Meteo-tsunami unleashed cargo ship

A tsunami-like wave ripped a cargo ship from its moorings before it struck the Fremantle rail bridge last month, according to a leading WA ocean scientist.

Two adjacently berthed ship at North Quay were pushed into Fremantle harbour when their stern lines broke during a storm on August 17.

The cargo ship AAL Fremantle collided with a bunker barge before hitting the rail bridge.

Chari Pattiaratchi, a tsunami expert who heads coastal oceanography at the University of WA’s school of environmental engineering, said wind was initially blamed for the failure of the moorings but further analysis of water levels and weather data showed a “meteo-tsunami” was the culprit.

The phenomenon occurs when a weather event, such as a thunderstorm, causes an abrupt change in atmospheric pressure – in contrast to conventional tsunamis, which are triggered by seismic activity.

“On August 17 a travelling pressure jump of two hectopascals generated wind gusts of up to 102km/h,” Professor Pattiaratchi said.

“In turn, water levels rose sharply in Fremantle’s inner harbour, where the AAL Fremantle was berthed.

“It snapped its moorings and was dragged upstream by currents of .06m per second, hitting a barge before crunching into the bridge.”

Train services between Fremantle and North Fremantle were suspended for two weeks while the bridge was repaired.

Fremantle Ports harbourmaster Allan Gray said it appeared “strong wave action” caused the ships to break free.

The subsequent failure of a bollard holding the AAL Fremantle’s stern lines is still being investigated.

Capt. Gray said Fremantle Ports was looking at engineering solutions to minimise vessel movements during meteo-tsunamis, which can occur several times per year.