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.
Luke Toombes and Rob Ayre
Many large dams are built as multi-purpose structures, providing both flood mitigation and bulk water storage, but requiring a trade-off in functionality between those purposes. In response to the Millennium Drought (2001 to 2009) closely followed by devastating floods in 2011, the State of Queensland initiated a comprehensive review of the operation of its flood mitigation dams. Part of this study involved development of an Integrated Assessment Methodology to provide an informed and unbiased assessment of the competing factors affecting dam operations. The methodology assessed the primary variables of flood damage and other impacts, future bulk water infrastructure and water security requirements in the form of a net present cost/benefit. The study concluded that modification of the dam flood release strategy to reduce flood damage during large events would come at the expense of increased frequency of minor flooding, or vice versa, with minimal net benefit. Similarly, reducing bulk water storage to increase flood mitigation would increase water supply costs by a similar magnitude to the flood damage prevented.
Przemyslaw A. Zielinski
Three aspects of the current engineering practice in using event trees in dam safety risk analyses are discussed in the paper. These aspects include assignment of probabilities for initiating events, treat-ment of dependencies in the event tree, and dynamic aspects of dam system behaviour and accounting for time. The paper discusses limitations of the methodology and common mistakes in engineering applications of event tree methods when assessing dam safety risks and making safety decisions for specific dams. Of particular importance is the discussion of incorrect interpretation of dependency structure when addressing common cause failure modes.
S. Suter, G. Singh, and M. Britton
Today, many organisations rely on hydrodynamic modelling to assess the consequences of dam break failure on downstream populations and infrastructure. The availability of finite volume shock-capturing schemes and flexible mesh schematisations in widely used software platforms imply that dam break modelling projects will be carried out differently in the future: Finite volume based platforms allow widespread application of shock-capturing methods and flexible mesh platforms can represent features in the study area more realistically and are more flexible thanks to varying mesh resolutions. Furthermore, the recent adoption of Graphics Processing Unit (GPU) technology in mainstream scientific and engineering computing will also significantly decrease computation times at relatively low cost.
This paper examines the application of finite volume, flexible mesh and GPU technologies to dam break modelling. One-dimensional (1D) modelling results are compared to those from two-dimensional (2D) finite difference and finite volume approaches. The results demonstrate that there are differences between modelling approaches and that the computational speeds of 2D simulations can be significantly reduced by the use of GPU processors.
William Ziegler and Heather Middleton
This paper presents the collation of over 20 years of data on vertical and horizontal movements around Cataract Dam in the Southern Coalfield of New South Wales, reporting subsidence that continues 25 years after extraction in the area ceased. The occurrence of increased vertical movement over old goaf areas as the result of extraction in the same seam at greater than 1km distance has been observed. Together with a change in the behaviour of measured head of water 6 years after extraction ceased in the area. These points raise the question, how long should subsidence monitoring continue after extraction has ceased in areas of important infrastructure?
Steven Slarke, Dr Martin Mallen-Cooper and Marcos Guirguis
Keepit Fishway Offsets
Fish passage structures are being provided by State Water Corporation as part of a strategic program to address fish passage barriers that triggered S218 of the Fisheries Management Act 1994 at Mollee Weir, Gunidgera Weir and Weeta Weir in the Namoi River. These sites are an offset for dam safety upgrade works on Keepit and Split Rock dams in the headwaters of the Namoi River. Rather than applying high-level fish lifts at the dams, the three lowland sites represent the top three ecological priorities in the Namoi River for fish passage facilities – a case of less cost for greater ecological outcomes. The objective of the fish passage facilities at these sites is to restore upstream and downstream fish passage for about fifteen native fish species. The key biological objectives are to pass adult and juvenile fish upstream and adult fish and larvae (which drift with the current) downstream.
Mollee Weir was constructed in 1973 on the Namoi River downstream of Keepit Dam, near Narrabri in northern NSW. The nine-metre high weir is used for irrigation and comprises a reinforced concrete structure featuring three bays with undershot gates and two piers. The upstream and downstream water levels are highly variable, with a maximum differential head of about six metres. Fish are unable to pass the weir during regulated and unregulated flows; even when the undershot gates are fully raised in high flows, due to high velocities in the opened weir. The weir’s large undershot gates are also a barrier to safe downstream fish passage during regulated flows. High water pressures and velocities beneath the partially raised gates create a high mortality rate for fish and larvae moving downstream.
Fish Passage and Regulator Structure
Designed for State Water NSW by URS Australia Pty Ltd in cooperation with Dr Martin Mallen-Cooper of Fishway Consulting services, Mollee Weir features a new fish lock for upstream-migrating fish and a dedicated overshot gate with dissipating pools for downstream-migrating fish, and was constructed during 2013 to 2014.
It is the tallest fish lock in Australia that is filled from the top.
The innovative design features two separate downstream fish holding bays and two fish lock entrance gates, to provide optimal entrance conditions at varying river flows and water levels.
To provide safe downstream fish passage at low to moderate river flows, a 4 m wide ‘downstream multi-function migration gate’ has been integrated beside the fish lock structure. This overshot gate also provides an attraction flow to the fish lock entrances, and tracks the upstream water level at high river flows to provide a high discharge pool and weir fishway as a bypass around the weir structure.
The Mollee Weir fish lock provides upstream fish passage for the full range of upstream and downstream water levels.