About the draft Study

• Why did Council undertake a flood study?

• The commissioning of the Draft Flood Study is a requirement of Councils under the NSW Government’s Floodplain Risk Management Program. The objective of a Flood Study is to improve understanding of flood behaviour to better inform flood risk management for property owners and publicly managed community assets. The overall Flood Management Program’s outcomes is to increase community safety whist mitigating damage to private and public assets.

  The initial stage of developing this Flood Study included seeking data from property owners on past flood events in late 2018.

• What is the overall process?

• The delivery of this Draft Flood Study comprises stages 1 to 2 in the five-stage process outlined in the NSW Government’s Floodplain Development Manual (FDM, 2005) These works include:

  Stage	Timeframe	Detail
  1	Late 2018	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.
  2	Drafted in 2019 Public Exhibition Period: 19 February to 1 April 2020	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.
  3	2020 - 2023	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.
  4		Floodplain Risk Management Plan (FRMP) – outlines a range of measures, for future implementation, to manage existing, future and residual flood risk effectively and efficiently.
  5		Plan Implementation – once the management plan is adopted, an implementation strategy (devised in Stage 4) is followed to stage components dependent on funding availability.

• Has there been past significant flood events?

• Significant flood events occurred in the Exile Bay catchment in the early 1970s, 1986, 1988 and 2018.

• How was the community collaborated with on the development of this study?

• In late 2018, Council informed the property owners and occupiers of the 2,145 properties located within the Exile Bay catchment of the development of the flood study. 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.

  Council’s consultants received 65 submissions:

  • 45% of properties have not been affected by flooding
  • 45% of properties had been affected by flooding in their yard/garage
  • 8% had been affected by flooding above their floor level
  • 50% of properties were aware of overland flow in Exile Bay
  • 37% were not aware of overland flow in Exile Bay.

  
  

• Has the impact of climate change or sea level rise been considered in the flood study?

• The potential impact of climate change has been considered and used in the modelling to predict the potential risk of flooding in the study area. The influence of climate change means that predicting of future storm events is becoming more uncertain. However, it is generally accepted that storm events will become more frequent and more severe.

• 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 an LGA.

  The following methodology has been used to identify individual properties within the preliminary Exile Bay FPA:

  • Mainstream Flooding: The 1% AEP peak flood level within Saltwater Creek, Edwards Park and Greenlees Park plus 0.5 m freeboard, then extending the level perpendicular to the direction of flow.
  • Overland Flow Flooding: Cadastral lots where 10% or greater of the cadastral lot is affected by 1% AEP peak flood depths of greater than 0.15 m.
• How can the height of a flood be predicted?

• The flood study undertaken by GRC Hydro used flood simulation models, which utilises historical records of rainfall, land surface and water 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. A 1%AEP flood event means 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. This information is overlayed onto maps that illustrate the water depths within the study area.

  Probabilities of experiencing a given size flood once or more in a lifetime. Modified from Floodplain Development Manual: the management of flood liable land, NSW Government, 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
  	at least once	at least twice
  10% (1 in 10 odds)	99.9%	99.3%
  5% (1 in 20 odds)	97.0%	86.4%
  2% (1 in 50 odds)	75.3%	40.8%
  1% (1 in 100 odds)	50.3%	15.6%
  0.5% (1 in 200 odds)	29.5%	4.9%

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

• Of the 2,151 properties within the Exile Bay catchment, only 254 properties were identified as being located in a flood planning area.

• My property is not in the study area but is near identified flooding areas, do I need to do anything?

• The Draft Flood Study does not directly affect you, but may contain useful background information to assist you in assessing and managing your risk. You are encouraged to view the study. It would also be worth visiting the website of the NSW SES (https://www.ses.nsw.gov.au/) to find some useful information on how to manage flood risk.

• 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 254 properties identified within the Flood Planning Area will be notified in writing in February 2020.

  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 SES has a useful website that provides advice on how to manage flood risk. Visit www.floodsafe.com for more information.

• How can I find out more information about Flood Planning?

• Additional general information on Flood Planning is available from the NSW Government Office of Environment and Heritage. You can also download a copy of the Floodplain Development Manual 2005, which includes the NSW Flood Prone Land Policy via the following link: http://www.environment.nsw.gov.au/floodplains/manual.htm

Information for Property Owners and Occupiers within the Flood Planning Area

• What is Council doing to protect our lots from flooding?

• Council cannot control a flood event, however we can provide information to residents to better equip them in the case of a flood. We will also be putting in place development controls that will provide guidelines for any future development. The aim of this is to minimise the future risks associated with flooding. For instance this study will be used to ensure that future development does not make flooding worse in your area. At later stages of the Floodplain Management Process, mitigation measures may be considered by Council. To view the Floodplain Management Process, visit www.collaborate.canadabay.nsw.gov.au

• What does it mean if my property has been classified as a flood control lot?

