Flooding at Narrabeen Lagoon

Northern Beaches Flood Warning and Information Network

Warringah, Manly and Pittwater Councils in conjunction with the State Emergency Service and Manly Hydraulics Laboratory have established an innovative online flood warning system for the region. 
The system involves the installation of a number of rainfall and water level gauges that provide live data on where and when flooding may occur, lagoon levels, weather warnings and any recent or forecast rainfalls - also handy for gardeners.
The information can be viewed via the attached links.

Flood Types

There are two kinds of floods that occur naturally in Warringah: flash floods and lagoon flooding.

Flash floods occur when heavy rainfall causes creeks to break their banks or the stormwater network to exceed its capacity and overflow. Flash floods happen suddenly with little or no warning and recede relatively quickly.

Lagoon flooding occurs when creeks and stormwater networks drain into lagoons during heavy storms and the level of the lagoons rise, inundating the floodplain. Lagoon levels drop when the entrance opens either naturally or mechanically and the water is released into the sea. The speed of this release depends on tides and wave movement, and can take a long time.

Floods can be dangerous. You should never enter flood waters.

One-in-100-year Floods

Floods can be defined by their probability to occur. A one-in-100-year flood is a flood with a probability to occur once in every 100 years. However, this doesn’t mean that if it floods one year, it will definitely not flood for the next 99 years. Neither does it mean that if it has not flooded for 99 years, it will flood the following year. Some parts of Australia have received a couple of one-in-100-year floods in one decade.

Flood Levels

In Australia, flood levels are measured at the mean sea level (Australian Height Datum – AHD). All flood levels, floor levels and ground levels are given in metres AHD. For example: if the floor level of your house is 6.7m AHD and the projected floodwater level of a one-in-100-year flood is 7.2m AHD, the floor of your house would be 0.5m under water in a one-in-100-year flood.

Probable Maximum Flood

A Probable Maximum Flood (PMF) is the largest flood that could occur and is very rare or unlikely. Despite this, a number of floods in Australia have approached the scale of a PMF, such as the Wollongong flood of 1998. Every property potentially affected by a PMF will have some flood risk.

Flood Prone Land

According to the NSW Floodplain Development Manual (2005), flood prone land is any land below the Probable Maximum Flood (PMF) level - land that has the potential to flood in the future.

What is a Flood Notation on a Section 149 Certificate?

Properties which are flood affected will be notified on the Section 149 (2) Certificates. There are two questions that relate to flooding: Question 7A(a) indicates whether the property is subject to flood related residential development controls and Question 7A(b) indicates whether the property is subject to flood related development controls for commercial, industrial or vulnerable development.

Sections of Dee Why, Narraweena, Cromer, Ingleside, Brookvale and Oxford Falls may be flooded during rare and severe storms. This information is notified to these properties on Section 149(5) Certificates, to keep all residents informed about the possible risks while Council carries out further investigations to improve the stormwater system.

Development in Flood Prone Land

There are conditions that must be met before you build in an area that is prone to flooding.

Flood Information Report

Do you need additional information on flooding for a particular property?      

The Flood Information Report will help you to prepare your information for a Development Assessment. You are strongly encouraged to lodge an application for a Flood Information Report for any proposed development applications within a flood zone. A fee is payable for this service, details are on the form.

Following the application submission, you will receive written advice that details the following information (subject to availability):

  • Flood extent maps (1 in 100 year, Flood Planning Level, Probable Maximum Flood)
  • ALS spot height maps
  • Depth mapping (where available)
  • Flooding cross sections for the property (where available)
  • Flood levels for the property (1 in 100 year, Flood Planning Level, Probable Maximum Flood level)
  • List of all flood related provisions and controls relevant to the development type
  • Flood explanation sheet
  • Guidelines for undertaking a Flood Risk Assessment report
  • SES flood information brochure

An example Flood Information Report is in the documents section.

Further details are available in the Flood Study reports, available through our Flood Studies page.

Floodplain Management Process

The NSW Government has developed a Flood Prone Land Policy which helps local governments to manage flood prone land. The aim of the policy is not to block development but ensure new development is protected from floodwaters and to ensure it doesn’t make flooding worse for other people in the floodplain.

This is achieved through the Floodplain Management Process which involves:

  1. Establishing a floodplain risk management committee that includes community groups and state agency specialists
  2. Compiling existing data
  3. Developing a flood study
  4. Developing a floodplain risk management study
  5. Developing a floodplain risk management plan
  6. Implementing the plan, which may include mitigation works, planning controls, flood warnings, flood readiness and response plans, environmental rehabilitation, ongoing data collection and monitoring

Floodplain Management Association

Warringah is a member of the Floodplain Management Association which promotes best practice in floodplain management.

In this section you can find

Flood Studies

Flood Studies

Warringah Council periodically reviews flood studies to ensure we have the most up-to-date information available for each floodplain.