The Challenge

Toowoomba Regional Council had recently undertaken flood studies of more than 30 towns in the region for the purpose of informing a planning scheme update, emergency management and infrastructure planning. The flood studies were undertaken as part of the SP051 Flood Studies project.
Although a very large undertaking, more than 50% of the Toowoomba Local Government Area (LGA) has not been modelled. Floodplain mapping of these areas is available, however, is either based on extreme design rainfall or floodplain delineation based on soil characteristics.
The purpose of this project was to develop rapid, low cost flood extent mapping for the 1% Annual Exceedance Probability (AEP) design flood for the whole of Council LGA. A previous GPU modelling exercise has been undertaken in 2014. Whilst useful at the time, this exercise utilised very coarse model topography derived from SRTM only (no LiDAR) and was based on a duration independent synthetic storm approach to model inflows.

Approach and Outcomes

The scope of this project was to develop direct rainfall TUFLOW GPU models for the whole of Council LGA and validate the models by comparing discharges and flood extents to the SP051 Flood Study model results. Due to the size of the TRC LGA, the area was split into thirteen model domains. The model validation was undertaken for the 1% AEP flood event. Several iterations were run to test the models’ sensitivity to adopted roughness values and rainfall losses to achieve the best possible fit to the SP051 Flood Study results. To maintain consistency with SP051 Flood Studies, model validation was undertaken using ARR 1987 Intensity-Frequency-Duration (IFD) data. At most locations, a good fit between modelled discharge hydrographs was achieved. The TUFLOW GPU flood extents also matched the existing flood study extents well.
Since the completion of the SP051 Flood Studies in 2014, the new ARR guidelines have been released in December 2016. The final GPU models were therefore set up using ARR 2016 IFD data and validated to flows extracted from the Regional Flood Frequency Estimation (RFFE) Model fort the 1% AEP design event. In addition to modelling and mapping of results for the 1% AEP event, GPU models were also set up for the 2%, 10%, 20% and 50% AEP design events to enable Council to run the models in-house at a later stage.
The GPU results for the 1% AEP event were combined with existing flood study results. The seamless 1% AEP flood extent layer generated as part of this project combines the best available flood information for the whole Toowoomba region. The mapped 1% AEP flood extent (at a 10 m resolution) will inform planning scheme updates, assist with targeting future studies and inform flood studies in areas not yet covered by detailed modelling.