• In a planning context, properties that have been identified as being within the flood planning area are classified as a flood control lot. This means that the land will be subject to flood-related development conditions to reduce the impact of flood. For example, if your property is identified, you will need to ensure that the floor level of any new building work is above the flood level to minimise the risk of loss to your property.

  Further, properties identified as being subject to flood related planning controls will be listed as a flood control lot on your 10.7 Planning Certificate, which are issued when people are buying and selling property.

• What is a Planning Certificate?

• A Planning Certificate provides land owners, prospective buyers and developers information about the land use zoning and development controls applying to the land, as well as information about potential development constraints.

• Will it affect my building insurance premiums?

• Insurance premiums are determined by individual insurance companies based on their assessment of risk and probability, and are outside of Council’s control. Only your insurance company can comment on this.

• My property has never been affected by a flood, so why is it now classified as a flood control lot?

• Floods do not occur in a regular pattern. It is not uncommon to experience a long period of no floods, and then a period of several floods. The Study has shown that there are limited records of past floods.

  Evidence suggests that if you live near a creek, river, stormwater canal or in a low-lying area, you may be at risk from flooding even if you have not experienced flooding in the past. Even if there is a lack of historical evidence of flooding at your property, it does not necessarily mean it is not at risk from potential flooding.

• Will being identified as being a flood control lot affect the value of my property?

• There are many houses across Sydney that are listed as being at potential risk of flooding. The value of houses can vary significantly depending upon a range of factors, such as site conditions, adjoining developments, market conditions and interest rates. Ultimately, it is the market that determines house prices and Council is not in a position to advise on the value of individual properties.

What happen's next?

• Community consultation

• Community members are encouraged to review the Draft Flood Study and provide their feedback during the Public Exhibition Period of 19 February to 1 April via:

  • 24/7 online: www.collaborate.canadabay.nsw.gov.au/exilebayfloodstudy or click on Exile Bay Flood Study
  • Email: feedback@exilebay.com

  Post:

  Strategic Asset Management and Innovation, City of Canada Bay Locked Bag 1470, Drummoyne NSW 2470

  If you disagree with the study’s findings, you will be required to provide detailed evidence of any purported inaccuracy in the Study by providing alternative modelling by experts if you disagree with the study findings.

• What happens next?

• At the end of the public exhibition period, Council will review the feedback received before finalising the Study. Once it has been adopted by Council, it will be used to guide the rest of the Floodplain Management process defined by the NSW Government’s Floodplain Development Manual. The results of the study will be used to manage the impact of flooding upon our community. To view the Floodplain Management Process, visit www.collaborate.canadabay.nsw.gov.au

Terminology

• Terminology Index

• What does “flood prone land” mean?

  “Flood prone land” is a specific technical term used in the NSW Floodplain Development Manual to refer to all land below the level of the Probable Maximum Flood.

  What is a Probable Maximum Flood Level?

  The Probable Maximum Flood (PMF) is described as the largest flood that could conceivably occur at a particular location and is modelled based on Probable Maximum Precipitation which is in turn calculated by methods developed by the Bureau of Meteorology.

  And what is a flood control lot?

  A flood control lot is a lot identified as being within the Flood Planning Area on the Flood Planning Area Map. It is generated by adding a freeboard to the modelled 1% AEP flood surface level.

  A flood control lot is land or property that is subject to flood-related development controls.

  What is a 1 in 100 year flood?

  This is largely superseded terminology used to indicate a flood that has a 1% chance of occurring in any given year. The preferred term is now 1% AEP. AEP stands for Annual Exceedance Probability. 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?”

  What are flood-related development controls?

  Flood related development controls, specify the special conditions for developing or building on a flood control lot. Typically these controls include condition that may require the floor level of new building work to be above a certain level.

  What is a Flood Planning Level?

  A Flood Planning Level (FPL) is the estimated height of a 1% AEP flood event, with freeboard added (see below). It is used as the basis to determine planning controls for flood control lots.

  What does ‘freeboard’ mean?

  The term ‘freeboard’ is used to describe a safety buffer, to account for unknown factors and uncertainties in the modelling that determines the Flood Planning Level and is typically applied as 0.3 or 0.5m above the 1% AEP Flood Level.

  What is ponding?

  Ponding generally describes relatively still water that remains for a time after a flood event because it is in a low-lying or trapped area and cannot easily drain away.

  What is mainstream flooding?

  Mainstream flooding is defined as water that flows over the banks of creeks, river canals and lagoons. In other words; when a creek or river breaks its banks.

  What is overland flow?

  Overland flow flooding is defined as surface runoff, typically excess rainfall or stormwater runoff from homes, driveways and other surfaces