What is a detention basin?

A holding pond that temporarily stores some of the stormwater run-off from a catchment during big storms. They’re located near waterways such as creeks, rivers, streams and lakes.

What is a catchment?

An area where water is collected by the natural landscape, usually surrounded by mountains or hills. In a catchment, rainwater run-off eventually flows to a creek, river, dam, lake, ocean or into underground waterways like springs. The proposed McMahon Street Detention Basin will be located in the Cabbage Tree Creek catchment.

How does Council know what works will help reduce the impacts of flooding?

Each year, Council spends millions of dollars on stormwater and floodplain management. Our team of flood experts prepare flood studies and floodplain risk management studies that help us understand the flood behaviour for a particular catchment and see if there are any ways of reducing flooding risk in an area. 

Floodplain risk management studies  include a plan of potential solutions aimed at reducing the existing and future flood risk. Examples of these solutions include: 

  • emergency response plans based on detailed understanding of flood behaviour
  • building new structures that collect and carry stormwater into drains or creeks, such as detention basins and swales, or improving existing ones to better manage stormwater and floods 
  • land zoning that says what can and can’t be built on flood-prone land
  • voluntary purchase of houses built in high flood risk areas

What did the Fairy and Cabbage Tree Creek catchment flood study find?

The Study was prepared in 2010 and the community helped by sharing information about their experiences of flooding in this area. We discussed the issues and potential solutions to the flooding problems. The McMahon St basin was identified as an option that should be further explored by Council, as it was supported by the community and recommended by the consultant as a way of reducing flood impacts in this catchment.

Why do we need a detention basin in this area?

This detention basin will help protect houses, roads and land from flooding by storing water that flows from  the steep land of the escarpment to the west, and releasing it slowly onto flatter land to the east.

What information do you want people to share with you?

It’ll be a while before we can build the basin, as there’s still lots of work to do before our plans are final. At this stage, we’d like to  hear from you about what you value about this area and how you might use this space, e.g. for walking. We want to make sure that our design considers how local residents enjoy this land. We’ll keep you updated as we progress with our plans for the next stage of this project, detailed design. 

What will happen to the trees if you build a detention basin here?

The existing mature trees in this area provide benefits such as shade, cooling, clean air, homes for native wildlife and making this area beautiful. The design we’re working on includes keeping these trees.

Will the basin be safe?

It’s being designed to prevent dam break during extreme storms by releasing water the basin can’t hold over a spillway, at a point that has the least impact on surrounding properties. During heavy rain, detention basins can quickly fill with fast-flowing water, so it’s safest to stay away from them when it’s raining. There’ll be warning signs at the basin to let people know this.

Who will maintain the basin?

The land around detention basins looks like parkland when it’s dry and people often use it for recreation. The main function of a detention basin however, is to protect from flooding, so we maintain these in a different way to how we maintain our parks. It’s normal for a detention basin to fill up with silt and sediment, particularly in cases where the basin is located close to the foot of the escarpment. Sometimes vegetation such as reeds will grow in them, like you might see in a pond. Once built, the basin will be regularly inspected by Council staff for upkeep and to make sure it’s still working well. 

What is run-off?

Rainwater that has not been absorbed by the soil or evaporated (turned into water vapour). It travels along the surface of the ground into places that collect water. It occurs when there is more water than land can soak up. 

What are peak flows?

Flooding is the process of water moving over land within a certain period of time. During every flood, there is a time known as the peak of run-off. This is the point in a flood where the maximum amount of water is flowing over a given piece of land. Basins extend the peak flow out over a longer period of time to reduce the worst flood impacts.

Why doesn’t Council clear out creeks?

Council is responsible for maintaining watercourses (including creeks, overflow paths or drainage pipes) on Council-owned land and has a maintenance program for this. The majority of watercourses throughout the city of Wollongong are on private property and the maintenance of these is the responsibility of the land owner. In these cases, Council is unable to perform any work on the watercourse. If you require advice on maintenance of watercourses, please contact our Customer Service team by phoning (02) 4227 7111 or emailing council@wollongong.nsw.gov.au

In many cases, vegetation in watercourses can help reduce flood risk by slowing down the flow of stormwater, and preventing it from arriving all at once in areas where the creek bed flattens out or where several creeks join up. This effect is very important because it helps prevent serious dangers like properties or roads being flooded by fast-flowing water in a short amount of time. 

As well as helping prevent the most serious flood hazards, appropriate creek vegetation adds beauty to our city, and provides vital habitat. It helps to manage water quality by filtering pollutants, regulates temperatures in urban areas and reduces erosion that can generate debris, or damage land, buildings, or roads.

We are continuing to work in the creeks on public land across our city, to reduce infestation by problematic creek vegetation such as weeds, and replace it with stable, low-maintenance vegetation. This enhances the many functions of  our urban creeks without increasing flood risk.

What other work have you done to reduce the risk from flooding in this area?

There’s another detention basin in Foothills Road, Mount Ousley:

Foothills Road Detention Basin, Mount Ousley

This basin is similar to what we’re proposing for this site. There are also some debris control structures to reduce the risk of blockage during storm events. We can’t prevent flooding, so we use planning controls to make sure that new developments are built in a way that minimises flood risk. Council also buys some high-risk properties under the Voluntary Purchase Scheme.

What can I do around my yard to help keep watercourses clean?

  • Be careful not to dispose of grass clippings and other garden cuttings in or near watercourses and remove any obstructions that may cause blockages or divert flood waters. 
  • Be aware of any drainage easements or overflow paths that affect your property. Seek Council approval before altering your driveway or footpath levels, as this may cause water to flow off the road and down your driveway.
  • Take care when planting trees near drainage pipes. Certain species with aggressive root systems e.g. Jacaranda, Poplar, Willow, Fig, Camphor Laurel and Rubber Trees can cause pipes to become blocked or cracked. 
  • Don’t 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. 
  • Don’t fill low-lying areas of your yard without seeking Council approval, as this may cause water to pond and increase flooding potential on your and your neighbours’ properties. 
  • Keep drainage inlets on your property clear of any rubbish or blockages. Remember, large paved areas will increase runoff, so you may need extra drainage. 

How can I join the conversation?

There are a number of ways to ask questions or share your feedback with us: