Find out more about the Overland Flood Studies

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Monday, 16 July 2018 3:53PM

Burwood Council recently completed Draft Overland Flood Studies for the Exile Bay/St Lukes/William Street Catchment, the Powells Creek Catchment, the Dobroyd Canal Catchment and the Cooks River Catchment. The purpose of these flood studies is to identify potential flood prone locations and to quantify the degree of flood risk that these areas might be exposed to. The properties identified in the studies could potentially be impacted by either Overland or Mainstream flood water.

At the Ordinary Council Meeting of 26 June 2018, Council resolved to publicly exhibit the Draft Overland Flood Studies for all of Councils Stormwater Catchments. The Public exhibition of the Draft Overland Flood Studies is your opportunity to comment on the technical aspects of the flood study.

You can view the flood studies on our Public Exhibition page - Click here to view.

What is the Broader Floodplain?

Generally the Broader Floodplain is any land susceptible to flooding by a Probable Maximum Flood event. The broader floodplain detailed in the Study includes areas where there is overland flow of stormwater traveling downstream as well as areas normally associated with flooding where the water level in a creek or river rises. This overland flow may come from the roadway or from nearby higher properties.

What is a Flood?

A flood is a relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam and/or local overland flooding associated with major drainage before entering a water course.

What is Local Overland Flow?

Local Overland Flow is the inundation by local runoff rather than overbank discharge from a stream, river, estuary, lake or dam.

What is Mainstream Flooding?

Mainstream flooding is the inundation of normally dry land occurring when water overflows the natural or artificial banks of a stream, river, estuary, lake or dam.

What is Major Drainage?

Councils have discretion in determining whether urban drainage problems are associated with major drainage or local drainage. For the purposes of the Floodplain Development manual major drainage involves:

  • The floodplains of original watercourses (which may now be piped), or sloping areas where overland flows develop along alternative paths once system capacity is exceeded and/or
  • Water depths generally in excess of 3 metres (in the major system design storm as defined in the current version of Australian Rainfall and Runoff). These conditions may result in danger to personal safety and property damage to both premises and vehicles and/or
  • Major overland flow paths through developed areas outside of defined drainage reserves and/or
  • The potential to affect a number of buildings along the major flow path.

What is Local Drainage?

Local Drainage is the smaller scale problems in urban areas. They are outside the definition of major drainage.

Why does flooding occur?

Flooding is a natural process. It happens periodically as a result of heavy rainfall in a catchment when the water level in a creek or river rises. Specifically, it occurs when the runoff generated from the storm exceeds the capacity of the drainage system. The effects of flooding in the Burwood LGA are magnified by the proximity of urban development to natural and modified creeks and channels. Floodwaters overflow the banks of creeks and channels inundating the floodplain which may include roads, parks, residential, and commercial properties.

What is Council doing to manage the flooding problem in Burwood LGA?

Burwood Council’s responsibility is to manage lands subject to flooding on two levels. Firstly, in accordance with the NSW Government’s Flood Prone Land Policy, Council is responsible for formulating and implementing Floodplain Risk Management Plans. These plans involve catchment-wide studies that identify significant flooding issues and floodplain management studies that identify potential flood mitigation solutions and strategies. Flood mitigation options could typically involve floodplain modification, property modification and emergency response measures.

Secondly, Council has a responsibility to ensure future developments are compatible with flood hazards and do not create flooding problems in other areas. As such Council may enforce planning (development) controls such as minimum heights of floor levels above ground level and prohibiting specific land uses in areas prone to flooding.

Why is Council conducting this study?

Flooding costs local government and property owners and occupiers a lot of money and imposes substantial intangible costs on the community, such as social and emotional costs. The main objectives of Floodplain Management are: ‘to reduce the impact of flooding and flood liability on individual owners and occupiers of flood prone property, and to reduce private and public losses resulting from floods, utilising ecological positive methods wherever possible’.

How are floodplains managed in NSW?

In NSW, Local government has the primary responsibility for controlling the development of flood prone land, but the NSW Government, through the Office of Environment and Heritage (OEH) and the State Emergency Service (SES), also has an important role to play in managing the flood risk across the State.

The NSW Government’s Flood Prone Land Policy is directed at providing solutions to existing flooding problems in developed areas and ensuring that future developments will not create flooding problems in other areas. The State Government subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist councils with their floodplain management responsibilities.

The NSW Government provides technical and financial support to local councils to develop Floodplain Risk Management Plans which consist of the following stages:

1. Flood Study
2. Floodplain Risk Management Study
3. Floodplain Risk Management Plan
4. Implementation of the Plan

Read more about these stages below.

What is a Flood Study?

