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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Damon Miller and Grant Jones
Mt Buller Alpine Resort has significant constraints on its capacity to store and supply potable water during times of peak demand, which impacts the ability to sustain and grow visitation to the resort, limiting optimal functioning and future development of the resort.
A proposed new 100 megalitre dam would primarily supply the resident and visitor populations with a reliable potable water source while also maintaining through snowmaking, the Resort’s amenity and functionality during winter for skiing and snow-play.
Standard dam engineering criteria of technical feasibility and environmental impact influenced the site selection and design of the new off-stream storage whilst additional key drivers unique to an alpine resort, framed around impact to existing skiable terrain and resort functionality, were critical to satisfy the resort stakeholders. The need to minimise the visual impact of the dam and integrate with the resort environment was also of high importance.
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.
Gary Gibson and Vicki-Ann Dimas
Earthquake recurrence models are based on observed seismicity, geological data and geodetic motion. They are particularly difficult to define in regions of low seismicity where the average recurrence interval between moderate to large earthquakes greatly exceeds the duration of the known earthquake catalogue.
The earthquake process may be considered as ongoing long-term deformation due to plate movement in the region about the fault, resulting in stress build-up, and a significant number of small earthquakes through the deformed region. Larger earthquakes occur at irregular intervals, with ruptures on the larger faults that release elastic strain energy from the region. Most strain energy release is during the large fault rupture.
This gives a wider range in hazard estimates compared with extrapolation methods, increasing hazard in regions of active faulting and reducing hazard where long-term geological stability can be observed. As dams are usually in regions with recent uplift, this method will tend to increase hazard estimates.
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.
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.