Posted by: coastlinesproject | September 8, 2015

Salisbury Beach; Rip Tides and Jetty Repair.

North Jetty

 

 

During the summer of 2015 lifeguards at Salisbury Beach State Park had to make 105 rescues. Usually they would only have to make 25. So, why did they have to make 4 times more rescues than normal than any other beach in Massachusetts?

 

The answer lay right beside them; where cranes, barges, excavators and trucks were busily placing 5-ton boulders on the Merrimack River North Jetty. Before this repair, energy from Northeasterly waves would be dissipated as the waves broke through low spots on the jetty. But now the jetty presented a straight, impenetrable wall without holes so the waves would ricochet off, scour out great scallop shaped portions of the beach and then return back out to the ocean as dangerous rip tides.

 

Even on calm days the area that has been scalloped out by storms could harbor these currents that develop quickly and shoot swimmers directly out to sea. It is impossible to swim against a rip tide and veterans know you have to turn parallel to the beach to find water calm enough to return to safety. But even veterans can drown in the grip of a rip tide.

 

But how did this situation come about? If the Army Corps had done a simple literature search, they would have discovered that the same problems occurred after they repaired the jetties in 1970.

 

Dennis Hubbard, presently chairman of the Department of Geology at Oberlin College, wrote a paper for the Corps in 1976. In it he explained that 90% of the time longshore currents travel along this section of beach from the center of Plum Island north. But when you have a Northeaster with waves higher than eight feet, the waves ricochet off North Jetty, scour sand off Salisbury Beach and then skirt around the harbor bar on southerly currents that can distribute the sand as far south as Annapolis Way, almost a mile away.

 

If the Corps had looked for his paper they would have also discovered why North Point has lost two and a half football fields worth of sand since the South Jetty was repaired in 2014. According to regulation and custom, the Corps should have conducted their own sediment flow analysis or at least made a literature search before undertaking these two $12 Million dollar projects. With a better design both of these unintended consequences could have been avoided.

 

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Dennis Hubbard will be giving a Storm Surge lecture at the Parker River Wildlife Refuge Center at 6:30 on October 21st.

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