FLOODING: What triggers warnings on the Northern Rivers
AS WET weather sets in and flood warnings remain in place for parts of the region, knowing exactly what determines a flood is vital information to know.
The Bureau of Meteorology website explains that flooding is among Australia's most deadly natural disasters, but it's also important for the life cycle of many plants and animals, and for agriculture.
Flood heights that trigger flood warnings on the Northern Rivers
Rivers and creeks each have individual heights that trigger flood warnings.
These are some of those heights in metres according to the Bureau of Meteorology.
Marshalls Ck at Billinudgel: Minor: 2.50 Moderate: 3.00 Major: 3.50
Brunswick River at Mullumbimby: Minor: 2.50 Moderate: 3.50 Major: 4.50
Wilsons R at Lismore (mAHD): Minor: 4.20 Moderate: 7.20 Major: 9.70
Richmond River at Wiangaree: Minor: 11.00 Moderate: 15.50
Richmond River at Kyogle: Minor: 12.00 Moderate: 14.40 Major: 16.00
Richmond River at Casino: Minor: 11.90 Moderate: 14.90 Major: 17.70
Richmond River at Coraki: Minor: 3.40 Moderate: 5.00 Major: 5.70
Lower Richmond River
Richmond River at Bungawalbin: Minor: 3.00 Moderate: 4.50 Major: 5.00
Richmond River at Woodburn: Minor: 3.20 Moderate: 3.70 Major: 4.20
What causes floods and how will you know if there's one on the way?
Flooding occurs when water escapes (or is released) from a watercourse (such as a lake, river or creek), or a reservoir, canal or dam.
Floods in Australia are predominantly caused by heavy rainfall, although extreme tides, storm surge, snow melt or dam break can also cause flooding.
More recently, coastal flooding as a result of sea level rise due to climate change is being considered in planning and land.
How rivers flood
Rivers are watercourses formed over thousands of years. They carry excess water from the land to the lowest point possible, often the sea. There are two main contributors to a river flooding in Australia: catchment (natural drainage area) moisture, and rainfall.
The land is a lot like a kitchen sponge - its capacity to absorb water depends how dry it is to begin with. For a river to flow, more rain needs to fall than can be absorbed by the soil. When the soil can hold no more water it's 'saturated', so water will flow freely across the ground and join streams, creeks and eventually, mighty rivers.
Rivers have a maximum capacity to carry water. If this capacity is exceeded, the river will eventually rise higher than its banks and flow out into areas adjacent to the river.
Many Australian rivers transport rainfall long distances across the landscape. While some rivers flood rapidly and discharge their water close to where the rain fell, other longer rivers may not flood for weeks or even months after the rain has fallen. The Murray River, Australia's longest at over 2,500km, may take months for headwater floods to reach the river mouth in South Australia. At a shorter timescale, river flood peaks can often be reached some days after the rain has fallen.
How are floods predicted
Long before rainfall is recorded on the ground it is the job of specially trained hydrologists to assess the potential for flooding and forecast river height at certain locations at a given time. Hydrologists use three main inputs for assessing flood potential:
1. forecast rainfall from our meteorologists;
2. observations from our vast flood monitoring network. Rainfall and river height data is collected from around 3500 stations across Australia owned and operated by the Bureau and partner organisations, including water management agencies and local councils; and
3. computer modelling from the Australian Water Resources Assessment Landscape model (AWRA-L). This model is run across the entire country every day and provides an early assessment of how wet or dry soils are and how quickly flooding might occur.