Ipswich City Council
Thagoona, QLD

The Thagoona area has been subject to extensive flooding in the past; most notably the recent events of November 2008 and January 2011 and to a lesser extent in November 1999, May 1996 and December/January 1991/92. Extensive flooding also occurred in 1974 however much less development and infrastructure existed in the area at the time. The November 2008 event is the largest event since 1974 and caused the most flood damage in the history of occupation of the area with widespread overland flow through residential and agricultural lots, inundating houses, sheds and vehicles and causing scour damage on many roads and driveways.

The purpose of this study was to increase the resilience to flooding of the community within the catchment through identifying subcatchments, drainage and flooding behaviour, flood risk issues, potential impacts of future development, resilience opportunities, mitigation options and development of a flood risk management strategy.

Hydrological modelling was undertaken using WBNM and hydraulic modelling through TUFLOW. The WBNM model incorporates 137 subcatchments (2785 ha) and was initially calibrated to the Rational Method. The hydraulic model extent was reduced as much as possible to ensure reasonable runtimes with the 3m grid size while still encompassing the key areas of interest without downstream boundary effects. LiDAR data was used to make the 3m DEM of the hydraulic model area (5km x 4km, 2 million cells). Recent survey was used to capture road crest lines and swale centrelines to overlay the DEM. The hydraulic model includes over 100 culverts, bridges and two sections of trunk drainage within the more recently developed areas of Thagoona. The SMS software package was used to create animations of the flood progression.

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