Understanding flood terminology and types

A river catchment is an area of land where rainwater collects and flows downhill into a river system. It is often bordered by hills and can span multiple local government areas (LGAs).

A flood is a high-water event that overtops natural or artificial banks in rivers, streams, estuaries, lakes, or dams. It can also result from local overland flooding, poor drainage, or elevated sea levels and waves breaching coastal defenses, as defined in the NSW State Flood Plan.

Riverine flooding occurs when heavy rainfall causes rivers and streams to overflow. Unlike flash floods, this type of flooding develops more slowly, usually taking over six hours to rise. It does not include floods caused by storm surges, high sea levels, broken dams, or overland drainage issues.

Flash flooding, often unexpected, is caused by sudden local or nearby heavy rainfall. It typically occurs in small catchments within six hours after rain, overwhelming natural or artificial drainage systems and causing rapid overland flooding and stream rises. The faster the water is moving and the deeper the water, the higher the flood hazard risk.

NSW SES understands that the effects of Climate Change will continue to increase the severity of disasters, including storm and flood disasters, and their impacts on life and property in NSW. It is understood that the changes to natural climate processes, temperatures, and pressure systems will continue to affect the extremity of Australia’s weather systems and ocean hazards. The impact of warmer temperatures and a drier Australian continent will lead to higher severity of hazards, including increased risk of thunderstorms and storm surges, flash flooding and large flood events, and impacts to oceanic climate systems, making rainfall and flooding even harder to predict.

A way of describing flood levels. It measures height above sea level rather than from the riverbed.

A Flood Watch is issued by the Bureau of Meteorology (BOM) 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 refer to the type of flooding experienced in the catchment being highlighted. The BOM issues a Flood Watch to provide early advice on a developing situation that may lead to flooding. A Flood Watch is not a warning of imminent flooding.

A Flood Warning is issued when the Bureau of Meteorology (BOM) 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 in some of the larger river basins. Flood Warnings will generally include specific predictions of the severity of expected flooding.