‘ King tides’ give glimpse of coming higher ocean levels

Flooding likely if coupled with storm surge

So- called king tides, which hit their month- long peak in B. C. beginning this week, offer residents an idea of what higher ocean levels will look like in the coming decades, says the City of Vancouver.

B. C. government estimates suggest the ocean- level rise by 2100 due to climate change will range from 80 centimetres at Nanaimo to 120 centimetres in the Fraser Delta.

King tides, also known as a perigean spring tide, are formed twice a year when the gravitational pull of the sun and moon reinforce each other.

Usual water levels at high tide are 3.4 metres to 4.3 metres in the Vancouver area, but a king tide can reach five metres.

Vancouver City water and sewers director Brian Crowe said the king tide by itself will not create flooding, but combined with storm surges could pose a problem.

A storm surge is primarily caused by high winds pushing on the ocean’s surface.

The combination of peak high tides and a storm surge has caused temporary flooding in the past in one of Vancouver’s low- lying areas at Kent Avenue South near Marine Drive, where water backed up in the drainage system from the Fraser River, said Crowe.

The road is being redeveloped at a higher elevation, he said.

“The ( king) tides themselves are pretty benign, but they can be interesting to see. People who watch that kind of stuff — maybe they walk the seawall all the time — they’ll notice higher than normal water levels,” said Crowe.

And they provide some insight into what can be expected in the future as sea levels rise and storm surges become more severe, both an effect of climate change, he noted.

“If, over future decades, sea levels rise so ( that) more often it’s at the level we see in these annual peaks, and more storms coincide with the highest natural tidal level, you can get more temporary flooding.”

It’s why the city is developing a climate change adaptation strategy and a strategic plan for coastal flood and erosion risk, said Crowe.

University of B. C. ocean sciences professor Susan Allen agreed that the peak tides that take place twice a year are a good chance to see what rising ocean levels will look like in the next few decades.

She also agreed that the risk of significant effects from a peak tide occur when there is a storm surge.

“Suddenly, that becomes a problem,” said Allen. “And if you go somewhere where the land is subsiding, you add that effect. Then you can have erosion problems.”

She noted that king tide is not a name used in any science textbooks.

Normally the highest tides are called spring tides, which does not refer to the season, but comes from the meaning of jump or rise, said Allen.