Why do floods occur?

Flooding occurs when there are large volumes of rainfall. In river systems it may take several hours or even days for flood water to rise and break its banks. Along creeks or overland flow paths, which have relatively small catchments, this happens more quickly and is often referred to as flash flooding.

Other factors that can contribute to flooding are the topography of an area, soil conditions and impervious ground cover (e.g. concrete). Urbanised areas typically have more impervious areas – footpaths, roads, driveways - which increase the volume and velocity of water flowing downstream during large storms.  

What are the risks associated with flooding?

Flooding can cause significant damage to property and risk to life. It may also result in the loss of valuables and disruption to essential services. Flood waters can become dangerous at relatively low depths:

  • an average sized person can be knocked over by as little as 150mm of moving water, and
  • a large car can float away in 600mm of moving water. 

What is a flood study?

A flood study is a comprehensive investigation of flood behaviour within a catchment. It provides detailed information about the nature of flood risk including the distribution, extent, depths, levels and velocity of floodwaters for a range of storm events, including a 100 year flood.

What is a 100 year flood event or 1% AEP?

A 100 year flood event is a flood that will occur on average once every 100 years. There is a 1% probability of it occurring in any given year. However, if an area has had a 100 year flood, it doesn’t mean that it would be another 99 years before the next one happens. For example, the last time the Brisbane River flooded before the 2011 disaster was in 1974. These were both 100 year events.

Annual Exceedance Probability (AEP) is usually expressed as a percentage and is the chance of a  storm of a given intensity and duration happening at least once in any year. A 1% AEP flood event is a 100 year flood event and is used as the flood standard for residential development controls.

Why has Council undertaken this flood study?

Flooding costs owners, residents and local government a lot of money. People affected by floods can also suffer significant distress and hardship.

The flood study is the first stage of an overall floodplain management plan for the catchment. The plan will identify ways of managing flood risk which may include:

  • controls on development
  • helping people at risk (e.g. flood warning systems) and
  • changing behaviour of the flood itself (e.g. upgrade of stormwater systems)

How was the flood study undertaken?

Council undertakes flood studies and management plans in accordance with the NSW Floodplain Development Manual (NSW Government, 2005) which outlines the framework that Councils need to follow.

In 2016, Council engaged a specialist consultant, WMA Water, to conduct the Alexandra Canal flood study. The consultants investigated rainfall data and undertook complex computer-generated flood modelling to determine flood behaviour. They also used information gathered from property owners and residents in the catchment which included survey responses, written accounts of flooding, plus videos and images of past storm events.  This information was used to verify the results of the flood model.

When was the last flood study for Alexandra Canal done?

This is the first comprehensive flood study of the Alexandra Canal catchment. In the past, Marrickville Council and Sydney Water conducted drainage studies to determine flood behaviour . This included a study of part of the Alexandra Canal Catchment in 1998.

How has Council determined if a property is flood affected?

Properties flooded in a 1% AEP storm event, where more than 10% of the property area is inundated and the flood depth is greater than 0.15 metres, are initially identified as being flood affected.

Each property is then reviewed to determine if it is either:

  • Exposed to water depths generally in excess of 0.3 metres in or adjacent to the property, as this may be dangerous, cause property damage or both; and/or
  • Located in the floodplain of an original watercourse (which may now be piped, channelised or diverted), or sloping areas where overland flows develop along alternative paths once the capacity of the stormwater system is exceeded; and/or
  • Located in a major overland flow path through developed areas outside of defined drainage reserves.

This process ensures that only those properties which are subjected to a genuine flood risk are identified.

For more information please refer to Council's Policy For Identifying Properties Subject to Flood Related Development Controls 

Why is Council identifying flood affected properties?

Flooding can be a serious threat to life and property.  The NSW Government requires Councils to study and identify land that has the potential to be affected by flooding. This is to ensure Council’s planning controls have adequate measures to prevent new developments and redevelopments of existing properties from making flooding worse.

Council notified me a few years ago that my property is flood affected – why is this happening again?

Council uses the latest information to identify flood affected properties. The current study uses updated modelling techniques which provide an improved understanding of complex flood behaviour. Because of this, the results of this study supersede previous flood work undertaken by Council.

Why were houses built in areas that flood?

The inner west has some of Sydney’s oldest suburbs and architectural styles including terraced houses which were built in the 19th century.  In the past, development occurred without much consideration of flooding because there were no flood records and little was known about flood behaviour. While the lowest lying land was kept clear of development, there was little understanding of how much water could flow through the catchment in rarer, more severe storms. It is only in the last generation that the rainfall data and computer technology has been available to better understand and calculate this.

There is no recorded history of flooding in my area, could I still be at risk?

Yes. Lack of evidence of historical flooding does not necessarily mean an area is not prone to flooding. If you live close to a creek, river, stormwater drain or in a low-lying area, you may be at risk of flooding even if you have not personally experienced it. Flooding can also occur on the sides of hills if the shape of the landscape concentrates overland flows on the way to the drainage network.

Will the study fix the flooding in my street?

The study is the first step toward managing flooding in the Alexandra Canal catchment. The next stage of the process is developing a plan for managing floods and Council is expecting to start this work in 2019. 

What can I do to prepare for a flood event?

Visit the State Emergency Service www.floodsafe.com.au to get advice on simple things you can do to prepare for a flood.

Will the value of my property be affected if it has been identified as flood affected?

Prospective buyers are able to find out if a property is flood affected because Council has a duty to disclose this information.  If you are concerned that the flood classification may affect the value of your property you should seek valuation advice. 

Can a flood affected property be developed?

Properties affected by flooding can, in most instances, be developed providing 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. You will need to 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?

Your insurer will most likely offer some flood cover although this may depend on the flood level information they have about your property. This may be different from Council‘s information because insurance companies make their own assessment. You should confirm the specific details of your situation with your insurer.

My property already has stormwater drainage – can it still be identified as flood affected?

Stormwater pipes are designed to take water away from dwellings and manage small flood events. In a large flood event (e.g. a 100 year flood), it is likely that the capacity of these stormwater pits and pipes will be exceeded. The Flood Study and Draft Report show where this additional water is likely to go.

The maps show that only a small part of my property is affected - why has it been identified 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 will be identified. Development controls are applied to a whole property, regardless of the extent of the flood.

What progress has Inner West Council made on managing floods?

Inner West Council has completed flood studies for around 75% of the local government area. Once the Johnstons Creek, Whites Creek and Alexandra Canal studies are complete, Council will have completed around 95% of flood studies in the area.

Inner West Council is also currently undertaking Floodplain Risk Management Studies and Plans for Leichhardt, the Marrickville Valley and the Hawthorne and Dobroyd Canals.