Whanganui's beaches could be at their most dangerous this summer, a coastal scientist says.

Rips in Whanganui last for roughly five months, and grow in size and strength through that time, Dr Roger Shand, director of Coastal Systems, told the Chronicle this week.

He was speaking after the death of 17-year-old Jordan Marshall, who drowned on Christmas Day after getting caught in a rip off Castlecliff Beach.

Dr Shand - who holds a science degree, majoring in mathematics and physical geography, a teaching diploma, a post-graduate diploma and a doctorate in coastal processes on New Zealand's west coast beach systems - worked on a report focusing on the Whanganui coast.


The report, published in 2004, studied images of the coast taken fortnightly over four years.

"We've done quite a lot of research on the beach on this particular coast," he said.

The government-funded study was sent to water safety organisations but was never acted on, something Dr Shand said was "a bit of a pity".

He said the findings could be used to predict when beaches were at their most dangerous, and organisations could apply for extra safety funding for those times and adjust their management of those areas.

"The beach at the moment is in one of those high-risk states.

"Controlling the whole thing is the fact that the sand bars form on the lower beach near the shore ... and then they migrate seaward."

A new bar would form as the other moved out to sea, but in the period just before a new bar formed, the coast "looks like a big, wide, flat beach".

"That's a very safe situation from a bathing point of view," Dr Shand said.

"Rips start after that ... basically, they get bigger and bigger and bigger. That's where they're heading at the moment."

He said the Whanganui coast was now "building toward its most extreme form of rips" and this would last for another month or two.

"We've got a period coming up, and it's going to be substantial.

"Surf Life Saving could make a specific case that the risks will be here this summer, we're going to need more funding, we're going to need more lifeguards."

The rips around Whanganui were at different phases, but the rip at Castlecliff which claimed the life of Jordan Marshall was in a more advanced stage.

He said this rip phasing occurred at roughly 25 per cent of New Zealand beaches, mainly west coast ones.

Dr Shand said people often thought a rip was a good spot to swim in because it looked calm.

Rips were also stronger when the waves were smaller. When the waves were large they would break over the rip and help push back towards the shore.

If someone was not a confident swimmer and couldn't recognise a rip, "frankly, I think you shouldn't go much more than your ankles".

"These things, they surge the water. If you get in to your waist at times the water's level, but it actually moves up to your chest - you're off your feet very quickly."

-Dr Shand's report - Rip-associated bathing hazards on beaches characterised by net offshore bar migration - can be read or downloaded at bit.ly/1mki47W