The assessment looked at current and future risks from rising sea levels, which is important for making policies and plans to manage surrounding low lying areas. It mapped areas potentially impacted by coastal storms as well as regular tidal inundation under various future conditions. It also looked at how the flooding might occur as backwater flooding, as high tides emerge through the stormwater drainage system.
The purpose of this assessment was to:
- Use up to date level data to map coastal and tidal inundation in the Cooks River.
- Identify priority areas for further investigation and adaptation planning.
- Identify assets and critical infrastructure exposed to these hazards now and in the future.
This study does not immediately change current planning and development policy, such as development controls or land zoning.
Limitations of the study.
- Catchment flooding not considered: This study does not consider flooding due to rainfall. Catchment flooding is addressed in the relevant flood study and managed under the Floodplain Management Framework.
- Sea level rise projections: The exact rate of sea level rise is uncertain, however the scenarios assessed in this study can be expected to occur at some point in the future. The exact timeframe is uncertain.
- Simplified stormwater mapping: We used a 2D mapping technique to determine which low-lying areas might be connected to the main waterway via stormwater pipes. More detailed investigation at certain sites may be appropriate to get a more detailed understanding of risk.
- Inclusions/exclusions: The presence of mitigation assets such as floodgates and stormwater pumps were not considered in the mapping. Some areas may be mapped as inundated, but current or future mitigation can reduce the impact.
Tidal Inundation & Coastal Inundation
The Coastal Management Act defines 7 ‘coastal hazards’ including:
- Tidal inundation
- Coastal inundation
| Tidal inundation | Coastal inundation |
 |
 |
| Permanent and cyclical | Temporary and event-based (like an East Coast Low) |
| Predictable with astronomical tide forecasting | Unpredictable |
| Initially low-level impacts, gradually worsening with sea level rise | Generally infrequent, becoming more frequent with sea level rise |
Sea Level Rise & Planning Horizons
The study mapped inundation at three sea level rise increments:
- +0.2m
- +0.4m
- +0.9m
There is uncertainty around exactly when these levels might be reached, but there is certainty that they will be reached at some point in the future. A conservative approach was used in this study. The timing is dependent on several factors including:
- Global emissions pathways
- Planetary ocean and ice systems’ response to warming
Overbank Flooding & Backwater Flooding
The study looked at two ways for tidal waters to reach low-lying areas:
- Overbank flooding
- Backwater flooding
Overbank flooding happens when the river water overtops the banks, spilling into low-lying land in the floodplain.
Backwater flooding happens when the bank is higher, but stormwater outlets are connected to land that is lower than the water level. This causes saltwater to flow back up the pipes causing tidal flooding in lower areas. This is already occurring in some of the most low-lying areas, and with rising sea levels this will become more Common. There are temporary management options such as specially designed floodgates that can prevent water from flowing up the pipes.
Eventually, areas can progress from backwater to overbank, changing what type of interventions are feasible.
The study mapped areas throughout the Cooks River catchment that can be expected to face this issue in the future. Overbank and backwater flooding are mapped differently. This is shown in the maps as yellow (backwater) and blue (overbank) areas connected by green points and lines (representing the stormwater system). A snippet of a map from the report is shown here.
Sea level rise and increased risk from tidal inundation is a concern throughout NSW, Australia and the world. There are several recent studies that have considered this hazard on a larger scale. Importantly, while these studies use different techniques, they all deliver the same message, and provide supporting information that can be used to make decisions for our future. Some of the key relevant studies include: