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.
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.
Bronson McPherson, David Guest, Barton Maher, Ian Tanner and Amit Chanan
There is significant community interest in the potential for water supply dams to be adapted for flood mitigation, particularly for major dams located upstream of flood vulnerable populations. There may be a number of large dams which have the potential to provide significant flood mitigation benefits to Australian communities if they can be adapted for flood mitigation functionality. Other dams already provide significant flood mitigation benefits, however their limitations are not properly understood by the general public. Two major dams located near a large urban town centre prone to flooding are examined as a case study and some international cases are presented.
Flood mitigation often has a different funding source to water supply. The funding arrangements for flood mitigation dam works can be complex, considering the potential stakeholders and somewhat intangible benefits. If the community wants to use a water supply dam to provide flood mitigation then who provides the funding for the modification works?
Paul Southcott, Tony Harman
This paper addresses structural behaviour of the Rowallan spillway walls and the learning that can be derived from this in the design of critical retaining walls in dams and how this can be applied both to remedial works and new work. The authors propose design criteria suitable for retaining walls in high hazard dams.
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.
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.