Flooding is among Australia's most deadly natural disasters—but it's also important for agriculture and the life cycle of many plants and animals.  

 

So, what are floods and what causes them? 

 

Put simply, a flood is when a normally dry area is covered in water. There are many ways this can occur and many types of flooding.  

 

Riverine flooding is perhaps the most common in Australia. Rivers are formed over thousands of years. They carry excess water from the land to the lowest point possible, often the sea. 

 

The two main contributors to riverine flooding are heavy rainfall and the lands capacity to absorb water. The land is a lot like your kitchen sponge, there is a limit to how much water it can soak up. Once the land is saturated, water flows across the land into our river systems. 

 

However, river systems have a maximum capacity to carry water. If this capacity is exceeded, the water will eventually rise higher than its banks and flow out into low-lying areas adjacent to streams and rivers. 

 

How quickly a river responds to rainfall and how long that flooding lasts can vary significantly. In flat, inland regions, floods may spread thousands of kilometres and last for weeks to even months. In the mountain and coastal regions, flooding is often less extensive and of shorter duration, but with higher flow velocities. 

 

Likewise, flash flooding happens very quickly and for that reason is the most dangerous type of flooding. They can occur in any part of Australia but can be a particularly serious problem in urban areas where the drainage system may not cope. 

 

Flash floods are of short duration within six hours of intense bursts of rain. The rain falls so quickly that it can't soak into the ground or drain away through normal channels. It can cause roads to become rivers and turn normally safe areas to raging torrents within minutes! 

 

In some coastal communities, storm surge can be a dangerous cause of flooding. It can occur when an intense low pressure system or a Tropical Cyclone, crosses the coast, raising sea levels that can swamp low lying areas up to a kilometre inland. 

 

The effects of a storm surge can be further intensified by exceptionally high tides, often referred to as a 'king tide'. These can be caused by the local topography and the positions of the Earth and moon relative to the Sun. 

 

A less common cause of flooding in Australia such as 'Snowmelt', when rapidly warming conditions cause snow to melt quickly into the river system.  

 

While we often associate flooding with damage, disaster and loss, flooding is a natural process that can bring benefits. 

 

Floods fill billabongs and wetlands, replenishing water bodies and contributing to the life cycle of many plants and animals. Floodwater also carries valuable nutrients and minerals important for agriculture. 

 

However, flooding is the second most deadly natural disaster in Australia. And all types of flooding can be dangerous. 

 

To find out more about floods watch our 'Understanding flood classifications' video and visit the Bureau website. 

 

FLOOD CATEGORIES 

 

If you live in a flood prone area, it’s important to understand when you need to be prepared and how intense a flood is likely to be. 

 

FLOOD CATEGORIES 

 

Riverine floods typically occur 6 or more hours after heavy rainfall. In Australia, we have 3 riverine flood categories: minor, moderate and major. 

 

 

Minor flooding refers to inundation of low-lying areas next to watercourses, which may require the removal of stock and equipment. 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. 

 

 

For moderate flooding the area of inundation is more substantial. Main traffic routes may be covered and some buildings may be affected above floor level. Evacuation of flood affected areas and removal of stock may be required. 

 

 

Finally, major flooding is very dangerous and may result in the extensive inundation of rural and urban areas. Properties and towns are likely to be isolated and major traffic and rail routes likely to be closed. Evacuation of people from flood affected areas may be required and utility services may be impacted. 

 

FLOOD WARNINGS 

 

The Bureau produces warnings and advice to local communities who are experiencing, or are at risk of experiencing riverine flooding.  

 

 

Flood Watches provide early advice of a developing situation that may lead to flooding. 

 

While Flood Warnings are issued when floods are likely to occur or are already occurring. 

 

Sometimes short bursts of heavy rainfall can lead to rapid onset flooding, known as flash flooding. When a weather pattern is identified as likely to result in heavy rain, Bureau meteorologists will issue a severe weather warning for heavy rain that may lead to flash flooding.  

 

Current flood watches and warnings, as well as severe weather warningscan be found on our warnings page or the BOM weather app.  

 

You can also access rainfall and river level data for around 3,500 locations across Australia. 

OUTRO 

 

Sadly, most fatalites around flooding are avoidable. 15cm of water on a road is enough to make your car float, so remember the best way to stay safe is to stay out of flood water and follow advice from local emergency services.