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.

Floods

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.

Flood Warning Products

The Bureau of Meteorology provides flood forecasting and warning services for most major rivers in Australia. These services are provided with the cooperation of other government authorities, such as the State/Territory Emergency Services (S/TES), water agencies and local councils. The Bureau delivers this service through Flood Warning Centres in Bureau Regional Offices in each State and the Northern Territory.

Flood Watch

What is a Flood Watch?

The Bureau issues a Flood Watch to provide early advice of a developing situation that may lead to flooding. A Flood Watch is not a warning of imminent flooding.

A Flood Watch provides information about a developing weather situation including forecast rainfall totals, catchments at risk of flooding, and indicative severity where required. The product also provides links to weather warnings, other Bureau flood-related products, and contact details and information of relevant emergency services.

Although there is uncertainty attached to a Flood Watch, its early dissemination can help individuals and communities to be better prepared should flooding eventuate. A Flood Watch may discuss possible snowmelt, local flooding or tidal impacts but a Flood Watch will not be issued solely on the basis of these phenomena.

When is a Flood Watch issued?

A Flood Watch is generally issued up to four days in advance of the expected onset of flooding. A Flood Watch can be issued before, during and after the rainfall has occurred, depending on the level of maturity of the flood warning systems and services, and flood impact information made available from the local emergency services or state agency.

Flood Watches are updated at least daily and finalised once all areas are covered by flood warnings or the risk of flooding has passed.

What action should I take if a Flood Watch is issued for my area?

If a Flood Watch is issued for your area we encourage you to look out for future Flood Watch updates and heed Flood Warnings if these are issued, and to follow the advice of the local emergency services. If flooding develops, consider making preparations to move livestock, family and possessions to higher ground.

Refer to the Bureau’s website for the latest rainfall and river level heights, as well as the latest weather forecast and warning information and, if possible, bookmark these pages for quick access.

What geographic areas are Flood Watches issued for?

The Flood Watch service covers all parts of Australia including areas without flood forecasting systems and data networks. These Flood Watch areas are based on riverine catchments but in some parts of Australia without well-defined rivers and streams flooding is predominantly overland flow making it difficult to determine the best boundaries to use.

The areas referred to in Flood Watches, as shown below, have been defined and reviewed in partnership with key agencies in each state and territory to ensure they are locally relevant.

State / Territory New South Wales Northern Territory Queensland South Australia Tasmania Victoria Western Australia

What is the difference between a Flood Watch and a Flood Warning?

A Flood Watch is issued when forecast rainfall information suggests that local and/or riverine flooding is possible across the Flood Watch area. A Flood Watch may cover a large area due to uncertainty associated with the location and amount of forecast rainfall. A flood watch may also make reference to the type of flooding that may be experienced in the catchment being highlighted. The types of flooding that may be referred to include:

Local flooding – For areas without a well-defined river, describes situations where intense rainfall is expected to cause high run-off volumes in small catchments or localised areas with minimal impact on main streams. The areas outlined in a Flood Watch may not be currently covered by a formal flood warning service.

Local and Riverine flooding - For areas with a well-defined river, describes situations where intense rainfall is expected to cause high run-off volumes in small catchments or localised areas with significant impact on the water levels in main rivers and streams.

The areas outlined in a Flood Watch may not be currently covered by a formal flood warning service.

A Flood Warning is issued when the Bureau is more certain that flooding is expected, often when rainfall has started to fall. Flood Warnings are more targeted and are issued for specific catchments or even sub-catchments in some of the larger river basins. Flood Warnings will generally include specific predictions of the severity of expected flooding.

There will be occasions when a Flood Warning is issued without it being preceded by a Flood Watch, largely due to the complexities of forecasting rainfall accurately. Weather models are excellent at identifying broader scale weather patterns but are not always able to represent the smaller scale features that can cause heavy rainfall, particularly in tropical areas. For this reason there will be times when heavy rainfall leading to flooding occurs but forecast models were not able to accurately identify this beforehand.

Flood Warning

Flood Warnings are issued by the Bureau to advise that flooding is occurring or expected to occur in a geographical area based on defined criteria. Flood Warnings may include either qualitative or quantitative predictions or may include a statement about future flooding that is more generalised. The type of prediction provided depends on the quality of real-time rainfall and river level data, the capability of rainfall and hydrological forecast models and the level of service required.

A quantitative or qualitative flood warning of Minor, Moderate or Major flooding is provided in areas where the Bureau has specialised warning systems. They provide advanced warning about the locations along river valleys where flooding is expected, the likely class of flooding and when it is likely to occur. Predictions of expected water levels and the timing of flood peaks are provided at key forecast locations.

The Bureau also provides generalised flood warnings when there is not enough data to make specific predictions or in the developing stages of a flood. They typically rely on forecast rainfall and knowledge of historical flood response. Generalised warnings contain statements advising that flooding is expected in particular river valleys but do not provide information about flood class nor precise locations.

As part of its Severe Weather Warning Service, the Bureau also provides warnings for severe weather that may cause flash flooding. State emergency services or local authorities may provide flash flood warnings in some locations.

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

Flood Classifications

The Bureau uses a three tiered classification scheme that defines flooding as minor, moderate or major at key river height stations. Each classification is defined by the water level that causes certain impacts upstream and downstream of the station. These levels have been determined based on standard descriptions of flood effects (see below), historical data and relevant local information. The classifications are revised from time to time by the Bureau’s partner agencies and affected communities.

Minor flooding

Causes inconvenience. Low-lying areas next to water courses are inundated. Minor roads may be closed and low-level bridges submerged. In urban areas inundation may affect some backyards and buildings below the floor level as well as bicycle and pedestrian paths. In rural areas removal of stock and equipment may be required.

Moderate flooding

In addition to the above, the area of inundation is more substantial. Main traffic routes may be affected. Some buildings may be affected above the floor level. Evacuation of flood affected areas may be required. In rural areas removal of stock is required.

Major flooding

In addition to the above, extensive rural areas and/or urban areas are inundated. Many buildings may be affected above the floor level. Properties and towns are likely to be isolated and major rail and traffic routes closed. Evacuation of flood affected areas may be required. Utility services may be impacted.

Reporting or alert level

Water level rises to this level may cause the isolation of stock in very low-lying areas. Typically below the minor flood class level, this is the level at which a river alert is issued (where available).

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 these predictions 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 during a flood event.

River Height Bulletins

A summary of observed river heights (metres) at selected locations and specified times within river basins.

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, Watches 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.

These include the Council, Police, and State Emergency Service in the local area. Bureau of Meteorology: Flood Warnings, Flood Watches and general information are available directly from the Bureau of Meteorology, including: On the web at: www.bom.gov.au/australia/warnings Through the Telephone weather warnings service . Flood Warnings and Flood Watches in most States are available on the Bureau of Meteorology’s recorded message service. Charges apply.

Flood Warnings, Flood Watches and general information are available directly from the Bureau of Meteorology, including: Radio: Radio stations, particularly local ABC and local commercial stations broadcast flood warning information as part of their new bulletins, or whenever practicable.

Further Information

The Bureau can be contacted using the feedback form or by calling your regional office.

More information on Australian rainfall and river conditions can be found at : http://www.bom.gov.au/australia/flood/

More information about the Bureau’s flood warning service and general flood preparedness can be found at: www.bom.gov.au/australia/flood/otherlinks.shtml

Further information or flood preparedness, safety and recovery for people who live in flood prone areas can be obtained from Emergency Services in each State and the Northern Territory.