Levees

Levees play an important role in floodplain management. They also have the potential to impact neighbouring properties, the community and the catchment generally.

This is why construction or modification of levees is regulated by state planning laws. The level of assessment required depends on the potential impacts of the levee.

About levees

Levees are raised embankments or earthworks built to protect parts of the floodplain from inundation. They are designed to withstand certain river heights, and will be overtopped by floodwaters which exceed this level.

Levees can provide significant reductions in damage and allow communities to function during long-duration floods, provided the structural integrity of the levee is not compromised. Levees generally have a finite design limit and freeboard (factor of safety) above this.

Levees are almost never designed to exclude the Probable Maximum Flood (PMF), and as such, will be overtopped at some stage. They should be designed to ensure that overtopping floods can enter the protected areas in a manner that reduces the associated consequences.

Levees are only as a good as their weakest link, which is often the lowest crest level. However, they may also fail if they are not adequately maintained. Levees can trap local stormwater, causing damage behind the levee, unless flood gates and pumps are provided.

Levees in Queensland

Levees are constructed on many different scales and for different reasons. In Queensland levees are sometimes built around entire townships by local governments, often with additional funding from the state and federal government, for protection from flood waters. Well known Queensland levee examples are in Goondiwindi, Charleville and Roma.

Small scale levees are commonly built by individual landowners, developers and/or farmers on their properties, to protect particular areas for example, crops from flooding. Sometimes a group of individuals will combine to construct a levee to protect a group of properties.

Regulation – Levees and state planning laws

Levees can play an important role in floodplain management. However levees can have a significant effect on flood behaviour, which may have adverse impacts on neighbouring properties and other areas of the floodplain, if not specifically accounted for. Because of this, construction or modification of levees is now regulated by state planning laws.

As background, prior to the Queensland Floods Commission of Inquiry established following the floods of 2010-2011, Queensland had no consistent policy or regulation to control the construction or modification of levees. The Commission’s Final Report proposed a regulated approach to address the impacts and risks associated with levee development.

The Queensland Government has implemented a statewide regulatory framework to manage the construction or modification of levee banks to ensure levee bank design and construction addresses potential impacts on neighbouring properties, the community and the catchment as a whole.

The level of assessment required depends on the potential impacts of the levee. Before building a levee, it is important to consider whether there are other options available. There may be alternatives such as changing the land use, relocating development to avoid the flood-prone area, raising building heights or making them flood resistant.

Levees in urban or town areas

Some examples of levees constructed for town protection include (local government area in brackets): Bedourie (Diamantina), Charleville (Murweh), Dirranbandi (Balonne), Goondiwindi (Goondiwindi), Mackay (Mackay), Proserpine (Whitsunday), Emerald (Central Highlands), Gatton (Lockyer Valley), Murgon (South Burnett), Millmerran (Toowoomba) and Chinchilla (Western Downs), as listed in the 2013 Regulation of Levee Banks in Queensland: Decision Regulatory Impact Statement.

Private levees in Queensland

Construction or modification of levees is regulated by state planning laws. The level of assessment required depends on the potential impacts of the levee.

Limitations of levees

Levees can provide protection to buildings, land and infrastructure, but only up to a certain size flood event. It is not practical to build levees high enough to keep out all floods. Every levee has a chance of being overtopped by floodwaters and as such, communities should be prepared to evacuate to reduce the exposure to flood risks.

When a levee overtops or fails, the water may rise very quickly in the areas which the levee was protecting. Many have been built by individuals privately and may not be properly maintained. Failure of levees occur regularly and can result in worse flooding in areas directly behind the failed levee that would occur naturally.

Levees can create a number of problems.

• Flood mitigation levees designed to provide protection from water breaking out of rivers and creeks may increase flood heights elsewhere. In some places this may be significant.
• If levees fail or are overtopped, the damage caused by the water’s breakout can be considerable if not specially accounted for in the levee design.
• When individuals or communities protected by a levee assume that the levee will protect against all flood, this may result in development in inappropriate locations.

Land-use planning controls may be needed inside the levee to limit development near any spillway and limit the impacts of local flooding from internal drainage issues to development within the protected area.

Emergency management planning should consider the ability to maintain the community behind the levee during an event. This may depend on the residual risk, the safety of occupying the area, the rate at which the protected area would fill if the levee overtopped, the ability to evacuate when the levee overtops, and the availability of essential community services.

Ongoing community education is required to ensure that the population is aware of the risk of overtopping and associated emergency management plans, and does not lapse into the common belief that a levee provides protection against all floods.

More information

For information, guidelines and applications in relation to Category 1, 2, and 3 levees refer to constructing and modifying levee banks.

Managing the Floodplain: A guide to Best Practice in Flood Risk Management in Australia - the Australian Disaster Resilience Handbook 7 - is a good source of information about flood risk and its consequences on the community and includes information about levees.