FLOODING

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. 

When was the last time it flooded?

A significant storm event which caused flooding and damage in the inner west occurred on 25 April 2015. This was approximately a two to five year, 30 minute flood event in the study area. 

What can I do to prepare for a flood event?

Visit the State Emergency Service www.floodsafe.com.au(External link) 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. 

FLOOD STUDY

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.

When was the last flood study for Hawthorne Canal catchment?

In 2013, the former Ashfield and Marrickville Councils jointly studied a range of storm events in the Hawthorne Canal Flood Study, which was adopted by both Councils in early 2015. The study identified a number of areas which are likely to experience flooding. These areas are concentrated around the Canal itself as well as a number of overland flow paths. 

Why did Council undertake 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)

Will the study fix the flooding in my street?

The study is the first step toward managing flooding in the Hawthorne Canal catchment. The next stage of the process is developing a floodplain risk management plan which Council is currently working on. 

FLOOD AFFECTED PROPERTIES

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 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.

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?

Council is not in a position to advise you on this matter. The methods and information used by insurance companies to set their policies varies between individual companies. You will need to speak to your insurer to find out how this will affect your premium. 

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.

FLOODPLAIN RISK MANAGEMENT PLAN

What is a floodplain risk management plan?

A floodplain risk management plans analyses flood behaviour and identifies and prioritises options to help protect people and property through better planning, emergency management and infrastructure works. 

What is a flood model?

Expert consultants create a computer-generated model of the area. The model shows the impact of storm events ranging from heavy rainfall to the most extreme flood. 

This includes:

  • where the water will run
  • how the existing drainage system will cope
  • whether property damage is likely to occur

Council’s consultants, WMA Water, will use the photos, video and information provided by residents to test the model developed and verify its accuracy.

Who is WMA Water?

WMA Water is a specialist engineering consultancy with expertise in floodplain management.

How can I find out if my property or business is in the Hawthorne Canal catchment?

The Hawthorne Canal catchment extends from New Canterbury Road at Lewisham to Dobroyd Point at Iron Cove. It includes parts or all of Petersham, Lewisham, Dulwich Hill, Summer Hill, Ashbury, Ashfield, Haberfield and Leichhardt. 

The canal was originally a natural waterway known as Long Cove Creek. During the 1890s the creek was straightened and given artificial banks to improve sanitary conditions for a growing population. View a map of the catchment

How will the information I provide be used?

Information, photos and videos from residents will be used to test the model developed in the earlier studies and verify its accuracy

What are the options for managing floods?

A range of options for managing floods will be investigated through the development of the plan. There are three main ways of managing flood risk:

  • controls on development

  • helping people at risk (e.g. flood warning systems)

  • changing the behaviour of the flood itself (e.g. upgrade of stormwater systems).

Each option will be evaluated by how much it:

  • reduces flood risk and losses

  • costs to implement

  • benefits or impacts society and the environment.

Council will prioritise flood management options and implement as funding allows. Implementation may need external funding, for example, through state government grants. 

When will Council implement the flood management options identified through the plan?

Options identified through the plan will be prioritised and implemented accordingly. Construction of flood management options is likely to begin from mid-2018. Depending on what the options are, they may need external funding to implement, such as through state government grants.

Will Council prepare floodplain risk management plans for areas outside the Hawthorne Canal?

Yes, Council is progressively completing flood management plans across the local government area. The progress of the flood planning process for each catchment in the local government area is outlined below: