(Also known as detention basin, retarding basin, storage basin)

What is a flood detention basin: A flood detention basin typically comprises of walls or embankments that provide an area for water storage, an outlet to control water flow and a spillway to pass water flows that exceed the basins design capacity. They can be dry or permanently wet and can be located online (i.e. along streams or watercourse) or located offline (located away from streams and watercourses). Like dams they operate by temporarily storing flood water which reduce downstream flood heights but can increase the time that land is inundated.

Improves community access and recreational use:

A flood detention basin may improve community access and recreational use. Why? A flood detention basin particularly in urban areas can covert private land to public land for varying uses. A dry detention basin for example may be utilised as sporting fields, skateboard parks, golf courses and general parkland. A wet detention basin for example can create nice picnic areas and recreational fishing opportunities. However, in some instances detention basin may turn currently utilised recreation areas into non-usable basins due to extreme safety risks during flooding.

Does not disadvantage individual members of the community:

A flood detention basin may cause equality issues and can impact individual members of the community. Why? Flood detention basins generally occupy large areas of land to provide sufficient water storage. As a result particularly in urban catchments, large areas of land need to be purchased causing the relocation and disruption of local community members.

Provides safety to the community during flooding:

Flood detention basins are generally neutral in providing safety to the community during flooding. Why? The flood detention basins “free” storage can be filled by flood waters and later released. This in effect reduces the flood water level downstream but increases the time that land is inundated up to the design height. However, these basins can quickly fill causing life threatening situations particularly if utilised for recreational purposes.  Please note: Flood detention basins need to be designed to accommodate different flood events by having varying outlet structures. It is not uncommon for detention basins to be designed to the 1% annual exceedence probability flood event and as a result they provide little benefit when larger flood events occur. Larger events that cause overtopping or failure can cause increased risk to life due to large volumes of water moving extremely fast.

Raises community awareness and understanding of the local flood risk:

Flood detention basins may improve community awareness and understanding of the local flood risk. Why? Flood detention basins are usually seen as parkland or sporting fields, but with signage can prove to be valuable flood reminders. However, they may also create a false sense of security for community members, as detention basins are usually only constructed to store minor to moderate flood events.

Does not threaten local plants and animals and their habitat:

Flood detention basins can both have negative and positive environmental and ecological impacts. Why? Inline flood detention basins can alter the ability for streams to regulate their flows causing erosion and scour, they can also cause habitat disturbance and alter migration patterns. On the other hand wet detention basins may create habitat for local plants and animals and multi level outlet controls can accommodate naturally fluctuating flow patterns.

Does not cause water quality issues:

A flood detention basin may cause water quality impacts. Why? Flood detention basins are typically designed in small rainfall events to remove pollutants and sediment. However, during large flood events these pollutants and sediments can be carried downstream in large plumes causing major water quality issues.

Initial Costs (i.e design/construction) require minimal council expenditure:

Flood detention basins have major initial costs. Why? Although flood detention basins are relatively simple to construct, feasibility studies and detailed design studies are vital to insure their placement and function does not cause adverse impacts both upstream and downstream. Detention basins typically costs around $10 per square metre for smaller basins and $5 per square metre for larger basins if Council owns the land (Brown et. al. 1997). However, the cost and availability of: materials; machinery; labour/ project management; design and feasibility studies, easements and/or the acquisition of land and legislative costs can skew this typical cost per square metre significantly.

Requires minimal ongoing council expenditure after implementation:

Flood detention basins have moderate ongoing maintenance costs. Why? As flood detention basin remain unused for long periods of time and are required to perform to a predetermined level at short notice, it is vital that ongoing maintenance is undertaken. Maintenance includes:  1) Inspecting for rabbit burrows, bank scour, cracking, build up of debris, slump or failure; 2) Repairing any faults that which would affect the capacity, and consequently the function of the flood detention basin. 3) Mowing and general maintenance of the flood detention basin and associated drainage systems.

Reduces flood damages to the community:

Flood detention basins have the ability to reduce flood related annual average damages for small areas of flood prone land (NSW Government 2005). Why? As detailed previously flood detention basins can reduce the height of floodwater downstream which can reduce flood damages particularly for more frequent flood events. Note: Detention basins are usually small and are designed to overtop in major floods. Failure can also occur which can cause catastrophic downstream damages if appropriate planning decisions have not been made.

Does not cause negative flood impacts to other areas (both upstream and downstream):

Flood detention basins have the potential to cause adverse flood impacts to other areas. Why? As discussed previously, detention basins are designed to be either inline or offline. Inline basins may cause water to build up and flood land upstream while offline basins may result in flooding of areas that had previously not experienced inundation.  As a result feasibility and detailed design studies for a full range of flood events (from regular to extremely rare floods) are required to insure their placement and function does not cause adverse impacts both upstream or to other areas.

Brown, W. and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic Region. Prepared for: Chesapeake Research Consortium. Edgewater, MD. Center for Watershed Protection. Ellicott City, MD