Flood Warning Services
The primary function of the Flood Warning Service Program is the provision of an effective flood forecasting and warning service in each Australian State/Territory. This service is provided in cooperation with other government agencies such as State/Territory emergency management agencies, water authorities and local Councils, coordinated through Flood Warning Consultative Committees and established cooperative working arrangements in each State/Territory.
The flooding of rivers following heavy rainfall is the most common form of flooding in Australia. Very high tides are another cause of flooding in coastal areas. Flooding of rivers in inland areas of central and western New South Wales and Queensland, as well as parts of Western Australia, can spread for thousands of square kilometres and may last for weeks or even months. In hilly or mountainous areas of these inland rivers, as well as in rivers draining to the coast, flooding can occur more quickly. As these rivers are steeper, flooding often lasts for only one to two days. Flash flooding usually results from relatively short intense bursts of rainfall, commonly from thunderstorms. This flooding can occur in any part of Australia, but is a particularly serious problem in urban areas where drainage systems may not cope and in very small creeks and streams. Flash floods tend to be quite local and it is difficult to provide effective warning because of their rapid onset.
The Bureau of Meteorology provides a flood warning service for most major rivers in Australia. This service is provided with the cooperation of other government authorities, such as the State Emergency Service (S/TES) in each State/Territory, water agencies and local Councils. The Bureau delivers this service through Flood Warning Centres and Regional Forecasting Centres in Bureau Regional Offices in each State and the Northern Territory.
The Flood Warning Service provides different types of information that depends on the type of flooding and the flood risk. The range of information, which may vary between States and areas within a State, includes:
An Alert, Watch or Advice of possible flooding, if flood producing rain is expected to happen in the near future. The general weather forecasts can also refer to flood producing rain.
A Generalised Flood Warning that flooding is occurring or is expected to occur in a particular region. No information on the severity of flooding or the particular location of the flooding is provided. These types of warnings are issued for areas where no specialised warnings systems have been installed. As part of its Severe Weather Warning Service, the Bureau also provides warnings for severe storm situations that may cause flash flooding. In some areas, the Bureau is working with local councils to install systems to provide improved warnings for flash flood situations.
Warnings of 'Minor', 'Moderate' or 'Major' flooding in areas where the Bureau has installed specialised warning systems. In these areas, the flood warning message will identify the river valley, the locations expected to be flooded, the likely severity of the flooding and when it is likely to occur.
Predictions of the expected height of a river at a town or other important locations along a river, and the time that this height is expected to be reached. This type of warning is normally the most useful in that it allows local emergency authorities and people in the flood threatened area to more precisely determine the area and likely depth of the flooding. This type of warning can only be provided where there are specialised flood warning systems and where flood forecasting models have been developed.
Major Causes of Death or Injury
Most flood-related deaths result when people attempt to drive, walk, swim or play in floodwaters. Depth and current are easily misjudged and floodwater will sweep away and submerge even very large vehicles. Therefore, do not drive or walk into floodwater, especially where a current exists, even if the water appears to be shallow.
Definitions and Terminology
Minor flooding: Causes inconvenience. Low-lying areas next to watercourses are inundated which may require the removal of stock and equipment. Minor roads may be closed and low-level bridges submerged.
Moderate flooding: In addition to the above, the evacuation of some houses may be required. Main traffic routes may be covered. The area of inundation is substantial in rural areas requiring the removal of stock.
Major flooding: In addition to the above, extensive rural areas and/or urban areas are inundated. Properties and towns are likely to be isolated and major traffic routes likely to be closed. Evacuation of people from flood affected areas may be required.
Observed River Height: Depth of water (in metres) at a river height measuring gauge located along the river. In most cases, a zero reading is the lowest water level that is reached during dry conditions. In many tidal areas, as well as a few inland areas, river levels are expressed in metres above mean sea level or Australian Height Datum (AHD).
Peak River Height: Highest river height (in metres) observed during a flood event at the specified site on the river.
Predicted River Height: Height (in metres) to which the river is predicted to rise at the river gauge referred to in the warning. The actual depth of flood water will vary across the floodplain. Knowledge of past flood events, as well as estimates of flood levels from flood studies, are used by local Councils, emergency services and landowners to determine which areas are likely to be flooded from the predicted river height. The accuracy of this prediction will depend on a number of factors, including the type of flood forecasting model and its input data. Predicted river heights are subject to forecasting error and are regularly updated as more information becomes available.
River Height Bulletins: A summary of observed river heights (metres) at selected locations and specified times within river basins. These bulletins are not available in all States.
Interpreting Flood Warnings
In order to get the most benefit from flood warnings people in flood prone areas will need to know what, if any, effect the flood will have on their property and some knowledge of how best to deal with a flood situation. Sources of such information could include;
- Flood Bulletins/Warnings issued by the Bureau and/or the local Council or emergency services which often contain details of areas affected by flooding, road closures and other advice on what the community should do if they are likely to be flooded
- Long term residents who may have experienced a similar flood in the past and remember how it affected them
- Local Councils that have conducted flood studies and have maps of areas that are likely to be flooded by a range of floods
- Information pamphlets - see Further Information section below.
Flood Warnings typically include a statement about both current and expected levels of flooding at key locations in the area covered by the warning, along with a weather forecast and the latest available observations of river height and rainfalls in the area. In the interpretation of warning messages, it is important to note that the predicted height is a river level above a certain datum, and not a depth of floodwater. The Bureau's role is to provide Flood Warnings, some of which contain forecasts of expected river heights. Other agencies (local Councils, S/TES, etc) are responsible for interpreting river levels into depths and areas of inundation. People living in floodprone areas should consult with these agencies to find out what level of warning service is operated for their area.
Accessing Flood Warnings
In each State, Flood Warnings and River Height Bulletins are available via some or all of the following:
- Local Response Organisations: These include the Council, Police, and State Emergency Service in the local area.
- Bureau of Meteorology: Flood Warnings and general information are available directly from the Bureau in each State.
- Radio: Radio stations, particularly local ABC and local commercial stations broadcast Warnings (and Bulletins) soon after issue.
- Telephone Recorded Information Services: Flood Warnings are available in some States on a Bureau of Meteorology recorded message service. Charges apply.
- Internet/World Wide Web Access: The Bureau's home page is www.bom.gov.au
For people who live in flood prone areas, more detailed information on flood preparedness, safety and recovery is available in the free booklet 'What to do Before, During and After a Flood' published by Emergency Management Australia (EMA) and available through the local State or Territory Emergency Service (S/TES) - check the telephone directory for the nearest office. Further information is available from EMA's Internet site: www.ema.gov.au