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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.
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.Learn more
Rob Ayre, Simone Gillespie
The Burdekin Falls Dam (BFD) is a SunWater-owned dam completed in 1987. BFD is located in North Queensland, approximately 180 kilometres south of Townsville. BFD is the largest dam in Queensland having a storage capacity of 1,860,000 ML and it has the largest spillway capacity in Australia. The Burdekin River basin drains an area of about 114,770 km2 which is nearly twice the size of Tasmania. Runoff in the catchment is very reliable and flows have overtopped the spillway every year, except one, since it was built. The volume of inflow into the dam during a flood event is considerable, and water spills from the dam for an average of three months each year.
SunWater is investigating the raising of BFD, to increase the storage capacity of the dam by two metres or approximately 30% of its current storage capacity to 2,446,000 ML. In addition SunWater are investigating provisions to further stabilise the concrete gravity main dam to improve dam safety performance by ensuring it complies with current guidelines. Design flood estimation has advanced since BFD was constructed, as the techniques for determining extreme rainfall have been progressively refined. To meet current Acceptable Flood Capacity (AFC) guidelines, the flood discharge capacity at BFD must be increased by 35%. However, whilst this estimate was derived in accordance with current relevant guidelines (ARR, Book VI, 2001) the size of the BFD catchment means that this particular catchment lies on the fringe of the applicability of these guidelines.
Of particular concern is the assignment of the Annual Exceedance Probability (AEP) of the Probable Maximum Precipitation (PMP), which is based upon catchment area. The adoption of the AEP of the PMP for BFD at 1 in 9,000 has implications for the application of risked based approaches for the design.
This paper discusses the existing methodology of design flood hydrology used in Australia and identifies areas of concern for the application of such techniques for large catchments. It also discusses the methodology SunWater utilised in an attempt to meet existing guidelines within these limitations.
Keywords: Burdekin Falls Dam, Flood Hydrology, Probable Maximum Precipitation, Annual Exceedence, ProbabilityLearn more