King Tides: An Astronomical Phenomenon 

King Tides: An Astronomical Phenomenon

By Mackensie duPont Crowley, communications specialist

December 2020 king tide in Jamestown
The basics of tides

Tides are the rhythmic rise and fall of water level in response to the gravitational forces of the moon and the sun. Narragansett Bay, like most places around the world, has two high tides and two low tides each day, also known as semi-diurnal tides. As the tide rises, salt water from the ocean enters the Bay. At that time, the current in the Bay travels from south to north (a flood current), and when tide falls, the water movement is north to south (an ebb current).

But what’s a king tide?

You may have heard this non-scientific term used in reference to an especially high tide, probably following a storm. In reality, a very particular set of astronomical circumstances make king tides, the highest high-tide and lowest-low tide of the year, both regular and predictable events.

King tides occur during a perigean spring tide. Let’s break down what that means:

  • Each month, we experience two spring tides that increase tidal ranges, during the new moon and the full moon.
  • The moon’s closest point to the earth during its 28-day elliptical orbit is called its perigee.

    Source: NOAA

In other words, a full or new moon must co-occur when the moon is closest to earth in its elliptical orbit to produce a King Tide. 

The effect of individual king tides may vary considerably; some may barely even be noticed, whereas others may cause coastal erosion, local tidal flooding and disruption to normal daily routines. This is particularly true when a king tide event coincides with a storm, because significant precipitation can impede water drainage, adding to the unusually high water level.

Why observe king tides?

King Tides are useful for us to see what conditions will be in the future. As sea level rises, raising tidal levels higher and further inland, king tides today will occur monthly, and eventually daily. For example, a king tide today is equivalent to a foot of sea level rise in the future. This glimpse of future everyday water levels allows us to plan for future sea level rise, and how tide waters will flood our roads, public parks, and other infrastructure. Observing king tides provides information to help decision-makers evaluate different adaptation techniques.

Help track king tides in Rhode Island
February 2021 king tide in Westerly

Rhode Island Coastal Resources Management Council (RICRMC) is leading the Rhode Island King Tides initiative to document the effect that extreme tide events have on our state’s beaches, coastal waterways, private property, and public infrastructure. Save The Bay is proud to join the URI Coastal Resources Center and the RI SEA Grant to promote CRMC’s MyCoast initiative. Through an easy-to-use app, MyCoast is a portal that collects and analyzes pictures and data relating to coastal events.

You can assist the Rhode Island King Tides initiative on MyCoast by downloading the app and documenting the effects that extreme tide events have on beaches, coastal waterways, private property and public infrastructure in your area. Your submitted photos will contribute to our long-term analysis of coastal vulnerability and planning initiatives with municipalities.

Learn more on the MyCoast website.

Other Resources:

Environmental Protection Agency: King Tides

NOAA High Tide Bulletin