Potential Flood Risks on the Gold Coast

Local flooding: An intense burst of rainfall over a short period of time may cause excessive run-off that builds up in a confined area and causes localised flooding. Inundation is expected to last only for a limited period of time until the run-off is able to drain away.

Regional flooding: Continuous heavy rainfall across a number of river catchments is likely to cause inundation across an extensive area. It may take a number of days for these floodwaters to subside.

Riverine flooding: While similar to regional flooding, riverine flooding occurs with excessive rainfall in one river catchment only and inundation is generally confined to areas linked with that river system.

During both riverine and regional floods, areas which normally experience local flooding are also likely to experience higher flood levels compared to those recorded in the main drainage paths such as creeks, canals, lakes and rivers.

All searches undertaken by Council attract a fee. Flood Searches currently cost $69.00 for processing the search application and providing a written report to the customer. The written report may prove valuable when purchasing a property, building a home or when applying for flood insurance. (The fee excludes the cost of collecting the data and producing the computer flood models.)

The Gold Coast Planning Scheme incorporates a set of flood maps which are available online.

A Flood Search system is in place to provide detailed peak flood levels (in AHD - Australian standard unit of elevation) which can then be compared with the resident's information about their property. It is recommended that a licensed surveyor be engaged to accurately determine property and floor levels.

As shown in the diagram, some parts of the property may be below the Designated Flood Level and therefore may be inundated during a 100 year ARI flood event.

AHD refers to Australian Height Datum. This is the standard elevation reference for mapping purposes adopted by the National Mapping Council of Australia. As a general guide, 0.0m AHD is approximately equal to mean sea level.

ARI = Average Recurrence Interval. When talking about floods, this is an indication of how frequently a flood of a particular size is likely to occur on average.

Therefore, a flood the size of a 100 year ARI flood is likely to occur once in 100 years on average, but it has a one per cent chance of occurring in any one year. A flood the size of a 10 year ARI is likely to occur once in 10 years on average, and it has a 10 per cent chance of occurring in any one year.

As this is often referred to as the "100 year flood", it is tempting to think that it will only happen once in 100 years.

A "one in 100 year ARI" refers to the magnitude or size of a flood. It means that a flood of that size or larger is likely to occur only once in 100 years, but it has one chance in 100 (one per cent) of occurring in any year. However, the rainfall events which cause floods are random and there is no guarantee when such a flood will occur, nor that it won't be much larger.

In fact, floods of that size have been known to occur more frequently than once in 100 years. Kempsey in northern NSW experienced two floods of this size in eight months in 1949 and 1950. During the 1890s the Brisbane River experienced three 100 year ARI floods over a period of five years. (Floodplain Management in Australia: Best Practice Principles and Guidelines, CSIRO, 2000)

Floods will happen. There is approximately a 50 per cent chance of experiencing a one in 100 year ARI flood at least once in a 70 year lifespan. (Floodplain Management in Australia: Best Practice Principles and Guidelines, CSIRO, 2000)

No. It is possible that floods of a much greater magnitude could occur.

The one in 100 year ARI flood event is the minimum standard used by Gold Coast City Council in determining the Designated Flood Level for habitable floors. The Designated Flood Level is applied when assessing development applications to ensure that habitable rooms in homes are built to a certain minimum level.

Many other Councils throughout Australia have also adopted the 100 year ARI flood event for the same purpose.

Many factors are considered when designing new roads, including potential regional flooding. Roads may be designed to allow floodwaters to continue to flow out of the river system into the ocean rather than temporarily holding them back. In many instances residential roads may be designed to a height above the one in 100 ARI flood event, others to one in 50 ARI or one in 20 ARI, to ensure evacuation routes remain passable during a regional flood event.

Temporary flood storage areas are areas that hold back some rainfall runoff for discharge later. This has the effect of reducing peak flood levels downstream. As the downstream flood levels subside all floodwaters progressively release and escape the river system.

Flood storage areas can be found naturally in floodplains or they can be constructed. The Merrimac / Carrara Floodplain acts as a natural flood storage area. Council's planning codes act to preserve its storage capacity.

Natural storage areas are often used to benefit communities as sites for a range of recreational activities. Parks, play grounds, sporting fields and golf courses are often designed to provide temporary flood storage.

Apply to Council for a Flood Search. You will receive a written report that will indicate flood levels generated by computer models and historical information if available. Where the report indicates no information available, Council has no information, but it does not necessarily mean that a property is immune from flooding.

Yes. Council engaged CSIRO to undertake a number of investigations into the possible affects of climate change as it relates to the Gold Coast Region.

Yes, but in a controlled way as articulated in the Council’s flood constraint code. The main aim of this control is to ensure that development in a flood plain does not cause any adverse flood impact on any other properties, on the city as a whole and on the Council capacity to exercise its responsibilities with respect to flood emergency management.

No. The spillway at Hinze Dam has no flood or flap gates that can control outflow. The geometry is fixed and the spillway has a central slot.

Flood modelling is a term that is used to describe the process of artificially trying to represent a river or floodplain system to determine the magnitude, extent and depth of flooding, how fast floodwaters rise and the implications for flood damage and emergency planning.

Yes, for the lower reaches of rivers. In the Australia Day 1974 flood, the Broadwater filled to a level of RL 1.65 metres (Australian Height Datum (AHD), or about 1.65 metres above mean tide level. Most of this water was flood water runoff from the rivers and earlier inflow from Moreton Bay.

In 1974 the Broadwater had limited tidal connections to the ocean, but since the construction of the Seaway, floodwaters can now escape to the ocean much faster. If the 1974 rainfall event reoccurred, it is estimated that the Broadwater flood levels would be 0.3 metres to 0.65 metres less than the levels that occurred in 1974.

However, the Seaway also means that the Broadwater is now much more susceptible to storm surge than it was in 1974.

Yes, some insurance companies offer flood insurance either as part of their home and contents policies or as an optional extra. Check your policy or speak with your insurance company to determine whether you are covered and 'exclusions' apply.

The Insurance Council of Australia has produced a brochure 'Flood Insurance: Are You Covered?' that can be downloaded form their website www.ica.com.au.

No. Council has provided its flood maps to Insurance Industry in the past. However, Insurance companies have their own means of determining flood risk for individual properties. Some companies have developed their own flood information while others seek information from the applicant, which may include the results of Flood Search.

Individuals can lodge an application for a Flood Search for a specific property with Council for an administrative fee of $53.00. 

This policy came into effect on 1 September 2003 as a joint initiative of the Departments of Local Government and Planning and Emergency Services. The purpose of the policy is to ensure natural hazards are adequately considered in Council's Planning Scheme. It allows a consistent approach to planning throughout the State. Details can be found at www.emergency.qld.gov.au.

No. Gold Coast City Council has contributed to developing this policy and its current Planning Code for Flood Affected Areas is more onerous in parts than the Planning Policy itself