Executive summary

The Review of Bulahdelah Floodplain Risk Management Study and Plan (FRMSP) consists of a two-stage review: the first stage consists of reviewing the flood study and the second stage consists of reviewing the floodplain risk management study and plan. This report documents the review of the flood study and has been prepared in accordance with the New South Wales Government’s Floodplain Development Manual (2005) A flood study is the first step of the floodplain management process set up to reduce flood risks and private/public losses resulting from flood while using eco-friendly solution where possible.

Manly Hydraulic Laboratory (MHL) were engaged by MidCoast Council (Council) to undertake the Review of Bulahdelah Floodplain Risk Management Study and Plan The original flood study was completed in 1991 using a 1D model and the FRMSP was completed in 2002-2003. The previous studies, while providing relevant information that relates to the flooding of Bulahdelah, were completed prior to the creation of the latest guidelines It was therefore required to update the model of the area using a two-dimensional TUFLOW model with inflow from an updated WBNM hydrological model for the estimation of mainstream (i.e. from the Myall and Crawford Rivers) and potential local overland (i.e. in Bulahdelah township) flood behaviour in the study area. This also took into account the performance of the stormwater drainage network including overflows from the drainage network.

The study outputs will inform the subsequent review of the FRMSP as well as decision making for investing in the floodplain; managing flood risk through prevention, preparedness, response and recovery activities; pricing insurance, and informing and educating the community on flood risk and response to floods. Each of these areas has different user groups or stakeholder, whose needs vary. Meeting the requirements of the identified end user groups and stakeholders in regards to flooding, is a key objective of this study.

The study has been overseen and guided by the Floodplain Management Advisory Committee of MidCoast Council, which includes representatives from key stakeholder and end user groups.

The flood study review has been completed to provide a detailed flooding assessment of Bulahdelah and the surrounding local catchment. The objective of this study is 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 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 hydraulic model was calibrated and validated against recent historical events including the March 2021 flood event. The model provided a fair match with available data such as recorded water levels at local gauges, historical photographs information provided by local community members and flood marks measured around Bulahdelah by MHL’s project team following the March 2021 event

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 Probable Maximum Flood (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 flashy with very short critical durations between 45 minutes and 2 hours to reach the peak level while the river catchment have 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 impacts on river flooding and low impact on overland flooding.

• Blockages of structures can have severe impact in areas with no gravity flow that only rely on the drainage network (e.g. ponding area), and maintaining the pits and pipes network is essential to avoid exacerbating the flooding in such 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 impacted by flooding and road closures around the catchments. Key infrastructure typically may have impeded access during severe flood events rather than being flooded, except during the PMF event.

Key issue areas potentially subject to deep flooding from the river include:

• Properties located west of Stroud Street and south of Jackson Street (including most properties along Jackson Street);

• Properties located in the two blocks enclosed within Prince Street, Richmond Street,

Alexandra Street and Jackson Street;

• Properties east of Stroud Street and west of Crawford Street between Meade Street and Jackson Street; and

• The area around the intersection of River Street and Richmond Street.

Key problem areas subject to shallower overland flooding include:

• Properties between Myall Street and Edgar Street at the southern end of the streets;

• Properties located along the drainage channel linking Stroud Street to Richmond Street via Alexandra Street;

• Properties along the flowpath linking Mackenzie Street to Crawford Street via Harold Street, Rosewood Street, Bulahdelah Way and Meade Street;

• Properties along the flowpath linking Bulahdelah Way to Crawford Street halfway between Stuart Street and Blanch Street, as well as properties along the western side of Crawford Street between this flowpath and Stuart Street; and

• Properties at the northeastern end of Suncrest Close.

Stroud Street, Jackson Street and Markwell Road near Markwell Bridge are inundated during a 20% AEP. 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