Wingecarribee Shire Council engaged Catchment Simulations Solutions to prepare a Flood Study for the Nattai River catchment. The extent of the catchment is shown in Figure 1 in report (refer Flood Study: Volume 2).

The objectives of the Nattai River Catchment Flood Study are:

• to review available flood-related information and data for the Nattai River catchment;
• to develop a hydrologic model to simulate the transformation of rainfall into runoff and generate flow hydrographs across the catchment;
• to develop a hydraulic computer model to simulate how the flows generated by the hydrologic model would be distributed/move across the catchment;
• to calibrate the hydrologic and hydraulic computer models to reproduce past floods; to use the calibrated computer models to define peak discharges, water levels, depths and velocities for the design 20%, 10%, 5%, 2%, 1% and 0.5% AEP floods, and the Probable Maximum Flood (PMF);
• to produce maps showing the extent, depth and velocity of floodwaters for the range of design floods; and,
• to produce maps showing flood hazard and hydraulic categories for the range of design floods.

In order to achieve these objectives, the Flood Study was completed in a staged manner. An overview of the outcomes of each stage is presented below.

Data Collection

Only a limited amount of flood-related information was available for the Nattai River catchment. Therefore, it was necessary to undertake additional data collection to ensure the study objectives could be met.

The additional data collection included the survey of 21 river cross-sections and 24 hydraulic structures (i.e., bridges and culverts). The extent of the survey information that was collected is shown in Figure 4 in the report.

A community information brochure and questionnaire was also distributed to 339 households and businesses within the Nattai River catchment (refer Appendix A). A total of 60 questionnaire responses were received. The questionnaire responses included photographs of past flooding within the catchment as well as descriptions of historic flood behaviour. This historic flood information was subsequently used to assist in the calibration of computer models.

Hydrologic Model

A hydrologic computer model of the Nattai River catchment was developed to define rainfallrunoff processes and generate discharge hydrographs at discrete locations across the catchment. The hydrologic model was developed using the XP-RAFTS software. The XP-RAFTS subcatchment layout is shown in Figure 5 in the report. There are no stream gauges located within the Nattai River catchment. Therefore, the XPRAFTS model could not be independently calibrated as part of the study. Nevertheless, a pseudo-calibration was attempted using the XP-RAFTS model with the TUFLOW hydraulic model.

Hydraulic Model

A fully two-dimensional hydraulic computer model of the Nattai River catchment was also developed as part of the Flood Study using the TUFLOW software. The TUFLOW model was used to route the flow hydrographs generated by the XP-RAFTS model across the catchment and produce information of flood levels, extents, depths and velocities. The layout of the TUFLOW model is shown in Figure 8 in the report.

A pseudo-calibration of the XP-RAFTS and TUFLOW models was performed using historic flood information gathered from the community questionnaire responses. The outcomes of the calibration determined that the XP-RAFTS and TUFLOW models were able to reproduced historic flood mark elevations to within 0.12 metres.

Design Flood Results

The calibrated XP-RAFTS and TUFLOW models were used to simulate design flood behaviour across the catchment for the 20%, 10%, 5%, 2%, 1% and 0.5% AEP events. The Probable Maximum Flood was also simulated. Peak floodwater depths and velocities were extracted from the results of the modelling and are presented in Figures 11 to 17 in the report. The results of the design flood simulations were also used to determine the variation in provisional flood hazard (refer Figures 18 to 23 in the report) and hydraulic categories (refer Figures 24 to 31 in the report) across the catchment.

The results of the design flood modelling show that:

• Flood behaviour across the Nattai River catchment is typically characterised by relatively shallow depths of inundation (i.e., < 0.3 metres). However, more significant depths are predicted along and immediately adjacent to designated waterways.
• The relatively shallow depths of inundation result in the majority of the Nattai River floodplain being exposed to a low provisional flood hazard.
• However, several sections of the catchment are predicted to be exposed to more significant floodwater depths and velocities and, consequently, a high provisional flood hazard during large events. This includes:
• Evans Street (between Payten Street and Colo Street)
• Mittagong Swimming Centre
• Oxley Road (between Nero Street and Murchison Street)
• A number of roadways are predicted to be overtopped during the 1% AEP flood. This would typically render the roadways impassable for at least 2 hours. This includes Range Road, Old South Road and the Old Hume Highway.
• The catchment incorporates a number of bridges and culverts. The results of a blockage sensitivity analysis shows that the severity of flooding upstream of these structures can be significantly increased due to blockage. This highlights the importance of routine maintenance on this infrastructure, particularly immediately after a flood.