What are mean tides? This sea level measurement helps California protect the coast
It is a situation of some gravity as the moon drags oceans around the world.
Tides vary in height as the moon’s orbit gets closer or further away because orbits are a squashed circle or ellipse.
In November, December and January, the sun, moon and earth align to produce the biggest tide swings of the year.
They are popularly known as king tides — but if you want to sound erudite, call them perigean spring tides.
If you missed seeing king tides in November, they are set to return Dec. 13, 14 and 15, and Jan. 11 and 12, 2021.
The king tides give us a preview of of rising sea level impacts.
Global mean sea level has risen by 8 inches since 1880 due to glacial melt and thermal expansion of warmer oceans. A third of that rise has been in the last 25 years.
Tides were a subject of interest to the Telegram-Tribune after California voters passed Proposition 20, the Coastline Initiative, in November 1972.
After a century of Californians filling estuaries and bays for the purposes of development, the California Coastal Commission was created and a period of coastal park acquisition began.
In 1976, the state legislature anchored the initiative in place with the California Coastal Act.
Since the area being regulated was defined in part by the tidal zone, Jim Hayes wrote this story on Jan. 13, 1973, edited here for length:
Mean tides — and how they got that way
The tide — whose ebb and flow once were secrets shared only by clam diggers, boaters, fishermen and lovers on moonlit nights — are fast becoming of crucial concern to just about everyone along the Central Coast.
Politicians and property owners, lawyers and lobbyists, enforcement officers and editorial writers, developers and folks who damn development: all talk glibly (and alas confusingly) in such tidal terms as “mean high water” and “mean low water.”
What does it all mean?
Whatever happened to good old easy-to-fathom sea level?
What did some 4,312,133 Californian’s envision as a “protected zone” when they cast their ballots in favor of Proposition 20 on the November ballot?
The oft-quoted official explanation in the state-distributed proposition booklet states, of the Coastline Initiative:
“…establishes permit area within coastal zone as the area between the seaward limits of state jurisdiction and 1,000 yards landward from the mean high tide line, subject to specified exceptions.”
Where, by the seaweed beard of Neptune, is the mean high tide line?
To look for it, we’ll have to learn just a smattering of tidal theory.
The tides, as the late Rachel L. Carson pointed out in “The Sea Around Us,” are “a response of the mobile waters of the ocean to the pull of the moon and the more distant sun.”
“Anyone who has lived near tidewater knows that the moon, far more than the sun, controls the tides. He has noticed that, just as the moon rises later each day by 50 minutes, on the average, than the day before, so in most places, the time of high tide is correspondingly later each day. And as the moon waxes and wanes in its monthly cycle, so the height of the tide varies. Twice each month, when the moon is a mere thread of silver in the sky, and again when it is full, we have the strongest tidal movements — the highest flood tides and the lowest ebb tides of the lunar month. These are called the spring tides. …
“And twice each month, at the quarters of the moon, we have the moderate tidal movements called the neap tides.”
Along the Central Coast, the tides are said to be of the “mixed type.” This means that there generally are tow high tides and two low tides in each 24 hours, but the morning tides are of different heights than the afternoon tides.
On the Atlantic coast, there are two equal highs and lows; on the Gulf of Mexico here is only one high tide and one low tide daily.
What’s called “higher high water” is the peak tide of any day.
On the Pacific Coast, mean lower low water is the most important reference plane for work involving the ocean and its bays and estuaries. This is the level from which all tides are measured. A “plus” tide is a specified number of feet above mean lower low water.
A “minus” tide, the kind beloved by clammers, is below mean lower low water. The large scale charts used by seamen also use this reference plane: Water depths are indicated in fathoms or feet above mean lower low water.
To find a reference point, we’ll have to look for what the U.S. Coast and Geodetic Survey called Bench Mark 4. It was in 1916 that surveyors for that agency of the Department of Commerce planted a bronze marker in a circular piece of concrete at the south east corner of the old Neptune Auto Court at the intersection of what now is Main and Dunes streets.
Basing their calculations on actual measurements of 220 high waters and 220 low waters observed between March and June 1919, the surveyors figured that the bronze disc — about the size of a tomato can lid — was exactly 81.08 feet above mean lower low water.
Now we can look for our old friend sea level — the datum from which all elevations on the land are figured: A table published by Coast and Geodetic Survey in 1951 indicates that sea level in Morro Bay is 2.01 above mean lower low water, or precisely 79.07 feet below the bench mark at the old Neptune Auto Court.
The ink isn’t dry on their appointments yet, they haven’t sat down for an official meeting and already the commissioners are being asked: Where are you going to draw the line?
We can suggest a beginning point: pick a time just after the full moon when the tides are at the spring. Then set up your transit with its plumb line pointing to that bronze disc near the office building of a weather beaten old auto court named for the sea god Neptune. Point your telescope of your instrument seaward—toward where the unconcerned waters are pulled to and fro by the distant moon and sun.
Look. And before you begin to measure, ponder the mystery of it all.
