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Brisbane — along with three quarters of Queensland — is under water today after the swollen Brisbane River peaked this morning at 4.65 metres, leaving thousands of houses and businesses flooded by the deluge. The floods are not expected to recede for some time. It all depends on further rain and tidal conditions — and the arrival of the king tide, the biggest tide of the season.

Some news outlets are reporting that a king tide has already occurred, while others are saying one is imminent. Twitter was also crackling with tweets this morning about a king tide, however it is likely that they had confused the river’s peak level with a king tide.

So how much of this was the result of excessive run-off and how much was due to a king tide? And have we had the king tide yet or is it still to come?

Crikey went to Tony Weber, a visiting fellow for the Integrated Catchment Assessment and Management Centre at Australian National University, for answers.

What is a king tide?

“The reason a king tide occurs is because the moon and the sun are both aligned at the same time, that’s what causes the greater than usual tide.

“Basically what happens is that’s it’s just the cycle of the normal tidal cycle coinciding with wherever the moon placement is. There can be other issues like prevailing winds but it’s typically the normal cycle of rise and fall associated with wherever the moon is in relation to a particular location.

“A king tide is just that which is higher than the normal tide level — what we call a mean high water spring. It’s usually the highest tide in the cycle.”

How often do they occur?

“It depends. It’s typically in the order of once every three to six months.”

According to some reports, there is a king tide expected in Brisbane soon, how high is the water likely to rise?

“Today’s tide level us not what we would typically call a king tide. The king tide is actually towards the end of the month, although the tides occurring at the moment in Queensland are higher than normal high tides.”

So when is the king tide expected in Brisbane?

According to Maritime Safety Queensland, the summer king tide is expected to be 2.65 metres and will occur on January 21 at 10:37 am.

Is that due in any part to the excessive run-off occurring at the moment?

“No, it’s just the normal tidal cycle. The run-off has nothing to do with the king tide. It’s the run-off on top of the tide that is the problem.”

How long do they typically last?

“The king tide level is probably only in the order of 30 minutes to an hour and that depends on where you are and how far upstream you are.”

What effect does the king tide have on a river already swollen by excessive run-off?

“The key issue is that it causes the run off that is coming down the river to slow down and that causes levels to rise because water becoming behind that water starts to build up. The challenge with the king tide when you have excessive run off is that it exacerbates the water level even further.”

Is there anything else to worry about?

“King tides are not the only problem that we have to worry about. We are very fortunate that we don’t have any storm surge with this flood. If this excessive run-off was the result of a cyclone then it would be even more serious. If we had a king tide, a storm surge and a river flood at the same time then that would be the worst case situation.”

Peter Fray

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