Frequently Asked Questions

What is a Flood Study?

What is a Floodplain Risk Management Study?

What is a Floodplain Risk Management Plan?

What is a Floodplain?

What is the Probable Maximum Flood?

What is the 100 year flood?

What is a flash flood?

Why do flood levels and information need to be reviewed over time?

Will my property value be altered if my property is flood affected?

Can a flood affected property be developed?

Will I be able to get house and contents insurance if my property is flood affected?

My property already has stormwater drainage so is it still included?

The maps show only a small part of my property is affected so why have I been classified as flood prone?


What is a Flood Study?
A Flood Study is a comprehensive technical investigation of flood behaviour in a particular catchment. The Flood Study provides detailed information on the nature of the flood risk including the distribution, extent, levels and velocity of floodwaters across the floodplain for various flood events including the 100 Year Flood and Probable Maximum Flood. 

What is a Floodplain Risk Management Study?
Following a Flood Study, the next stage in the floodplain risk management process is a Floodplain Risk Management Study.The purpose of a Floodplain Risk Management Study is to identify, assess and compare various risk management options and consider opportunities for environmental enhancements as part of mitigation works.

The management study draws together the results of the flood study and data collection exercises. It provides information and tools to allow strategic assessment of the impacts of management options for existing, future and continuing flood risk on flood behavior and hazard. It also allows for a robust assessment of the social, economic, environmental and cultural issues and costs and benefits of all options.

What is a Floodplain Risk Management Plan?
The purpose of a Floodplain Risk Management Plan is to provide a strategic management plan which best balances social, economic, environmental and cultural interests in the floodplain; optimizes the use of community infrastructure; minimizes the risk to people, property and emergency response personnel; formalizes and prioritizes mitigation works; and forms the basis for development controls that will be used to assess future development in the floodplain. 

What is a Floodplain?
A floodplain is an area of land that has been formed by flooding over time and is still subject to periodic inundation. Floodplains are also referred to as flood prone land.

Floodplains or flood prone land is all land affected by flooding up to and including the Probable Maximum Flood (PMF) event (or the highest conceivable flood that nature can produce). Only land that is above these levels is truly flood free.

What is the Probable Maximum Flood?
The Probable Maximum Flood (PMF) is the largest flood that could conceivably occur within a 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. 

What is the 100 year flood?
A 1 in 100 year flood uses 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.

It should be noted that if a 100 year flood is experienced in a certain year, that does not mean that there will not be another 100 year flood occurring in the same year or during the next 99 years. Floods do not happen like that. Some parts of Australia have received a couple of 100 year floods in one decade or even a year apart. On average, if you live to be 70 years old, you have about a 50/50 chance of experiencing a 100 year flood.

What is a flash flood?
Flooding that is sudden and unexpected is referred to as flash flooding. It is usually caused by slow-moving thunderstorms that deposit an extraordinary amount of water in a relatively short period of time. 

Why do flood levels and information need to be reviewed over time?
There is a chance that floods of any size will occur in the future. As the size of a flood increases, the chance of it occurring becomes smaller. Because some rare types of floods have not occurred for over a century, the height of future floods is predicted using computer models. These models simulate different flood levels and velocities for a variety of different sized floods. 

Given the importance of accurately predicting flood levels and information, Council engages experts to establish and operate these computer models. From time to time, computer models are reviewed, and predicted flood levels may change slightly. The reason why the models are revised can include:

  • New floods occur, providing additional data to fine-tune the model.
  • Flood mitigation works undertaken may change flood levels. 
  • More advanced computer models become available.
  • Development within the floodplain (which may be outside Council’s control).

Will my property value be altered if my property is flood affected?
If your property is classified as being flood affected, a prospective buyer of your property could have previously found out this information if they made enquires themselves with Council. The property market determines the value of any residential property. Individual owners should seek their own valuation advice if they are concerned that flood classifications may influence their property value. 

Can a flood affected property be developed?
Properties affected by flooding can be developed (in most instances) so long as acceptable risk management criteria are demonstrated and applied. The planning controls applicable to a flood affected property are specific to the proposed land use, the floodplain and the extent of flooding on your property. Council’s development controls make reference to the 100-year flood level and the flood risk precincts. You must therefore obtain and use this information when preparing a development application for your property. 

Will I be able to get house and contents insurance if my property is flood affected?
As a result of the Queensland floods in 2012, many insurance companies started offering wider cover although the extent of cover, particularly for very flood prone properties, may differ between insurers. The most likely situation is that your insurer will now offer you some flood cover although this may be dependent of the flood level information that the insurer has for your property (this may not necessarily be the same information available from Council). If flood cover is offered, the classification of your property within a Flood Risk Precinct per se, is unlikely to alter the availability of cover. Insurance policies and conditions may change over time or between insurance companies, and you should confirm the specific details of your situation with your insurer. 

My property already has stormwater drainage so is it still included?
Stormwater pipes are designed to take water away from dwellings and deal with small flood events. In a large flood event (100 year flood event), it is likely that stormwater pits and pipes will be full. The Draft Overland Flow Flood Study and Mapping Report shows where this additional water is likely to go. The type of rain needed to produce this type of flooding has not yet been seen in the area. 

The maps show only a small part of my property is affected so why have I been classified as flood prone?
The maps highlight which property blocks will be affected. Even if a small portion of the property is affected, the whole property area will be coloured in. Development controls are applied to a whole property and cannot be split, regardless of flood extent. We are inviting affected property owners to information sessions, either face to face or by telephone.


Contact
Drainage Investigation Engineer
Phone: 02 9366 3666
Email: floodplain@botanybay.nsw.gov.au