Whole of basin flood modelling

Subsequent to the findings of the Queensland Floods Commission of Inquiry, Kellogg Brown & Root Pty Ltd was commissioned by the Department of Natural Resources and Mines to prepare whole of basin flood maps for 16 coastal river basins ranging in size from 1600 km2 to 142,700 km2 to improve flood emergency awareness in Queensland. The maps and animations developed provided an appreciation of flood magnitude for both the 1% Annual Exceedance Probable flood event and an extreme event intended to approximate the probable maximum flood. The study was completed within 10 weeks and the maps and flood animations produced were subsequently used during Tropical Cyclone Marcia to provide situation awareness to emergency managers and non-technical responders. Two-dimensional hydraulic model grid sizes ranged from 30 m to 100 m and hydrology was undertaken using rain-on-grid techniques. This paper discusses the development of the models, source digital elevation model (DEM) constraints, the implications of hydraulic discontinuities of dams and waterfalls, the application of direct rain, losses, and the practical constraints of modelling large basins. Outcomes included confirmation of previously suspected inter-basin transfers, the vulnerability of cities, and a methodology to determine critical flood durations for sub-catchments and the whole catchment. Recognising the methodological limitations discovered during the program, the authors undertook a series of sensitivity tests and a verification program using outputs from an earlier study of the large Fitzroy River basin. The results, the need for further research, and possible uses are discussed in the paper.