September 9, 2016
On September 9th the engineering firm GZA presented their short-term solution to protect the houses on Plum Island’s North Point. It was an almost perfect example of mimicking nature to slow erosion.
Another company had proposed using synthetic material and spending $500,000 to build a system that would last until the Army Corps of Engineers could find a long term solution. It had been determined that that solution was too expensive and not temporary enough.
So what David Smith and Anders Bjarngard had done was look at the unique natural features of the area and then decide the best way to mimic them. There had formerly been about 400 feet of well-vegetated primary and secondary dunes protecting the neighborhood.
Such sand dunes are nature’s way to protect a barrier beach. They are perfectly designed to flex and absorb the energy of a storm. And after the storm passes they start to rebuild. As soon as you encase sand in a synthetic fiber it loses the ability to move. In effect you have created an immovable seawall, which might make people feel safer but it will actually increase erosion by causing scouring around the ends of the sandbag seawall.
But natural sand dunes are not just big piles of sand. Instead of being encased by an outer coating of material they are bound together internally by a matrix of roots and rhizomes. Dune grass is a unique plant that has evolved to live symbiotically with the dynamic environment of a barrier beach.
A winter storm can bury dune grass under a foot of sand and instead of dying the grass simply grows up through the sand leaving a thick latticework of spreading rhizomes behind, which then binds the sand together internally. There is nothing that humans can do to surpass this elegant design.
So GZA decided to build two dunes, a primary dune nearest the water and a secondary dune nearest the houses. Residents would almost certainly see waves eroding the primary dune during the coming winter but it would not be because the dune had failed, but because it was doing its sacrificial job.
But the first year would be the most critical one because the dune grass will have not had enough time, or storms, to grow their deep interconnected rhizomes. Plus the so-called king tides would fall in November so that October, November and December would have all have ten-foot tides and about ten days of over nine foot tides. It was almost certain that a storm would occur during one of those windows of vulnerability.
But there was also a bright side to these conditions. We would only need four-foot waves to start pushing sand through the jetty. It would collect as about an acre of sand, four feet deep on the riverside of jetty during these winter months. That would be about 5,000 cubic yards of sand, or about the same amount of sand that GZA planned to add to the dunes every year to mimic the effects of winter storms.
The storms would also cause the jetty to continue to settle. It had already settled two feet in 2016. All it needed was to settle about another two feet and there would be enough sand for the beach and the dunes to start regrowing. Eventually it would regrow the 400 feet of sand that it had before the jetty was repaired.
But there was another way to keep the sand flowing. During the summer month the City of Newburyport could use a small dredge to pump sand through the jetty or a Bobcat to push it over the top. There were precedents for this in other parts of the country. The sand would then flow down along the jetty and around the spur to where bulldozers could add it to the dunes. It could also be stockpiled for future emergencies.
The city would have to get permission from the Army Corps of Engineers but this would probably not be as daunting as it sounds. After all it was the repair of the Corps’ jetty that caused the erosion in the first place.
If it did prove difficult to get permission to pump sand over the jetty, the city could always pump it around the jetty overland and deliver it directly onto the new dunes where it would again mimic the effects sand blown in by dry summer winds of wet winter storms. All in all it was a superb design that used appropriate technology, local materials and ongoing maintenance to provide an inexpensive short term solution so that nature could provide the long term one for free.
William Sargent’s latest book, Plum Island; 4,000 Years on a Barrier Beach, available in local bookstores and at www.ingramcontent.com.
Bill Sargent will be leading a beach walk starting from the Plum Island Point lighthouse Sundays at 2pm. Cost $10.