Feb 18 2013

Unintended consequences of jetties

Published by at 2:41 pm under Cape Cod

I read with interest the news reports of the massive erosion to Sandwich's beaches following the blizzard that hit that northside coast head on with 70 mph winds and big storm surge inflated seas two weeks ago. What I was unaware of was the simmering resentment by the town over the big pair of jetties that guard the entrance to the Cape Cod Canal, jetties built at the turn of the century when the canal was completed, put there by the Army Corps of Engineers to keep the entrance open and flowing.

I mention this only because of the widely held belief here in Cotuit that the Wianno Cut and its two jetties are responsible for the present situation where Dead Neck is starved of sand from its eastern end by the jetties, a situation the owners of the island -- 3 Bays and Mass Audubon -- believe can only be temporarily relieved by dredging sand from the growing western, Cotuit-end of the island and piping it back to the eroded section. As I've written before here, the littoral drift of sand is not a straight-forward, perpendicular dynamic where waves push and pull sand in and out of a beach, but where sand flows along the beach, driven by the prevailing winds and currents. A jetty interrupts that flow, backing up sand "upstream" while starving everything "downstream.

The situation in Sandwich is deplorable enough that the town is telling the Feds to fix it since the Canal and its jetties are Federal property. Not so with the Wianno Cut, where I believe the cut was sponsored and paid for by the town (initially with a wooden, planked jetty) at the urging of the people of Osterville who wanted easy access to Nantucket Sound.

Quoting the Cape Cod Times (paywall in effect):

"A big frustration for some is the U.S. Army Corps of Engineers. Town officials believe a source of the erosion problem is the Cape Cod Canal, which is controlled by the Corps. A jetty system built in the early 1900s to keep sediment from building up in the canal is starving the beach of sand needed to replenish the dune system.

"The end result: Scusset Beach has too much sand and the coastline from Town Neck to Spring Hill doesn't have enough, Selectman James Pierce said.

"I agree with others," Pierce said. "The feds caused the problem and the feds should pay to fix it.""

And speaking of Nantucket, fans of whaling history know the story of how that port's dominance of the whaling fishery ended in the mid-19th century when its harbor entrance was obscured by a sandbar, leading the novel and desperate measure of building a floating dry dock known as "The Camel" to lift whaling ships over the bar and into the city. The failure of the harbor and the Camel solution are widely regarded as the reason New Bedford took over from Nantucket as the primary whaling city on the East Coast. There's a great article on the Nantucket Bar and efforts to overcome it at the Nantucket Historical Society's website.

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