13 Conclusion

The flood study review has been completed to provide a detailed flooding assessment of Bulahdelah and the surrounding local catchments The previous studies, while providing relevant information that relates to the flooding of Bulahdelah, were completed prior to the creation of the latest guidelines The objective of this study is therefore to improve understanding of flood behaviour and impacts, and better inform management of flood risk in the study area. The study also provides a sound technical basis for any further flood risk management investigation in the area.

The key components of the flooding assessment included:

• Review of previous studies and available data

• Community consultation

• Hydrological analysis and modelling

• Hydraulic analysis and modelling

• Calibration and validation of the hydraulic model

• Sensitivity analysis

• Flood mapping

• Description of consequences of flooding

• Development of a draft flood study review report followed by a final report

The flood maps appended to this report are presenting the flood levels, depths and velocities for the critical duration and rainfall pattern of a full set of events including the 20%, 5%, 1%, 1 in 200, 1 in 500 AEP and PMF events and represent an envelope of the critical duration/pattern of a selected representative catchment for the Bulahdelah township and the critical duration/pattern for the Myall River. The township catchment is very reactive with very short critical durations between 45 minutes and 2 hours to reach the peak level, while the river catchment has typical critical durations ranging between 6 hours and 18 hours

Sensitivity analysis highlighted the following points:

• Tailwater conditions (including sea level rise) typically have relatively small impact on river flooding levels due to a number of natural choke points located at various locations along the Myall River raising the level much higher than the downstream level However, for smaller catchment events, that do not lead to a major response in the overall river catchment flooding, changes in Bombah Broadwater levels will propagate directly up the Myall River.

• Changes in roughness or in antecedent conditions of the catchment (wet/dry catchment leading to varying losses) could have moderate impact on river flooding and low impacts on overland flooding

• Blockages of structures can have severe impact in areas with no gravity flow that only relies on the drainage network (e.g. ponding area); maintaining the pits and pipes network is essential to avoid exacerbating the flooding in those locations. It is also noted that no blockage can lead to small increases in flood level at the downstream end of the township.

The above results allowed the definition of the flood hazard (i.e. H1-H6 flood life hazard categories) and hydraulic categories in the Bulahdelah catchment. These have been created and mapped to inform development control planning.

Results of the model allow the identification of main flooding areas, key infrastructure assets impacted by flooding, and road closures around the catchments. Key infrastructure typically may have access issues during severe flood events rather than flooding issues, except during the PMF event.

Apart from Stroud Street, road closures predominantly occur on secondary roads with most of the major road closures occurring for the PMF only. It is also noted that given the flashy behaviour of the catchment, flooding and road closures in the upper township catchment would be of relatively short duration while flooding of the areas surrounding the river may last from several hours to a few days

© Crown 2021

Classification: Release by consent

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