How were the flood mitigation options identified?

The flood mitigations options were identified by Cardno in consultation with Council staff and the State Emergency Services. The options were identified by:

  • Determining locations where flooding occurs most significantly or frequently;
  • Developing a preliminary list of flood mitigation options for each of these locations; 
  • Creating a short list of options by identifying those likely to be most effective and feasible to implement; and
  • Modelling shortlisted options to determine which are the best.

Of the 70 options identified at the preliminary stage, 28 options have been shortlisted and modelled. 

How will the flood mitigation options be assessed and prioritised?

Council and Cardno have undertaken a preliminary assessment of all options based on the criteria below.  

The preliminary performance of the options against economic, social and environmental metrics can be found on each of Option Plan. Each option is rated as Good, Moderate or Poor. These rankings are relative to each other for comparison purposes only.

Options that are 'Good' in all criteria will perform best, while options that are 'Poor' in all criteria will perform worst. Options which may be ranked 'Poor' in one area may still perform well over all. 

What are a ‘1 in 2 year’ and ‘1 in 100 year’ flood?

A 1 in 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.

A 1 in 2 year flood is a flood that will occur on average once every two years. 

What about Planning and Emergency Management options?

Planning and Emergency Management options will be exhibited later in the year as part of the Draft Marrickville Valley Flood Risk Management Study and 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. 

Will Council prepare floodplain risk management plans for areas outside the Marrickville Valley?

Yes, Council is progressively completing flood management plans across the local government area. The progress of the flood planning process in each catchment area is detailed on Council’s website.

What is a flood model?

Expert consultants use a software program to create a computer-generated model of the area. Storm events ranging from heavy rainfall to the most extreme flood are then modelled to show where the water will run, how the existing drainage system will cope and whether property damage is likely to occur. Council’s consultants Cardno will use the photo and video evidence as well as stories from residents to test the model developed in the earlier study and verify its accuracy.

Who is Cardno?

Cardno is a professional infrastructure and environmental services company that have been appointed by Council to help prepare the floodplain risk management plan.

What does Council already know about flooding in the Marrickville Valley catchment?

In 2010 Council examined the nature and extent of flooding in the Marrickville Valley for a range of storm events for the Marrickville Valley Flood Study adopted by Council in 2013.The study identified a number of areas which are likely to experience flooding, and in some cases widespread inundation, in a significant storm event. These include the Marrickville Industrial Area and many residential streets. You can view a map showing the extent of flooding in a 1 in 100 year flood event in the Marrickville Valley here.

How can I find out if my property or business is in the Marrickville Valley catchment?

Marrickville Valley includes parts or all of Petersham, Stanmore, Enmore, Newtown, St Peters, Tempe, Marrickville and Dulwich Hill. The valley floor was originally a freshwater wetland known as Gumbramorra Swamp that drained in a southerly direction towards the Cooks River. The catchment now drains into the Cooks River via four outfalls; the Eastern, Central and Western Channels, and the Malakoff Street Tunnel. View a map of the catchment.

How will the information I provide be used?

Stories from residents along with photo and video evidence will be used to test the model developed in an earlier study  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 primarily three ways of managing flood risk to reduce losses from flooding:

  • 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.

    Through this evaluation a list of prioritised options will be recommended for implementation in the catchment. Options may need external funding to implement, such as 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 on a priority basis. Construction of flood management options is likely to begin from mid-2019. Depending on what the options are, they may need external funding to implement, such as through state government grants.