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.
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Janice Green, Cathy Beesley, Cynthia The, Catherine Jolly
Design rainfall estimates are essential inputs to the design of infrastructure such as gutters, roofs, culverts, stormwater drains, flood mitigation levees and retarding basins. They are also integral to large dam spillway adequacy assessments undertaken to determine the flood magnitude that existing dams can safely withstand.
The previous design rainfall estimates for probabilities from the 1 year Average Recurrence Interval (ARI) to the 100 year ARI were derived by the Bureau of Meteorology (the Bureau) in the early 1980s using a database comprising primarily of Bureau raingauges and techniques for statistical data analysis that were considered appropriate at the time. More recently, estimates of rare design rainfall estimates for probabilities from 100 year ARI to 2000 year ARI have been derived for each state, with the exception of the Northern Territory, using the CRC-FORGE method.
As part of the revision of the 1987 edition of Australian Rainfall and Runoff: A Guide to Flood Estimation being undertaken by Engineers Australia, the Bureau conducted a five year project to revise the design rainfall estimates for probabilities from 1 year ARI to 100 year ARI. The new design rainfall estimates are based on a greatly expanded database which incorporates data collected by organisations across Australia. These data have been analysed using contemporary statistical methods that are appropriate for Australian rainfall data. These new Intensity-Duration-Frequency (IFD) design rainfalls were released in July 2013.
Over the next 18 months, the Bureau will be deriving design rainfall estimates for probabilities more frequent than 1 year ARI and revising the existing estimates of the CRC-FORGE rare design rainfalls. The estimates for more frequent design rainfalls will replace the current ad hoc estimates that have been derived by organisations in the absence of other estimates. The revised rare design rainfall estimates will replace the current estimates that were derived on a state by state basis and which, for most states, are now in need of revision as a result of the release of the new IFDs.
Peter Hill, David Stephens, Kelly Maslin, Rachel Brown, Simon Lang, and Chriselyn Meneses
There has been a growing awareness of the potential dam safety risks associated with hydraulic structures in urban environments such as retarding basins, water quality detention basins and recreational lakes. This has required estimates of rare and extreme floods for urban catchments and there are a number of important characteristics of urban catchments which distinguish them from rural catchments such as impervious areas, lack of streamflow data, blockage of structures and complex hydraulics. This paper describes the key considerations for flood estimation in urban catchments and draws examples from a number of current flood studies for urban catchments in Canberra.
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.
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.
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.