Do You Live Behind a Levee?
There are some 116 levees in NSW that provide protection to communities and land from flooding. It is important that the NSW SES engages with communities that live behind a levee to ensure that these communities know their levee and understand their risk.
What is a levee?
A levee is a man-made structure built to contain, control or divert the flow of water in order to provide protection to towns and/or agricultural land from flooding. Levees are designed to hold back a certain amount of floodwater. Some levees have been carefully built to a design, while others have been quickly constructed prior to floods and still remain. Though the area behind the levee is protected from flooding from the river, stormwater pipes or smaller streams not being able to drain into the river may lead to flooding from behind.
How is a levee designed?
Levees can be made out of earth or concrete. The crest is the top and the design height is the height of the floodwater the levee is intended to hold back. The freeboard is added to the levee to ensure it can withstand a flood that has reached the levee's design height; it takes into account factors such as wind or wave action of the water, erosion or settling of the earth over time. Freeboard should not be relied upon to hold back water.
Why does a levee fail?
Depending on a levee's age and its condition, water can seep through the levee leading to flooding in the area it has been built to protect. This can happen slowly or suddenly. Fast moving water, trees and animal holes can all lead to a weakness in a levee that can cause it to fail.
Why do levees overtop?
When a flood occurs, floodwater might flow over the crest of the levee - this is called overtopping. When a levee overtops it will cause flooding in the area behind the levee. Overtopping can be slow or fast and can also cause erosion leading to a full breach of the embankment and rapid high speed flooding behind the levee.
Is it time to move to safety?
Emergency Services make decisions about if or when people need to evacuate based on flood conditions. As exit routes can be cut off due to floodwater before a levee overtops or fails, you may be asked to evacuate several hours before.
If it looks like flooding may occur, start preparing your house in case you have to leave. Place possessions off the ground with electrical goods in the highest locations, anchor objects that are likely to float and cause damage and put important papers, valuables, photos and other special items into your emergency kit.
Understand your risk
If you live behind a levee you need to understand your risk, remember no levee is flood proof.
Find out:
- The height of floodwater the levee is designed to withstand
- How your neighbourhood and nearby areas will be affected
Take action:
- Develop an emergency action plan to follow in case of flooding
- Know your evacuation route
- In times of flood stay aware - listen to the radio, TV or check the Internet for warnings
