Understanding the hydraulic impact of hydrologic uncertainty in the urban Gowrie Creek catchment

Understanding the hydraulic impact of hydrologic uncertainty in the urban Gowrie Creek catchment

Estimating peak flow rates from a catchment has long been a focus of engineering hydrologists and is fundamental to the design of flood protection infrastructure. Understanding the uncertainty associated with peak flow estimation is, however, often neglected by practitioners. Without a detailed understanding of uncertainty, the risks associated with major flooding in urban catchments cannot be completely understood. In January 2011, Toowoomba experienced one of its worst floods on record with Gowrie Creek breaking its banks in multiple locations, resulting in at least four deaths. Peer review of subsequent hydrologic modelling of the catchment recommended gaining a better understanding of the loss parameters adopted and the resulting uncertainty associated with these parameters. The industry popular design event method uses the traditional initial/continuous loss model. Adopting a dynamic loss approach within a continuous simulation model overcomes this limitation, while allowing for the simulation of multiple rainfall iterations and in turn a better understanding of hydrologic uncertainty. This research first calibrated a continuous simulation hydrologic model by simulating 11 selected storm events (with peak flows ranging from as low as 9 m3/s and as high as 600 m3/s). By disaggregating 100 years of daily rainfall to sub-daily (six minute) rainfall multiple times using the method of fragments and simulating these within the calibrated continuous simulation hydrologic model, it was possible to produce multiple iterations of 100 years of stream flows. Flood frequency analysis of these stream flows produced peak flows at the outlet of the catchment for various annual exceedance probabilities with uncertainty. Finally, this research developed a two-dimensional hydraulic model of the Gowrie Creek catchment and simulated hydrographs correlating to the peak flows for each hydrologic model iteration to determine the impact hydrologic uncertainty has on the flood extent within the urban Gowrie Creek catchment. The hydraulic impact of hydrologic uncertainty was found to be significant and would have a direct impact on urban planning.

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