Williamtown Salt Ash - Flood Study Review

The objective of the study was to assess the impacts of climate change on the baseline 1% AEP
flood condition within the Williamtown / Salt Ash Flood Study area. Central to this was the
development of a new two-dimensional hydraulic model of the study area, in order that the impacts
could be properly assessed.
In completing the flood study review, the following activities were undertaken:
• Review of relevant studies regarding flood conditions and climate change impacts within Port
Stephens;
• Site inspection to confirm the presence and configuration of key hydraulic structures;
• Merging of the existing Williamtown / Salt Ash Flood Study and Williams River Flood Study
modelled to produce a composite model capable of properly assessing the impact of climate
change in the study area;
• Updating of model topography with available LiDAR survey data;
• Calibration of 1% AEP design event flood levels with the 4.84m AHD level from the Flood
Frequency Analysis at Raymond Terrace;
• Prediction of design flood conditions in the catchment using the developed model; and
• Production of design flood mapping series.
The climate change scenarios that were considered were combinations of 2050 and 2100 sea level
rise conditions with baseline, +10% and +30% flood flows. A sea level rise of 0.4m by 2050, results
in around a 0.2m increase to the 1% AEP flood level in Fullerton Cove. A sea level rise of 0.9m by
2100, results in around a 0.6m increase to the 1% AEP flood level in Fullerton Cove. For the 1% AEP
event peak flood levels in Fullerton Cove increase by around 0.1m and 0.3m for the 10% and 30%
flow increases respectively.
The dominant flooding mechanism (in terms of peak design water levels) for the Williamtown / Salt
Ash locality is mainstream Hunter River flooding. Under these conditions, Hunter River flooding
results in Fullerton Cove filling and discharging into the Tilligerry Creek floodplain, under crossdrainage
structures and through overtopping of Nelson Bay Road. The baseline flood level within the
Tilligerry Creek floodplain is increased from 1.2m AHD to 2.6m AHD, under the worst case climate
change scenario.
The flood levels along Windeyers Creek are driven by flow conditions in the Hunter River. Hunter
River flood water provides a backwater influence in Windeyers Creek, which fills the storage area to
the east of the Pacific Highway. The total volume of water flowing from the Hunter River along
Windeyers Creek determines the flood level reached in the storage area. A higher flood level in the
Hunter River will result in a higher flood level in the storage area. At this location, the sea level rise
scenarios have little impact on peak flood levels. There is only a small difference between flood levels
for the baseline condition and the 2100 scenario. However, the increased flood flow scenarios do have a significant impact, with peak flood levels increasing by around 0.2m and 0.6m for the 10% and
30% flow increases respectively. The baseline flood level at this location is increased from 4.4m AHD
to 5.2m AHD, under the worst case climate change scenario.
The flood study review will form the basis for the subsequent floodplain risk management activities,
being the next stage of the floodplain management process.

Tags: Salt Ash, Williamtown, hydraulic model, hydrologic model
Organization: Port Stephens Council