Understanding the King of Tides

A young man watched curiously as I snapped photos of the inundated marsh bridge. Unless one wanted to remove his shoes and socks, roll up long pants, and get wet while possibly stepping on unseen hazards, the route from Barfield Park across the creek to Simmons Cemetery was impassable. A lengthy run had brought him to the south side of the marshy waterway and now, trying to return home, his route was partially under water.  

 

He looked at the water level and asked two great questions. “Does it get this high very often?” and “Will it go down soon or should I start walking the long way around?”  

 

The answers were “Not very often, but predictably,” and “I would start walking.”  

 

“Nature Notes” is not an opinion column, so, leaving the heated topic of climate change alone for now, let’s wade into the age-old workings of gravity, heavenly bodies and their orbits and Charleston’s “King Tides.”

 

Growing up on the Isle of Palms, I had more friends who fished, hunted, shrimped, etc., than I had who did not. And all of those activities were inextricably linked to the tides, both to their times and to their magnitudes. We kept close track of the times of high and low tide and were always mindful of when the next abnormally high tides would occur. Oystering or flounder gigging depends on tides and winds being just right.

 

There are many resources, printed and online, where monthly “tide tables” for most locations can be found. The time and height, in feet above mean low water level, of each day’s high and low tides are known long in advance. They are a function of the earth, moon and sun, their elliptical orbits and their gravitational effects on each other.  

 

There are unpredictable factors that will change expected tide levels, too. In our area, these are high winds and low pressure associated with tropical storms or hurricanes or steady east or northeast winds along the coast. Either of these can help push and hold water inshore, possibly increasing already higher-than-normal water levels. The onshore winds played a part in some of our highest tides this September and October.

 

Picture the earth’s daily rotation (see the graphic on this page) as the moon follows its monthly course around our planet. The gravitational pull of the moon creates two bulges of water, one nearest to it and one on the opposite side of the planet. Left to its own devices, that gravity creates two full cycles, or two high tides and two low tides, per day. These are Charleston’s “semi-diurnal” tides. In other areas, like along Florida’s Gulf Coast, geographical constrictions can change the cycle dramatically.  

 

While we have two cycles of six feet each day, Destin, Florida, has one (diurnal) cycle of only one foot. But the moon’s path around the earth is elliptical, meaning the moon’s distance from earth changes throughout the month. So, therefore, does the amount of gravitational pull on our planet and so does the height of the associated tide. When the moon is closest to us, at perigee, and when the sun, moon and earth are aligned, adding the sun’s effect, too, we have a new moon or a full moon, strong gravitational forces

and perigean spring tides. (Oddly, “spring” refers to the springing forth of the water, not the season. These tides happen throughout the year.)  In Charleston, they have long been referred to as “King Tides.”

 

We should all learn about how the tide cycles affect the Lowcountry. It is a requirement if one spends any amount of time on the water. The Revolutionary War’s Battle of Sullivan’s Island was lost, in part, due to tides as the British troops could not cross Breach Inlet and the British fleet ran aground on hidden sandbars near Charleston Harbor.  

 

Even for those who don’t fancy aquatic pursuits, it can be useful knowledge.  

 

NOAA has set general flooding guidelines for Charleston of seven feet for minor flooding, seven and one-half feet for moderate flooding and eight feet for major flooding. They don’t happen every month, but seven-foot tide predictions are not that rare. Throw in a little easterly wind and the U.S. 17 “Crosstown” could close, or there might be no beach to lie on when you reach the Isle of Palms. These tidal changes can happen pretty quickly, too. It’s even possible that you might get stuck on the wrong side of a bridge in your very own neighborhood.