Councils in New South Wales have responsibility for identifying and managing flooding, and informing the community. Councils follow the NSW Flood Prone Land Policy, which outlines how Councils should manage flooding to reduce the risk of flooding to people and properties. It also sets consistent guidelines such as using the “1 in 100 year flood event” as the industry standard for design and planning purposes. One of the most important steps in this process is increasing our community's awareness of flooding so that people are better able to understand and plan for the flood risks they face.

The study includes the suburbs of Balgowlah Heights, Clontarf and the parts of Balgowlah, Fairlight, Manly and Seaforth that drain to Middle Harbour, North Harbour and the Tasman Sea (eg Manly flats, North Head, Ivanhoe Park area, etc.)

The draft Manly to Seaforth Flood Study Map shows land that would be affected by the Probable Maximum Flood (i.e. the largest flood that could conceivably occur). Properties affected by the Flood Planning Level (i.e. the 1 in 100 year flood plus freeboard) are also shaded on the map. 

A 1 in 100 year flood is a major rain event that occurs on average once every 100 years, i.e. there is a 1% chance of a flood of this size occurring at a particular location in any given year.

Some parts of Australia have received a couple of 1 in 100 year floods within a decade of each other. 

The Flood Planning Level (FPL) is the height used to set floor levels for houses in flood prone areas. It is defined as the “1 in 100 year flood level” plus an appropriate freeboard (see explanation of ‘freeboard’ below).

It is used for planning purposes, and land below the FPL is considered to be flood affected and therefore subject to flood related development controls. The FPL can be calculated as:

FPL = 1 in 100 year flood level + freeboard (typically 0.5m)

Freeboard is included in the Flood Planning Level as a buffer, to account for factors such as unforeseen blockages, waves from vehicles (such as SES rescue trucks), other localised hydraulic effects and uncertainties in the modelling and determination of flood levels. Freeboard is typically 0.5m above a flood level, for areas of shallow flood depths in the draft Manly to Seaforth Flood Study a reduced freeboard of 0.3m has been applied.

For most people, there is nothing for you to do in response to this flood study, as there is no immediate change to your situation. However property owners who are planning redevelopment of their property may need to take some additional steps as part of the consent process because flood related development controls apply. This could include the requirement to have the floor levels of new residential developments set at or above the Flood Planning Level and out of flood danger. 

If there is significant rain at the top of the hill, the rain will run downhill through low lying gullies, channels and surface depressions. The speed of floodwaters and depths may be less at the top of the catchment than further down the hill, but still be significant enough to pose a risk.

Councils prepare Flood Studies and Plans according to the NSW Government’s Floodplain Development Manual (2005), and implement associated recommendations with the financial and technical assistance of NSW Government through its Flood Prone Land Policy. Land use planning through development controls is one of the most effective means of managing flood risk in the catchment. Council has implemented the Northern Beaches Flood Information Network which includes live rain gauges and lagoon water level gauges, which are used by government agencies during flood events and are publically available at: http://www.mhl.nsw.gov.au/users/NBFloodInfo/ Council has also undertaken a range of targeted flood and coastal storm awareness programs for aged-care and childcare facilities.

Specifically in the Manly to Seaforth Study Area, Council has also recently constructed an underground stormwater detention system at Manly Oval to reduce flood inundation of the Raglan St and Pittwater Rd, Manly intersection. The Manly to Seaforth Floodplain Risk Management Study will outline what further can be done to manage flooding in the study area, through the identification, assessment and comparison of various risk management options. 

Studies show that an actual flood event, rather than a flood planning notation on a Planning Certificate, is more likely to have an effect on property values. 

Individual insurance companies typically identify Flood Prone Land and assess risk through their own flood studies, analysis and flood mapping exercises, irrespective of whether Council has undertaken a flood study. These calculations are outside Council’s control. The information is then used to set policies and premiums. 

If it is a life threatening situation call 000.

In the event of floods, storms or tsunamis, please contact the State Emergency Service (SES) on 132 500 or visit their website at ses.nsw.gov.au. 

The State Emergency Service has a useful website providing advice on how to manage flood risk. Visit www.floodsafe.com.au  for more information.