NAROOMA visitor Jenny Edwards, here in early May, was surprised by the exceedingly high tides.
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The local tide charts were forecasting high tides of a moderate 1.3 metres, but Jenny observed ‘King’ tides of around 2 metres.
“With a clear sunny day, barometric pressure more or less medium, only light winds and those offshore NW, I could not see any reason for the big variation,” Jenny said.
Jenny contacted the Narooma News to find out if we knew of any reason for the variation.
We in turn contacted Narooma ‘tide-watcher’ Greg Watts for any insights. Greg too had noticed the higher than normal tides.
Greg was unsure. He thought it may have been a lunar perigian tide effect, or perhaps higher tides than tide chart predictions due to sea level rises from global warming, as reported by ABC News at the time.
The riddle was solved at Eurobodalla Council’s information night on their Coastal Zone Management Plan, held in Narooma last Thursday.
The consultant preparing the plan, Pam Dean-Jones, presented information on a Coastal Trapped Wave event that had recently been observed running up the East Coast.
Greg Watts explained it this way:
“Imagine you are sitting in a bath and you push water away from you. The bath water, constrained by the walls of the bath, forms a wave that travels to the end of the bath before dissipating.
“For the Coastal Trapped Wave, the sides of the ‘bath’ are the coastline and the north-south flowing East Australian Current (EAC), a kilometre or two offshore. The wave is created down in Bass Strait.
“Southerly changes passing through the Strait create surges of ocean water, continuing 2-3 days after the front has passed.
“The surge flow is caught between the EAC and the continent and just like in the bath, is forced up the coast.
“After the forcing ceases the wave continues to travel forward, the surface moving faster than water dragging on the continental shelf below. The wave can reach as far up the coast as Queensland before it finally dissipates.
“The wave created by this surge has a height of only 40 cm but amazingly a wavelength of 2000 kilometres - or 3-6 days between wave ‘peaks’.
“In effect it is a tidal surge of 3-6 days duration. Other phenonema such as tides and cyclonic weather conditions - the East Coast Lows that we have recently been experiencing - can exacerbate the rise in tides caused by the Coastal Trapped Wave.”
According to Greg, the “Coastal Trapped Wave” phenomenon we experienced in Narooma in early May increased tide levels by 40cm.
“We were lucky high tides at the time were only 1.3 metres,” he said.
“If we were experiencing king tides or storm surges as well, many of the sea walls and estuary infrastructure such as jetties and boat-sheds around Wagonga Inlet estuary would have been over-topped and low lying areas inundated by the ocean.”
According to the CSIRO, Coastal Trapped Waves are an uncommon but regular occurrence along the east coast of Australia.
