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Flooding

What is flooding?  

It is hard to define flooding simply. For Council’s flood planning and management purposes, flooding is defined as:  

Relatively rare and high flow which overtops the natural or banks in any part of a stream, river, estuary, lake or dam, and/or overland flooding associated with major drainage before entering a watercourse, and/or coastal inundation resulting from super-elevated sea levels and/or waves overtopping coastline defences excluding tsunami.  

Flash flooding is flooding which is sudden and unexpected.  It is often caused by sudden local or nearby heavy rainfall.  Often defined as flooding which peaks within six hours of causative rain.  

Floodplain is an area of land which is subject to inundation by floods up to and including the probable maximum flood event, that is, flood prone land.  

Floodprone land is land susceptible to flooding by the Probable Maximum Flood (PMF) event.   

Flood liable land is synonymous with flood prone land (i.e. land susceptible to flooding by the probable maximum flood (PMF) event).   

Floodway storage areas  are those parts of the floodplain that are important for the temporary storage of floodwaters during the passage of a flood.  

Flood Planning Levels are the combination of flood levels (derived from significant historical flood events or floods of specific AEPs) and freeboards selected for floodplain risk management purposes, as determined in management studies and incorporated in management plans.   

Freeboard provides reasonable certainty that the risk of exposure selected in deciding in a particular flood chosen as the basis of the FPL is actually provided.  It is a factor of safety typically used in relation to setting of floor levels.  Freeboard is included in the flood planning level  

Probable Maximum Flood is the largest flood that could conceivably occur at a particular location.  

The insurance industry has its own definitions of flooding (and stormwater) and you should make your own enquiries about how your insurance company classifies flooding (and stormwater), and how this relates to your insurance provisions.  

Local drainage is only designed and intended to manage lesser and more frequent rainfall events. Local drainage problems can occur anywhere and are not included in Council’s floodplain management planning.  

What is Floodplain Management?  

Floodplain Management is making balanced decisions about the economic, social and environmental costs and benefits of allowing a broad range of activities to take place on a floodplain.  

There are many different aspects of floodplain management. These include:

  • Calculating flood information such as heights, extents and potential risks to life and property of flooding, including estimating the effects of possible future climate change and sea level rise.
  • Finding and evaluating complete systems of managing the risks, which would include consideration of emergency management, examining if it is feasible to reduce the flooding, looking at ways of living with the risk, and how to manage future development
  • Consulting with the community when planning for floods
  • The elected Council adopting a Floodplain Risk Management Plan.
  • Finding the funding and resources needed to implement this Plan over time.

Why does flooding occur?  

Flooding is a natural random event that occurs in all parts of Australia. In general, flooding is a rare event. The nature of flooding may change with possible future climate change and the changes that human activity makes to catchments and topography.    

Flooding can result in property damage and even loss of life.  

Who is responsible for managing floods?  

Flood management is a shared responsibility.  

Management of large scale (and generally rare) flood events is a shared responsibility between Government and the community.  

The NSW Government Floodplain Development Manual states: “The management of floodprone land is, primarily the responsibility of Councils”, but adds that other agencies such as the NSW Department of Planning and the NSW State Emergency Service have important and complimentary roles.  

Councils are responsible for the preparation of Floodplain Risk Management Plans and the SES (as the combat agency for dealing with floods) for the preparation of Local Flood (emergency) Plans, and in the management of floods when they occur.  

In addition owners and occupiers of flood prone properties - are required to play a significant role in flood awareness, preparedness of your property and appropriate flood response.  

Our activities to manage flood risk  

As the regional drainage and floodplain management authority Council delivers a variety of projects and services to minimise the impacts of flooding.  

These include:

  • identifying the extent of flooding in storm scenarios ranging from frequent to less common, which helps us determine and prioritise flood risks that should be reduced or eliminated
  • assessing and placing conditions on development assessments, which ensure flood risks are considered when developing or redeveloping land in flood-prone areas
  • installing, upgrading and maintaining infrastructure like stormwater drains and retarding basins, which aim to minimise flood risks and impacts.

Is my property flood affected?  

Only land that is higher than all future possible floods is completely flood free.  

To find out if your property is potentially flood affected, apply for a 149 Planning Certificate.  

If the Planning Certificate states the property is flood affected, Council is able to provide approximate flood level heights and/or depths for 1:5, 1:20 and 1:100 ARI (Average Recurrence Interval) flood events. Flood heights are given as metres/AHD (Australian Height Datum) and flood depths refer to actual water depth in metres over the subject property.  

To obtain approximate flood height levels and/or depths for a particular property please print and fill out the Flood Level Enquiry Request Form.  

What is the probable maximum flood (PMF)?  

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.  

The NSW Government Floodplain Development Manual requires all possible floods up to and including the PMF to be considered in floodplain planning. It states: “The PMF … provides an upper limit of flooding and associated consequences … . It is used for emergency response planning purposes to address the safety of people.”  

History has shown that extreme flood events can and do happen (e.g. the 1990 flood in Nyngan, the November 1996 flood in Coffs Harbour, the August 1998 flood in Wollongong, the 1998 flood in Katherine, QLD’s Central Coast in April 2008 – and it is believed some parts of Newcastle in the 2007 floods)  

What is the 1 in 100 year flood?  

There is no such flood as a “one hundred year flood”, even though this is a commonly used term in everyday language. All floods are different. We do not know when the next flood will occur, or how big it will be. But we can estimate what the likelihood (probability or chances) of a certain size flood are at a given location for a given period of time. So for example, if your area has had a (so called) “1 in 100 year” flood, it is a fallacy to think you will need to wait another 99 years before the next flood arrives. Floods do not happen like that. Some parts of Australia have received a couple of “1 in 100 year floods” in one decade. On average, if you live to be 70 years old and live at the same location at or below the “1 in 100 year” flood level, you have a better than even chance of experiencing a “1 in 100 year” flood in your lifetime.  

We should better refer to a flood that has a 1 in 100 chance of being equalled or exceed in any given year, or a flood that has a 1 in 10 chance of being equalled or exceed in any given year, or a flood that has a 1 in 500 chance of being equalled or exceeded in any given year, all at some chosen location – and so on.  

What is a floodplain?  

A floodplain is an area of land which is subject to inundation by floods up to and including the probable maximum flood event, that is, flood prone land.

Cessnock has many types of waterways and water bodies ranging from lagoons, creeks, rivers, dams and overland flow paths.  

These are surrounded by steep catchments that produce fast flowing water in periods of heavy rain. This stormwater is collected and contained by these waterways.  

However, in periods of extensive or heavy rainfall, the waterways cannot contain all the stormwater and it therefore spills out onto the adjoining land – known as a floodplain.

Floodplains or flood prone land includes 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 floodplain risk management process?

For more detailed information regarding the Floodplain Risk Management Process, please refer to the NSW Floodplain Development Manual 2005.