Environment & Waste


What is flooding?

Flooding is defined as a relatively high water flow that overtops natural or artificial banks in any part of a stream, river, estuary, lake or dam.

Flooding also includes local overland flow paths outside the defined drainage reserve and/or coastal inundation resulting from:

  • Heavy or prolonged rainfall; 
  • Super-elevated sea levels; and/or
  • Waves overtopping coastline defences (including projected sea level rise).  

Floodin in Wisemans Ferry 1986

Floods in Gosford City

The most common forms of flooding in the Gosford region are flash flooding, due to intense rainfall within the catchment, tidal action, such as King Tide, and storm surge which is associated with storms off the east coast of Australia. Storm surge is a common cause of flooding in the Brisbane Water estuary and along the coastal foreshore areas.

Flooding along Gosford’s southern boundary, the Hawkesbury River, can result from heavy rainfall in the upper reaches of the catchment beyond Windsor.

Due to the immense size of the catchment, this flooding may take up to three days before reaching the downstream areas of Broken Bay. Storm surge can also play a part in keeping flood heights elevated in Broken Bay and the Hawkesbury River.

The most frequent flooding in the Gosford region is flash flooding. It is sudden and unexpected and is caused by sudden local or nearby heavy rainfall which generally peaks within six hours of the onset of rain.

History of Flooding in Gosford City Fact Sheet

Living with floods

Flooding is generally a rare event and it is not always possible to predict when, where, or how big the next flood will be. However, the likelihood of different sizes of floods and their consequences can be estimated by computer flood modelling. This information is essential for council to plan appropriately for development.

Floods can bring considerable environmental and human benefit.  Groundwater is replenished, wetlands recharged, alluvial soils renewed and flood debris provides habitat and food for many species.  For the floodplains to be both healthy and productive, periodic flooding is essential.

However, there are also negative consequences as a result of flooding.  This includes potential risk to life and financial losses through damage to buildings and/or the building contents, public infrastructure, utility services and the environment.

Flooding also has the potential to disrupt essential services and cause human trauma. This can be especially true in many of Gosford’s suburbs where flooding can occur very quickly and with little or no warning due to the steep nature of the surrounding topography.

There are no quick fix solutions to the flooding issues in Gosford.  The complete elimination of flood risk on the floodplains of Gosford is an impossible task. 

It would inevitably entail the construction of massive drainage/flood mitigation structures and/or the purchase of all flood prone land in existing urbanised areas. This would be both cost-prohibitive and practically impossible. 

It is therefore necessary to find solutions to manage and live with the flood risks throughout Gosford.

Reporting a flood event to council

Council relies on information from the general public to monitor flood issues in Gosford City.  

If you have experienced consistent minor flooding on or near your property which has originated from a local stormwater drain, natural stream or estuary please reported it to council. 

This information is important to help identify new or repeated areas of flooding, assist with determining flood heights for future flood events, and identifying areas for priority investigation and potential works to alleviate the problems.

Flooding issues can be reported by contacting Gosford City Council.

Common flooding terminology

Probable maximum flood

The probable maximum flood (PMF) is the largest flood that could conceivably occur within a particular catchment, and is a very rare and unlikely event. However, when undertaking a Floodplain Risk Management Study, council looks at all storm events up to and including the PMF.

1% AEP or 1 in 100 year flood

A 1:100 year Average Recurrence Interval is the result of statistical data which estimates the probability that a particular rainfall event (or intensity) will be equalled or exceeded at a particular place within a certain period of time i.e. 100 years.

This is also referred to as the 1% Annual Exceedance Probability flood which is a flood that has a 1% chance of reaching or exceeding a particular magnitude in any one year.

It should be noted that if a 1% or 1 in 100 year flood is experienced in a certain year, that does not mean that there will not be another 1 in 100 year flood occurring in the same year or during the next 99 years.

Flood planning and management responsibilities

Council has a duty of care to manage lands subject to flooding within its Local Government Area.

Under the NSW Government’s Flood Prone Land Policy, the local government has responsibility for managing flood liable land.

Council is responsible for formulating and implementing Floodplain Risk Management Plans in accordance with the policy.  

The policy aims to reduce private and public losses resulting from flooding and encourages the development of:

  • Solutions to existing flood problems in developed areas; and
  • Strategies for ensuring that new development is compatible with any identified flood hazards and does not create additional problems in existing developed areas.

Gosford City Council, with the assistance of an Advisory Committee, is responsible for this task in each drainage catchment.

It obtains assistance from State and Federal Government in the form of technical advice and grant assistance funding for studies and works.

Managing Flood Risk Fact Sheet

Managing Flood Risk DVD

The Managing Flood Risk DVD is an industry first developed to address a gap in knowledge identified within the floodplain management industry.

As a management tool it provides valuable insight for communities and government agencies to better understand and manage the risks of potential future flooding emergencies within communities’ right across Australia.

The information provided in this DVD is aimed at various levels of government, Councillors and floodplain committees, engineers, planners, students, Emergency Services volunteers, and local service providers.

Flood Emergency Contacts
Flooding on Private Property
Flood Mapping

Also In This Section