Why did Council undertake a FRMS&P?

    It is a requirement of local councils under the NSW Government’s Floodplain Management Program. The objective is to improve understanding of flood behaviour to better inform flood risk management for property owners and publicly managed community assets. The Program’s overall goal is to increase community safety while mitigating damage to private and public assets.

    What is the overall process?

    The delivery of this Draft FRMS&P is stages 3 & 4 of a five-stage process outlined in the NSW Government’s Floodplain Development Manual (2005) These include:

    Completed 2017

    Data collection – collection of all applicable data to be used for the ensuing stages of the studies and community consultation to inform the community of the study and collect information from them on previous flood events.

    Completed 2017

    Flood Study – a comprehensive technical investigation of flood behaviour that provides the main technical foundation for the development of a robust floodplain risk management plan.

    Draft Study Completed 2022

    Floodplain Risk Management Study (FRMS) – assess the impacts of floods on the existing and future community and allows the identification of management measures to treat flood risk.

    Draft Study Completed 2022

    Floodplain Risk Management Plan (FRMP) – outlines a range of measures for future implementation to manage existing, future and residual flood risk effectively and efficiently.

    Plan implementation – once the management plan is adopted, an implementation strategy (devised in stage 4) is followed to stage components.

    Has there been past significant flood events?

    Significant flood events occurred in the catchment in 1986, 1988, 2018 and 2020.

    How was the community collaborated with on the development of this FRMS&P?

    As part of Council’s 2017 Flood Study, approximately 28,000 letters were sent to residents asking for flooding information. During the public exhibition period another 5,600 letters were sent out to residents whose property is located within the probable Maximum Flood (PMF) area.

    In October 2019, as part of the current FRMS&P, 3,500 letters were sent to residents asking for further flooding information to update the Flood Study to the current standard.

    These community members were encouraged to share their experience about historic flood events, including sending in images of their property and surrounds when flooding had occurred. This community experience was used to calibrate the Flood Model so that the study reflected reality.

    Has the impact of climate change or sea level rise been considered in the FRMS&P?

    The effects of climate change on flooding has been included in the computer modelling. Climate change is predicted to lead to more frequent and more severe rainfall events, which will increase the occurrence of flooding. Sea level rise will also worsen flooding although due to the area’s topography, very few locations in the LGA will see worse flooding with higher sea levels.

    What is a Flood Planning Area?

    The Flood Planning Area (FPA) defines properties that are subject to flood related development controls. The FPA is a key planning tool for managing and mitigating flood risk in a local government area. The following methodology has been used to identify individual properties within the preliminary FPA:

    • Category A: Located on/in the proximity of an identified/designated floodway; and
    • Category B: Located on/adjacent to localised flood with significant flood depth or flow (that is not part of the identified/designated floodway).

    What is a 1 in 100 year flood?

    This is largely superseded and confusing terminology used to indicate a flood that has a 1% chance of occurring in any given year. The preferred term is now 1% Annual Exceedance Probability (AEP). It does not mean that if a flood occurs in one year, it will not flood for the next 99 years. There is an informative article on the Bureau of Meteorology website titled “Why do 100 year events happen so often?”

    How can the height of a flood be predicted?

    The FRMS&P undertaken by GRC Hydro used flood simulation models, which utilises historical records of rainfall, land surface and catchment data for the study area. Detailed information is produced when the model is run to reflect different flood scenarios like a 1% AEP flood event.

    This information is then overlayed onto maps that illustrate the water depths within the Study area.

    How often do Floods occur?

    A 1% AEP flood event is colloquially known as a 1 in 100 year flood event that has the probability of occurring 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. This does not mean that if a location floods one year that it will not flood for the next 99 years. Nor, if it has not flooded for 99 years that it will necessarily flood the next year. Some parts of Australia have experienced more than one ‘1 in 100 year’ floods within a decade of each other.

    Below is a table that shows the standard AEP floods and the chance of occurrence over a 70 year period.

    Probabilities of experiencing a given size flood once or more in a lifetime. Modified from the NSW Government’s Floodplain Development Manual: the management of flood liable land (2005)

    Chance of a flood of a particular size being exceeded in any one year

    Chance of experiencing a flood in a 70 year period

    Chance of experiencing a flood in a 70 year period

    10% (1 in 10 odds)
    5% (1 in 20 odds)
    2% (1 in 50 odds)
    1% (1 in 100 odds)
    0.5% (1 in 200 odds)

    How many properties have been identified that are at risk of flooding?

    Of the 11,473 properties within the North Sydney local government area, only 745 properties were identified as being in the FPA.

    What do I need to do if my property has been identified as being affected by potential flooding and how can I minimise the impact of flooding?

    All property owners and occupiers of the 745 properties identified within the FPA will be notified in writing by Council.

    You do not need to take any immediate action as the likelihood of flooding in the area has not increased based on the Study. However, because the study identifies the potential risk of flooding to your property, you can be better prepared if this does occur. For instance, it may be a good idea to ensure valuable items are not stored under your house or on the floor in the garage. It is better to store these items in a location where they are less likely to be impacted by a flood, should one occur.

    The NSW State Emergency Service (SES) website provides advice on how to manage flood risk. Visit www.floodsafe.com.au for more information.