Explainer: king tides
02 January 2018
They're natural, predictable, and ancient, but so-called 'king' tides hit the headlines time and again. What causes these tides, how will you know one is coming and what happens to our coastlines when they coincide with severe weather?
What are king tides?
While not a scientific term, 'king tide' is widely used to describe an exceptionally high tide. These tides are a natural and predictable part of the tidal cycle. The time of year they occur varies by location and between years.
Image: Large and powerful waves combined with king tides at Narrabeen, NSW, causing coastal erosion in February 2020. Credit: Nick Moir Photo.
What impact can king tides have?
King tides have very noticeable effects where the ocean meets the land at beaches, estuaries, harbours, and other coastal locations. With water levels much higher than normal, flooding and erosion can occur in low-lying coastal areas.
The tides can also be disruptive or dangerous for coastal activities such as boating and rock fishing. Water may cover rocks and other hazards that are usually visible, or make waterways underneath bridges or other pathways temporarily inaccessible. On the other hand, the extremely low tides that accompany king tides can expose hazards and render river entrances inaccessible. The currents during the mid-tide (in between high and low tide) will also be the strongest of the year, posing danger to boats crossing river mouths, or to swimmers from rip currents.
King tides can pose an even bigger problem if they coincide with hazardous weather and wave conditions such as tropical cyclones, East Coast Lows, or storms. The combined effect of large swells and a king tide can elevate the sea level above the expected height, causing substantial erosion and damage to coastal property and infrastructure.
In early June 2016, large waves and storm surge from an East Coast Low combined with king (or close to king) tides, resulting in substantial and widespread coastal damage in New South Wales. There was significant coastal erosion on Sydney’s Northern Beaches at Collaroy and the Central Coast at Wamberal. Many other coastal areas also experienced coastal erosion or inundation.
While king tides have always occurred and are not a result of sea-level rise, their impact and reach will be greater as sea levels continue to rise.
Image: Coastal erosion and damage to properties at Collaroy Beach in June 2016. Credit: UNSW Water Research Laboratory
What causes tides and king tides?
Tides are the alternating rise and fall of surface water levels across oceans, beaches, estuaries, and harbours. They are caused primarily by the gravitational pull of the moon, which is strong enough to displace the oceans towards the moon. A similar displacement away from the moon occurs on the other side of the earth where the centrifugal force exceeds the moon’s gravitational pull.
The same forces are at play as the earth revolves around the sun (although to a lesser extent because the sun is so far away). The sun's gravitational force is greatest when the earth is closest to the sun (perihelion; in early January) and least when the sun is furthest from earth (aphelion; in early July).
The tidal effect varies with the alignment of the sun and moon. It increases when the sun and moon, and their gravitational forces, line up (i.e. during a new moon or full moon). These more extreme tides are known as ‘spring tides’ (as in ‘springing forth’) and they occur every 15 days. Spring high tides will cause the surface water level to be a little higher than average, and spring low tides to be a little lower than average.
When the sun and moon are at right angles to each other (i.e. during a first- or last-quarter moon) the tides will be more moderate. The sun's mass allows it to exert enough gravitational force on the oceans that it negates some of the effects of the moon's pull. These more moderate tides are called ‘neap' tides and they occur seven days after a spring tide. High tides will be a little lower and low tides a little higher than average.
There are other factors involved, including the moon's declination (angular height above the equator), local geography and topography, and water depth. All of these factors combine to create a complex tidal system across the world's oceans.
Image: The gravitational forces of the moon and sun cause 'spring' (more extreme) and 'neap' (more moderate) tides.
King tides are typically the highest spring tide of the year. These exceptional tides happen because the distance between the moon and earth varies due to the moon's elliptical orbit. The gravitational force is greatest when the moon is closest to the earth (perigee) and least when it is furthest from the earth (apogee). King tides often occur when the earth, moon and sun are aligned at perigee and perihelion.
How can I find out about king tides at my location?
Check tide predictions for tide heights and times at your location. The timing and expected height of king tides are very predictable although their actual height will depend on the local weather and wave conditions on the day.
Knowing when and where king tides occur can help you be better prepared. Homes and businesses in low-lying areas can take precautions (such as using sandbags) to prevent flooding. Boaters and other coastal users should consider changing their activity to a safer time in the tidal cycle.
Image: Tide predictions web page showing tide heights and times for a selected location.
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