Posted by: coastlinesproject | January 29, 2016

Plum Island; A Weir jetty to the Rescue.

A Weir Jetty to the Rescue;

Saving Northern Reservation Terrace

 

January 10, 2016

 

On January 10th a moderately strong storm slammed into the East Coast. It was arguably the first storm of our strange El Nino year. The mid-West had already been whacked by a three-day rainstorm that drowned 25 people near the Mississippi River, and California was suffering mudslides from almost weekly El Nino storms.

 

New Englanders were just relieved that the storm didn’t drop any snow. In fact it set several heat records with temperatures close to 60 degrees during what should have been the coldest month of our year.

 

I wasn’t able to drive to Plum Island until after the storm had moved on into Canada drawing a frigid west wind in its wake. But the waves were still 10 feet high riding on top of a nine-foot high tide. It was creating an unnerving spectacle.

 

The waves were sliding imperceptibly through the jetties, but when a single wave hit a shallow shelf of sand it would rear up like a galloping white charger trailing a cloud of rainbow filled spray. Then it would smash against the dune and withdraw with a great tangle of roots and dunegrass in its maw.

 

The storm had torn through 19 feet of the six-foot high dunes that run along the entire length of North Point. And the dune had already retreated back by 80 feet in just the last five months. A second January storm would wash another 15 to 20 feet of sand dunes out to sea. This was probably the fastest eroding beach in New England if not along the entire East Coast.

 

But all the erosion was not happening in exactly the same manner. Storms pushing waves from the Southeast would carve more out of the north side of the beach while storms pushing waves from the Northeast waves would carve more out of the southern side of the beach.

 

Thus, each storm acted like a saw cutting first one side of the beach then the other, so the remnants of hurricanes Joaquin and Patricia had each carved about 20 feet of sand off the north side of the beach in October and November while waves arriving from the Northeast had carved 10 feet off the south side of the beach in September and December and 35 feet in January.

 

At this rate the ocean would be lapping at the foundations of about 20 houses on Northern Reservation Terrace by 2017, and become a major problem for 148 houses thereafter. It would make Newbury’s erosion problems pale in comparison.

 

Meanwhile Newburyport’s Mayor Holladay had requested that the Merrimack Beach User’s Alliance take up the issue at its February meeting. It was fortunate she had acted so swiftly to try to address citizen’s concerns.

 

Congress might have to convince the Army Corps of Engineers to find a solution, or the Corps could do through an addition to its former repair. Whatever the case, the future of Northern Reservation Terrace was entirely in their hands.

 

The solution that would make the most sense would be for the Corps to install a rubble weir on the landward side of the jetty. The lowered area would then allow the longshore currents to carry sand through the jetty to rebuild North Point. But it would not let sand fill in the channel because the river’s outgoing current is stronger than the tide’s incoming current. So the sand would flow back through the jetties and offshore.

 

Such systems have been installed in several other areas of the country. One is being considered for Sandwich, Massachusetts that faces the same problem as Plum Island.

 

When the Corps repaired the jetties that stabilize the Cape Cod Canal the repair prevented sand from bypassing the canal to build up Sandwich’s beaches, so that several of her neighborhoods were being eroded almost as fast as those on Plum Island’s North Point.

 

It would be a fairly straightforward engineering problem to install a weir or even to use pumps to pump a slurry of sand and water over the jetty. The far more time consuming problem would be to navigate the complicated red tape to get it done.

 

But there was some money to get the process started. The Corps’ Northeast District has made a request to DOTS, the Dredging Operations Technical Support program in Vicksburg Mississippi to prepare a report for possible solutions. It was a doable project, but the clock was ticking fast.

 

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William Sargent’s latest book, Plum Island; 4,000 Years on a Barrier Beach is available in local bookstores and through Amazon.com.

 

 

 

 

 

 

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