaged NSW Public Works to undertake a condition and risk assessment of its weir portfolio. The assessment, consisting of a background review, site inspections, stability assessment and a collaborative risk workshop, examined risks to the structures, to Seqwater personnel and the public. Assessment of the risk consequences used Seqwater’s recent experience with repairs to weirs damaged in the 2011 and 2013 Queensland floods. The assessment was completed in 2013 and identified more than 1000 risks. It was recommended that more than 600 moderate to high risks be reduced through a prioritised program over the next 10 years. Weir performance following the 2011 and 2013 floods is also discussed.