A Flood Study is the first stage in the development of a Floodplain Risk Management Plan for a particular catchment. The Flood Study is a comprehensive technical investigation of flood behaviour for a particular catchment. The Study shows the distribution, extent, levels and velocity of floodwaters across sections of the floodplain for different flood events including the 1 in 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 the 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 behaviour and hazard and includes the social, economic, ecological and cultural issues in addition to an assessment of costs and benefits of all options.

What is a Floodplain Risk Management Plan?

A Floodplain Risk Management Plan formalises and prioritises mitigation works and other floodplain management measures that are recommended in the Floodplain Risk Management Study. The Plan is formally adopted and implemented by Council.

What is the 100 year flood?

A 100 year flood is the flood that will occur or be exceeded on average once every 100 years. It has a probability of 1% of occurring in any given year. If your area has had a 100 year flood, it is wrong 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 more than one 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 Probable Maximum Flood?

The Probable Maximum Flood (PMF) is the largest flood that could theoretically occur within a particular catchment, and is a very rare and unlikely event. Despite this, a number of historical floods in Australia have approached the scale of a PMF. Every property potentially affected by a PMF will have some flood risk, even if it is very small. Under State Government changes in 2001, councils must now consider all flood risks, even these potentially small ones, when managing floodplains.

What is the Annual exceedance probability (AEP)

The Annual exceedance probability (AEP) is the chance of a flood of given or larger size occurring in and one year, usually expressed as a percentage. e.g. if a peak flood discharge of 500m3/s has and AEP of 5%, it means that there is a 5% chance (that is one-in-20 chance) of a 500m3/s or larger events occurring in any one year.

Why were houses built in areas where it floods?

Flood problems often occur in many places around Sydney. While parks and canals were built where most of the water naturally flowed, and the lowest lying land was kept clear of development, it was not understood back then just how much water could flow through the catchment in the rarer storms. It is only in the last decade that this has been properly appreciated and the rainfall data and computer technology has been available to better understand and calculate it.

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

Lack of evidence of historical flooding does not necessarily mean the 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 from flooding even if you have not experienced it personally. Flooding can also occur on the sides of hills if the shape of the landscape concentrates overland flows on their way to the drainage network.

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.

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.

What if I want to carry our building works on my property?

When you make major modifications to your building (e.g. knock down and rebuild) you will have to make the property comply with any new requirements for building or development that may now apply to your property. Generally this means that rebuilt houses and house extensions take into account the flood risk in their design and thus reduce the risk of damage to the property as a result of flooding.

Will my property be rezoned?

There is no intention to rezone any land within Burwood LGA as a result of a flood study. The current zoning that applies to your land will remain unchanged.

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

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). 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 rain events. In a large flood event (100 year flood event), it is likely that stormwater pits and pipes will be full. The Draft Flood Study Report shows where this additional water is likely to go.

What solutions are available?

There are many different ways in which flood risks can be managed. Which ones are the most effective, practical or acceptable to the community will depend on the nature of flooding, the local topography, existing development and future aspirations for a locality.

The floodplain risk management process has yet to begin in the Burwood LGA.

Will Council upgrade existing drainage infrastructure?

The floodplain management process allows Council to identify deficiencies in its drainage system and investigate potential upgrades of this system. If upgrades are recommended as part of the floodplain process, Council will prioritise these projects and plan for them to be undertaken as soon as funds are available.

What can I do to minimise flooding?

Flooding is a significant issue which affects the entire community, and actions by individuals may have serious consequences on others within the catchment. To play your part:

  • Be aware if your property is affected by flooding or contains a potential overflow path;
  • Be aware if a drainage easement affects your property
  • Be conscious of flow paths around your dwelling and keep them clear - be careful not to dispose of grass clippings and other garden cuttings in or near the watercourse and remove any obstructions that may cause blockages
  • New fences for properties subject to flood related planning controls need to comply with Council requirements
  • Do not construct raised gardens or plant significant trees or vegetation within flow paths - Certain species such as Jacaranda, Poplar, Willow, Fig, Camphor Laurel, rubber Trees and other types with aggressive root systems can cause pipelines to become blocked or cracked
  • Do not perform any significant work (earthworks, creek bank protection, bridges, piping etc) to the watercourse through your property without first consulting Council
  • Do not lay any pipes, construct a bridge or divert a watercourse without first consulting Council - unapproved work can increase flooding for both you and your neighbours
  • Do not fill in low lying areas of your yard without seeking Council approval, otherwise this may cause water to pond and increase flooding potential on both your property and your neighbour’s.

With your help, we can minimise flood risks and damages.

What can I do to be flood prepared?

Visit the NSW State Emergency Service website at www.ses.nsw.gov.au for further information regarding flood preparation measures, in particular the NSW Flood Safe Guide.

Is there a map of the areas identified as potentially flood prone?

Yes, please click on the link below to view the map.

View map.