Living on a sandbar

Last week’s storm here on Cape Cod knocked a few trees down, peeled back some shingles on the roof of the sail loft, and put a few people in the dark. It also messed up the beaches, pulling a house into the water on Nantucket, and causing at least three significant breaches, or blow-outs, in spits from Nauset (see the post below), to Nantucket’s Smith Point, to Martha’s Vineyard, where once again Chappaquiddick is an honest island.

Part of the fun of living in a world defined by sand and surf is the occasional remodeling that nature performs under the cover of a hurricane or winter gale. The islands are especially sculpted by the wind and waves, with houses dropping off of bluffs and washing out into the Atlantic, entire sand spits vanishing, and new sandbars emerging to become islands like Tuckernuck and Muskeget, perhaps some of the most unstable, undependable land in America. What one storms piles up, another washes away, and Cape Codders come out of their homes following a “good one” looking forward to a beach walk along the shore to see what was lost and what was gained.

When I was in college I was fortunate enough to take a geology course called Beach Processes and Sedimentation — a deep dive into coastal geomechanics which taught me about things like berms and littoral drifts. I was over my head half the time, especially when modeling fluid dynamics, but it is a fond memory.

My favorite piece of trivia from that course was the discovery of gastroliths at the base of Gay Head on the Vineyard, fossilized stones carried there in the stomachs of seals.

But I digress.

My major project for that class was a look at the history of the Popponesset Spit here in Cotuit and the evolution of Sampson’s Island and Dead Neck, the former pair of islands united into one at the head of Cotuit Bay. I wish I had a copy of that paper, researching it was a bear and involved a lot of time in the map collection of the Sterling Memorial Library. I was able to find this material on the NOAA historical collection, doing the research in a couple hours where it took me a couple weeks in 1979.

There’s a few things to note. Sand moves in a predictable path along a beach — this is the “littoral drift” — sand doesn’t come into a beach perpindicularly, washing straight back out, but it moves sideways, drawing from one source to deposit to another. Hence, if you look at a map of Nantucket, there is an impossibly narrow spit of land — Great Point — sticking out like an appendage. And on Cape Cod, a similar, but bigger structure exists in Provincetown. Hanging south of Chatham is Monomoy Island.

Here’s good old Wikipedia with a diagram:

This slow motion in one direction leads to some interesting beach formations and relations between neighbors. If you are fortunate enough to have the cash to live on the water (or stupid enough), you also have the cash to try to pull a King Canute and tell the tide not to eat away at your very expensive shorefront. This leads to otherwise rational masters of the universe writing impossibly large checks to coastal engineering firms for the construction of sea walls and the wonderfully named man-made structure called a “groin.”

A groin is a little jetty made out of piled up boulders that stupid beachfront owners built after the turn of the 19th century to hold the sand in place, to back it up and hold it.

Here is an aerial photograph of the phenomenon, best observed on the New Jersey shore, called the “Jetty State” by some who like to fish for striped bass from those same rock piles.

See how the sand builds up at the top of each segment of beach but gradually wastes away at the bottom? This is messing with nature’s mojo, and once one greedy person puts in a jetty, everyone “downstream” of the flow of sand will need to do the same.

Here’s what has happened in Cotuit over the years, I think this is as good a picture of what happens when mankind starts trying to stop the sea from doing what it does best.

Here is a closeup of a nautical chart from 1857 when my Great-great grandfather Thomas Chatfield was alive.

There’s a few things to point out. Where there are numbers there is ocean — those mark depths in fathoms (multiples of six feet). If you look to the top left of the numbers, there is a 5 sitting between a little tear-drop island and one like looks like a fat J. That J is Sampson’s Island. To the right of it is a long sand spit known as Dead Neck — “neck” being Cape Cod parlance for long sand spit, dead probably referring to the fact that not much could grow there. The little island to the left was called The Chicken Coop — doubtlessly because someone put their chickens out there to keep them safe from foxes and raccoons. To the west of the Coop is a little salt pond called Rushy Marsh Pond. You can see it was open to Nantucket Sound. Further south there is another “neck,” this one extending roughly south to north. That is the Popponnesset Spit.

So, study that picture. Because that’s basically what Cotuit, Mashpee and Osterville’s shoreline looked before people started messing around with groins and jetties and dredges.

Now let’s skip forward thirty years to 1889. Not a lot of change, just a little more detail.

And let’s advance to 1933 after the first wave of summer people have moved in. This was Cotuit when my father was born, before the famous hurricane of 1938.

Whoa. Big change right? Lets start at the lower left, Popponesset Beach and the spit has doubled in size and is nearly as far north as Rushy Marsh Pond. The Chicken Coop is gone. Vanished. Sampson’s Island is now part of Dead Neck, and Dead Neck has been separated by Osterville to the east by the Wianno Cut. Wow. Also note how far west Sampson’s is in relation to Bluff Point behind it. It barely protects the harbor inside from the Sound.

Now fast forward thirty years and at least four hurricanes later to 1966. The biggest change is the Popponesset Spit.

The spit has been breached but its northern end has been pushed into Rushy Marsh Pond, sealing it shut and creating an appendix-like little cove between it and Oregon Beach (named by the Army during WW II when landing craft exercises were conducted at Camp Can-Do-It in preparation for D-Day). I am eight years old. This is the Cotuit I first recall.


