What Is a King Tide? Definition, Risks, and Climate Change Impact

Flood On Road During Hurricane

Maurice Alcorn / Getty Images

King tide is a non-scientific term for an exceptionally high tide. They’re also sometimes referred to as perigean spring tides. The water level of a king tide will be significantly higher than other high tides throughout the course of a year.

King tides might be an unusual event, but they can be predicted along with every other high and low tide, thanks to年度潮汐表for the U.S. coastline provided by the National Oceanic and Atmospheric Administration (NOAA).

What Are Tides?

要完全理解what a king tide is, it’s important to know how tides in our oceans work in general. Tides are the rise and fall of ocean levels. Out at sea, they’re not very noticeable, but where the ocean and earth meet, the different levels of tides are much more obvious. Most coastal areas have two high and two low tide events over a lunar day (24 hours and 50 minutes). This means high and low tides are a little later each day.

Tides are due to the gravitational pull that both the sun and the moon exert on the Earth. As the moon is closer to the planet, its influence has a stronger effect on tides than the sun. The highest tides occur when the Earth, moon, and sun are all in alignment.

Tidal ranges around the world show a lot of variation. The largest tidal variation in the U.S. is found near Anchorage, Alaska, with atidal range of up to 40 feet.

King Tides and Perigean Spring Tides

According to the Environmental Protection Agency,king tides and perigean spring tidesare effectively different names for the same phenomenon.

The term perigean refers to when the moon is closest to Earth—at itsperigee—and exerts its strongest gravitational pull. This usually occurs every 28 days. When perigee happens at the same time as a new or full moon, then the gravitational pull is greatest, leading to perigean spring tides, or king tides.

In perigean spring tides, the term "spring" refers to the motion, not the season.

How Often Do King Tides Occur?

The number of king tides per year depends on a range of factors including the location, tidal range, and local weather conditions. Most locations will experience king tidesonce or twice a year. The National Oceanic and Atmospheric Administration (NOAA)predicts tides for the U.S. coastline, and king tides are included in their predictions.

The Effects of King Tides

King tides can cause localized tidal flooding, as well as endangering shoreline developments, housing, habitat restoration, and infrastructure.

The effects of king tides are dramatically increased if they occur at the same time as cyclones or storms. This can be seen in the video below, where a coastal storm combines with a king tide to create localized coastal flooding.

On a positive note, the extreme low tides that accompany king tides can also reveal areas of the shoreline that aren't usually exposed during regular tides. Observations of these areas can give extra clues as to the health of the marine organisms living on our shores.

Initiatives likeThe California King Tides Projectencourage members of the public to safely take and then upload their photos of king tide events around the region.

King Tides and Climate Change

Flooding due to high tides, in particular king tides, is already an issue for coastal communities. These effects will only be increased as climate change causessea levels to rise,这意味着国王潮汐inlan将达到更远d. According to the EPA, the unusually high water level of king tides will eventually be theeveryday tidal level.

"Over time, sea level rise is raising the height of tidal systems ... King tides preview how sea level rise will affect coastal places. As time goes by, the water level reached now during a king tide will be the water level reached at high tide on an average day," says theEPA.

Cottages along a sandbar backed by salt marshes. Beresford, New Brunswick, Canada

by Marc Guitard / Getty Images

Chronic flooding has some major impacts for the infrastructure of local communities. King tides can help us identify locations that will be more prone to coastal flooding in the future, helping inform planning to keep coastal communities as safe as possible.

Initiatives likeThe King Tides Projectaim to track the impact of king tides and help coastal communities better understand the impact of climate change on their local environment.

King Tides Around the U.S.

Throughout the U.S. some locations are particularly well known for their king tides. These include Florida, California, and查尔斯顿, South Carolina.

For example, in Charleston the average high tide reaches around5.5 feet. King tides can reach 7 feet and more. This can cause an increase in major flooding events.

Originally written by
Melissa Breyer
Melissa Breyer
Melissa Breyer is Treehugger’s editorial director. She is a sustainability expert and author whose work has been published by the New York Times and National Geographic, among others.
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