Flooding - Frequently Asked Questions

• What is a 100 year flood?
• How are flood levels measured?
• What is the Probable Maximum Flood (PMF)?
• Why is Council studying flooding?
• How many flood studies have been completed in the Warringah area?
• What types of flooding do we have in Warringah?
• My property was never classified as ‘flood prone’ or ‘flood liable’ before. Now it is. Why?
• How much water will be on my property when it gets flooded and where can I get information about flood levels on my property?

What is a 100 year flood?

It is defined as the probability or likelihood that a location will experience a flood of a particular size, in any one year. If a location has a one per cent chance of flooding each year, this can also be expressed as having:

• a one-in-100 chance of flooding in that location in any year
• betting odds of 100-to-one against a location being flooded in any year

However, this doesn’t mean that if a location floods one year, it will definitely not flood for the next 99 years. Nor, if it has not flooded for 99 years, will it necessarily flood this year. Some parts of Australia have received a couple of 100-year floods in one decade.

 

How are flood levels measured?

Flood levels are provided as a level relative to Australian Height Datum (AHD). AHD is the reference point for all elevations on the ground. In Australia this reference point is generally mean sea level which is taken as 0 metres AHD. All flood levels, floor levels and ground levels are provided in metres AHD. For example, if the floor level of your house is 6.7m AHD and the height of floodwaters for the 100-year flood is 7.2m AHD, the depth of water would equal 0.5m (7.2m AHD -6.7m AHD).

 

What is the Probable Maximum Flood (PMF)?

The PMF is the largest flood that can occur. It is a very rare and unlikely flood. Despite this a number of historical floods in Australia have approached the scale of a PMF such as the Wollongong 1998 flood. Every property potentially affected by a PMF will have some flood risk even if it is very small.

  

Why is Council studying flooding?

Warringah has experienced large flood events in the past such as large floods in April 1998 and March 2003. A series of smaller floods have also taken place and even these threatened people and property. The main objective of studying flooding is to reduce the impact of flooding on people in the floodplain and to reduce private and public losses from floods which helps us keep you informed and aware to reduce flood damages. It will also help to make sure that any new development does not make flooding worse.

The State Government has produced guidelines Floodplain Development Manual – the Management of Flood Liable Land – April 2005 to assist Councils in the management and development of flood prone land. This sets out guidelines on how to identify the flood risks and the most suitable options for the local area.

 

How many flood studies have been completed in the Warringah area?

Warringah Council has conducted five flood studies.

• Manly Lagoon

           * Manly Lagoon Flood Study 1992 (PDF, 8.6MB)

           * Manly Lagoon Floodplain Management Study (PDF, 4.3MB)

           * Manly Lagoon Floodplain Management Plan (PDF, 1.3MB)

           * Bangaroo Street, North Balgowlah, Flood Investigation 2006 (PDF, 1.1MB)

           * Ryan Place, Overland Flood Study (PDF, 16.5MB)

• Narrabeen Lagoon

           * Narrabeen Lagoon Flood Study 1990 (PDF, 2.5MB)

           * Narrabeen Lagoon Floodplain Risk Management Study (PDF, 3.5MB)

           * Narrabeen Lagoon Floodplain Risk Management Plan, vol 1 (PDF, 183KB)

           * Narrabeen Lagoon Floodplain Risk Management Plan, vol 2 (PDF, 522KB)

• Dee Why and Curl Curl Lagoons
  * Dee Why and Curl Curl Lagoons Floodplain Risk Management Study and Appendices vol 1 (PDF, 1.9MB)
  * Dee Why and Curl Curl Lagoons Floodplain Risk Management Study and Appendices vol 2 (PDF, 2MB)
  * Dee Why and Curl Curl Lagoons Floodplain Risk Management Study and Appendices vol 3 (PDF, 2.6MB)
• South Creek
  * South Creek Flood Study (PDF, 29.3MB)
  * South Creek Bank Management Plan (PDF, 8MB)
  * South Creek Floodplain Risk Management Study (PDF, 9MB)
  * South Creek Floodplain Risk Management Plan, (PDF, 2.7MB)
• Frenchs Creek, Belrose

          * Frenchs Creek Flood Study - Final Report (PDF, 15.6MB)

          * Frenchs Creek Floodplain Risk Management Study (PDF, 8.6MB)

        

Council is reviewing its flood studies to update or undertake additional work to provide a predicted flood outline based on current catchment conditions. This will also use recent studies to try to predict the effect climate change may have on the catchment. The flood extent and level provided by Council may change in the future.

Warringah Council is working in partnership with Manly and Pittwater Councils to develop a new Flood Study, Floodplain Risk Management Study and Floodplain Risk Management Plan for Manly and Narrabeen Lagoons - in accordance with the NSW Floodplain Development Manual (2005).

The NSW State Government Office of Environment and Heritage provides technical and financial assistance to councils for the Flood Studies, Floodplain Risk Management Studies and Plans.

Each Flood Study has a specific project webpage:

• Manly Lagoon Flood Study
• Narrabeen Lagoon Flood Study

 

What types of flooding do we have in Warringah?

There are two main types of floods that occur naturally in Warringah:

Flash floods usually occur during storms with heavy rainfall when creeks break their banks or the capacity of drainage systems is reached. Often they are sudden and unexpected, with little or no warning. Flash floods typically dissipate quickly for example Frenchs Creek.

 

Lagoon flooding occurs when creeks drain into lagoons and the level rises. It often takes a long time for the water to be released into the sea because of tides and waves. Lagoon flooding can also be made worse when severe winds create waves on the lagoons. Lagoon flooding is usually confined to the lagoons and their floodplains. It is important to remember that large floods are inevitable and in many cases, will overwhelm the existing stormwater network and force open lagoons.

My property was never classified as ‘flood prone’ or ‘flood liable’ before. Now it is. Why?

Council typically identifies the area affected by the 100 year flood as 'flood prone' or 'flood liable'. The State Government changed the meaning of the terms ‘flood prone’, 'flood liable’ and ‘floodplain’ in 2001. Before this, these terms generally related to land where ground level was below the 100 year flood level. Now these terms relate to all land that could possibly be flooded.

The government changed the definition of these terms because there was always some land above the 100 year flood level that was at risk of being flooded in rarer and more extreme flood events. History has shown that these rare flood events can and do happen (eg November 1996 flood in Coffs Harbour, the August 1998 flood in Wollongong, the 2002 and 2008 floods in Europe).

As Council follows State Government procedures and develops flood studies, it gains a better understanding of the flood behaviour of the catchment in particular the velocities and depth of flood water. Council is then better prepared to make decisions on the best risk management option for the area.

During the identification of the flood prone lands, Council undertakes community consultation and public exhibition before the final document is adopted by Council.

Insurance policies and conditions in relation to flooding may change over time or between insurance companies and you should confirm the specific details with your insurer.

 

How much water will be on my property when it gets flooded and where can I get information about flood levels on my property?

Council is progressively undertaking reviews of its existing flood documents. The new documents will detail the depth and velocity for each property in the floodplain.

Council has some maps that show flood information for properties identified on floodplains. For information, email Council at council@warringah.nsw.gov.au or phone 9942 2111. Council’s flood studies are located at Dee Why Library.