Understanding flood terminology and types

A river catchment is the land area where rainwater gathers and flows into a river. Natural features, such as hills or ridges, help direct the water into that specific river system and the catchment may cover parts of several local government areas.

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 happens when heavy rainfall causes rivers and streams to rise and overflow their banks. In places where rivers are also affected by the tide, like in Lismore, high sea levels, storm surges or tides can make these floods worse. Unlike flash floods, riverine floods usually take longer than six hours to develop, but they may still be influenced by tidal conditions as tidal influences prevent waterways from running out to sea.

Flash flooding happens when heavy rain makes water levels rise quickly, overwhelming drains, creeks, or small rivers up to 6-8 hours from heavy rainfall. Flash flooding is often associated with overland flooding (sudden flooding in urban areas) but it's important to understand that flash flooding can happen in rural areas, particularly low-lying areas, small culverts or bridges. Understanding your local area and when a bridge or road may be cut off goes a long way. If you are not familiar to the area, have a chat to your neighbours or call council for advice.

Climate change is affecting weather patterns, leading to heavier rainfall and more extreme flooding in some areas. Warmer temperatures mean more moisture in the air, increasing the risk of intense storms. We also know that cyclones are decreasing in frequency but are shifting further south. We are particularly susceptible to coral sea cyclones, east coast lows and atmospheric rivers. 

Australia’s natural climate cycles influence rainfall and flood risk. Long periods of drought can make soil hard and water-repellent (hydrophobic), causing rain to run off quickly instead of soaking in, which can make flooding worse. 

Rising sea levels are also increasing the risk of storm surges, which occur when strong winds from cyclones and severe storms push ocean water toward the shore, causing sudden coastal flooding. Low air pressure in storms can also make sea levels temporarily rise even higher. 

Advances in weather forecasting and flood modelling are improving predictions, but climate change is shifting flood patterns, making some areas more vulnerable to severe events than in the past. 

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