The Lake Cathie and Lake Innes catchment is a moderate sized estuary that spans approximately 92 km² of which 25 km² is storage (i.e. lake or swamp area). The lakes are linked together by Cathie Creek to form an intermittent estuarine system which enters the Pacific Ocean at the village of Lake Cathie.
The entrance of Cathie Creek to the ocean is affected by a naturally elevated sand berm. A berm is an elevated sand dune that blocks tha creek to the sea. This naturally occurring berm results in an estuarine system which is predominantly closed to the ocean for varying periods of time. This berm creates two different flooding conditions.
- Catchment Runoff Flooding:
The berm height typically influences the water levels in the lake when runoff, as a result of rainfall in the catchment, occurs. When the entrance is closed to the ocean, the water in the catchment is trapped and backs up. This trapped water has the potential to rise to a level which can inundate property (public and private), assets (including roads etc), open space and the like.
- Elevated Ocean Condition Flooding:
Flooding in the lake system can also occur when rainfall runoff coincides with elevated oceanic conditions (i.e. storm surges). When the ocean level is raised due to storm conditions the water within the lakes can become trapped and back up. This trapped water has the potential to rise to a level which can inundate property (public and private), assets (including roads etc), open space and the like.
The Department of Crown Lands has issued Council with a short term licence (RN: 618957) to open the lake for flood mitigation purposes. The Licence relates to the protection of public infrastructure and private property in accordance with the Review of Environmental Factors - Lake Cathie Flood Mitigation.
Lake Cathie Flood Study
The Lake Cathie Flood Study report defines the flooding characteristics of the lake system for two scenarios (ie. lake open and lake closed). This report sets the 100 year flood level for the lake under various stages of lake entrance condition.
Following the study a blanket 1:100 Yr Flood Level of 2.63m AHD is now applied to the developed areas of Lake Cathie.
Image: Lake Cathie Floodplain map.
This page was last updated on: 11 May 2020