Above is 1972’s chart. The spit right below the word Highlands is shrinking — the cartographer has left a vestige below it — a sandbar or shoal. Sampson’s Island is no longer named, it’s all just Dead Neck, which has grown a little to the west closer to the mainland.

And here we are today. Dead Neck is almost touching Cotuit. The channel is tiny. Oregon beach shows no evidence that it was once fronted by a long neck of sand. Rushy Marsh Pond hasn’t had an influx of saltwater is probably forty years and plans are afoot to breach it to freshen its contents.

Let me point out a very interesting demonstration of the disruptive power of a groin.

See the last groin in the sequence of four? It effectively starves the beach to the north, creating a small “button-hook” effect.

Sorry to go on so long. I’m just a beach geek.

Moving beaches and strong storms always make me think of Lord Byron’s lines from Childe Harold’s Pilgrimage:

Roll on, thou deep and dark blue Ocean – roll!
Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin – his control
Stops with the shore; -- upon the watery plain
The wrecks are all thy deed, not does remain
A shadow of man’s ravage, save his own,
When for a moment, like a drop of rain,
He sinks into thy depths with bubbling groan,
Without a grave, unknell’d, uncoffin’d, and unknown.


Author: David Churbuck

Cape Codder with an itch to write

0 thoughts on “Living on a sandbar”

  1. Good God, David, I’m speechless at this post and at your command of hydraulic geology. This is one of the most impressive posts I’ve ever read on the web/
    Way to go!
    So why hasn’t most of the Cape just washed away like some of the Central California coast has (an area called “Los barrancas.”

    Outstanding David. I’m in awe.

    Jim

  2. Impressive indeed David.

    “…This leads to otherwise rational masters of the universe writing impossibly large checks to coastal engineering firms for the construction of sea walls…”

    Sounds like a job for me. How do I get hooked up as one of those coastal engineers? I love building stuff out of concrete, and I really like the idea of receiving an impossibly large check. : )

  3. Excellent lesson in geology.

    It is interesting to be able to see the changes in the coastline in and around Cotuit over the last 40 years or so. I remember the spit outside of Rushy Marsh at Oregon beach and the huge drop-off that was there at the time.

    Also interesting to see how much smaller the cut into Cotuit Harbor by Sampsons island has become.

  4. Some of my first cape memories are of swimming at the streep drop off inside the northern spit that sealed off Rushy Marsh Pond.

    Thank you for a great lesson and post. Izzy and I are queued up for a late may circumnavigation of Monomoy.

  5. Given the growth of Dead Neck and the vanishing of the Popponesset Spit, Chicken Coop, and the Rushy Marsh appendix — the question is this? Where did the sand go?
    The tides are ambivalent here — the current rushing out of Popponesset Bay and Cotuit Bay are pretty fast and will insure some degree of an open channel. But imagine how the tidal dynamics were before they cut through the Wianno Cut? Seapuit would have been a torrent emptying out West Bay. I think that Seapuit flow took the path of least resistance once it hit Cotuit Bay and flooded out through the Pirate’s/Cupid’s Cove cut.

    The tidal flow in Nantucket Sound is East/West — ebbs west, floods east, but the prevailing winds are from southwest, meaning, I surmise, the sand is moving from the south to the north. That would say the sand that formerly comprised the spits off of Oregon are now the considerable sand bar off of Loop and the massive extension of Dead Neck towards Cotuit.

    I dunno. Wish there were historical aerial photos and charts from the 1889 to 1933 gap to explain what year Dead Neck closed up and why. I had always assumed it was changed by the hurricane of 1938, but obviously some event between 1889 and 1933 made the massive changes.

    My theory, and I’m sticking to it, is the jetties at the Wianno Cut and the groins south of Oregon forced the changes. Get rid of them and things might restore.

  6. Good Evening – it is 10:35 p.m., Sunday, June 10. I have just return from my home in Popponsesset very dismayed. Our beach is continuing to erode at a rapid pace. Once one of the longest stretches, it is almost divided into 2 pieces. I was told by a neighbor that winter storm bring the sand in and summer storm wash it toward Cotuit. Is this true?

  7. Susan — the prevailing summer winds are from the Southwest, which indeed would trend the sand towards Cotuit. I’d look hard at the breakwater at the end of Wading Place Road as the culprit. Aerial photographs indicate it is starving the sand from flowing to the north and replenishing the spit. Where is the breach? I wasn’t aware there was one this spring.

  8. There is an ugly, untold truth about the Rushy Pond Project. With all the knowledge and experience of the detrimental effects jetties and groins cause on beaches, the Town of Cotuit embarked in the construction of yet another stone jetty by Oregon Beach, the former crown jewel of Cotuit beaches.
    In no time the once sandy beach North of the rigid jetty, has eroded leaving a man made retention cliff and a fringe of rocks on its place.
    Furthermore the jetty, which was built over private beach rights without notification or consent, now helps guide a stream of dirty water from the bacteria filled pond and into the ocean.
    The beautiful Oregon beach as we once knew it, is no loger available to be freely enjoyed by walkers, fishing enthusiasts and runners as the eye sore barrier of stones and dirty water is now on the way.
    Saddly due to the tampering with mother nature, the whole area is now much more vulnerable to flooding, than it ever was.
    I surely wouldn’t let my loved ones swim into that newly created sewer like beach environment.

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