The unprecedented rain event in Townsville caused the north Queensland city's Ross River Dam to reach more than 200 per cent capacity, prompting authorities to open the spillway gates to their full capacity.

The release caused 1,900 cubic metres of water per second to flow downstream, inundating several Townsville suburbs.

The Ross River Dam is a gated dam and was built in 1971 for flood control and water storage, with a catchment area of 760 square kilometres.

The dam had a $115 million upgrade that was finished in 2007 to install three spillway gates to control the flow downstream and increase its capacity to 233,187 megalitres.

On Monday afternoon the dam was at 229 per cent capacity and slowly falling, down from a peak of 244.8 per cent.

The dam is owned and managed by the Townsville City Council with operational help from SunWater.

How does a gated dam work?

The full supply level of a dam, when it is at 100 per cent capacity, is the approved water storage level of the dam for drinking or irrigation purposes.

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So when the level rises above that in a gated dam, then controlled releases are conducted for flood mitigation and the dam's structural safety.

For un-gated dams, operators have no control over the water that flows over the spillway once the dam's water level surpasses the full supply level.

University of Queensland's School of Civil Engineering Professor, Tom Baldock, said the elevation of the dam and the size of the reservoir could create more storage space.

"The storage capacity is dependent on reservoir geometry, so it's usually given a storage volume and as the level of the dam goes up, the area of the reservoir is increasing rapidly," he said.

"The side slopes [of a reservoir] tend to be quite flat so you don't have to increase the height of the water to get a much bigger volume resulting from the increased surface area of the reservoir.

"That means you can go from 200 to 250 per cent capacity without a very large rise in the water level — it's a non-linear rise in the water level."

Data from the Australian National Committee on Large Dams said the reservoir in the Ross River Dam could hold up to 803,565 megalitres of water in flood mitigation.

The Ross River Dam Emergency Action plan stated the dam's full supply level is when the elevation level (height of the river) reached 38.55 metres and the gates are fully opened when the elevation level reaches 43 metres.

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Will the dam fail?

Bureau of Meteorology senior hydrologist, Chris Leahy, said a dam was not compromised when its capacity was more than 100 per cent.

"The way it works with these dams is that 100 per cent is the force of flow level for the water supply requirements for the dam, and then anything above that is used for flood storage," he said.

"Flood storage will only be held in there for a short period of time — there's certainly no risk to the structural integrity of the dam at those types of levels [when it's over 100 per cent], but you'll only keep the water in there for a short time before it's discharged out."

The Ross River Dam underwent an upgrade that finished in 2007. ( Supplied: Meridian Helicopters )

The Ross River Dam's emergency action plan said it was currently at Alert Level STAND UP-3.

However, if the height of the river (EL, elevation level) goes above 43.6 metres and the flow rate exceeds 2,100 cubic metres per second, that raised the alert level to STAND UP-4.

The final alert level — STAND UP-5 — means dam failures are extremely likely.

There is no rating that triggers dam failure.

However, the emergency action plan states under Stand Up-4, that the vicinity of the dam must be evacuated if the river height reaches 47.5 metres.

Is this a record flood?

The height of the Ross River at Ross River Dam peaked at 42.99 metres on Sunday night at 10:00pm, the Bureau of Meteorology said.

Floodwaters flow fast over Aplin's Weir in Mundingburra near Townsville on February 3. ( Facebook: Donna-marie Audas )

The current flooding smashed the previous record for the dam, which reached a height of 40.73m on March 21, 2012.

It also smashed rainfall records, according to the Bureau of Meteorology 1,134 millimetres had fallen at Townsville Airport in the last 10 days to 9:00am Monday morning — breaking an 80-year record.

Far north, Ingham had also broken its all-time 10-day rainfall record, with 1,197 millimetres recorded.