Why does Council study flooding?

Under the NSW Government’s Flood Prone Land Policy, NSW Councils have responsibility to identify and manage flood affected areas to reduce the risk to people and property as a consequence of flooding.


What is the difference between a Flood Study and a Flood Risk Management Study and Plan?

A Flood Study provides technical information on the likelihood and characteristics of flooding within a catchment. A Floodplain Risk Management Study and Plan considers methods to reduce the risk associated with flooding on people and property.

How can I find out if my property or business is in the study area?

The study area includes the suburbs of Ashfield, Haberfield, Summer Hill and parts of Petersham, Lewisham, Dulwich Hill and Croydon.

View a map of the Dobroyd Canal catchment

View a map of the Hawthorne Canal catchment

How are flood risks managed?

Emergency management measures seek to help people at risk during a flood. This includes sharing information with the State Emergency Service, flood warning systems and identifying evacuation routes and locations of emergency refuge centres.

Development controls consider the way in which land is used, and any specific requirements for the buildings that are built upon it. They are typically addressed in the Local Environmental Plans and Development Control Plans. Development controls do not prevent building on flood affected land, but act to ensure that any new works are sensitive to the impact and risk of flooding on the property.

Infrastructure modification measures change the behaviour of flood waters through targeted public infrastructure works. This includes stormwater upgrades, additional underground pipelines and introducing detention basins and levees.

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.

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. 

How were the flood mitigation options in the draft plan identified?

The flood mitigations options were developed by WMA Water in consultation with Council staff, State Emergency Services, and through community consultation in early 2017. The options were identified by: 

  • Determining locations where flooding occurs in the 1% AEP event using the Dobroyd Canal  Flood Study (WMA Water, 2014) and the Hawthorne Canal Flood Study (WMA Water, 2015)
  • Consulting with the community to determine key areas of concern and identify options
  • Developing a preliminary list of flood mitigation options for each of these locations; 
  • Reviewing all proposed options and creating a short list by identifying those likely to be most effective and feasible to implement; and
  • Modelling shortlisted options to determine if the option would result in a beneficial reduction to flooding

How have the flood mitigation options been assessed and prioritised?

All of the options were assessed for their economic, social and environmental benefits. The economic benefit is determined by comparing the potential financial saving that the option would provide in a flood event with the cost to implement it. The outcome of this was combined with expected social and environmental benefits to provide an overall ranking to compare all options against each other. The outcomes of this multi-criteria assessment can be found in section 11 of the report.

Who is WMA Water?

WMA Water is a professional Civil Engineering and Water Services consultancy that has been appointed by Council to help prepare the floodplain risk management study and plan.

Why are the areas of Leichhardt draining to Hawthorne Canal not considered as part of this Study?

The areas of Leichhardt that drain to Hawthorne Canal were previously considered as part of the Leichhardt Floodplain Risk Management Study and Plan, which analysed and reviewed flood mitigation options for the whole of the former Leichhardt Council Local Government Area.

The draft plan shows a new pipeline within my property – what does this mean?

The draft plan only shows new pipelines within private properties where there is already an existing pipeline. Enlarging or replacing existing pipelines is only shown where it is considered to provide a significant reduction in flooding. The draft plan is a concept so the route of any new pipeline is indicative only. The exact route would be determined as part of a detailed analysis.  In some cases it may be possible to undertake work when a property owner is developing the land.

The draft plan shows a detention basin in a nearby park – what is a detention basin?

Detention basins are areas where stormwater is temporarily stored during large storm events. The water is then slowly released at a controlled rate so that any potential flooding of downstream areas is mitigated. Detention basins are usually located in public parks and are constructed by forming low mounding around the edges to force the water to pond temporarily within the park. This won’t have any negative impact to people using the park and its facilities.  


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

The draft Hawthorne Canal and Dobroyd Canal Flood Risk Management Plan does not recommend a specific works plan for implementation. Instead the Plan recommends a series of structural and property modification measures to be considered as part of Council’s forward planning for future capital works or otherwise undertaken when the opportunity arises through redevelopment, park upgrades, and pipeline replacement. Depending on the size and complexity of the options, external funding through state government grants or coordination with other government agencies may be required.