Dredging Popponesset Bay To Provide For The Spit
By: Brian Kehrl
The cutter sweeps back and forth across the channel like an enormous, ultra-powerful vacuum cleaner, ingesting everything it comes across. The metal machine gnaws at the sand to loosen it up before it is drawn about 20 feet away, to the bowels of a chugging, 700 horsepower diesel engine.
The suction then turns to an expulsion, and the sand that has filled in the entrance of Popponesset Bay over the last year is pushed through a wide black rubber pipe, first to a booster pump tucked just inside the bay and then along the nearby Popponesset spit.
On the other side of the process, a volcano of blackish watery sand erupts, piling up under the staircase down from Wading Place Road, at the root of the spit. A few dozen seagulls are feasting on the worms, fish, and other marine life that were taken on the ride with the sand. They are barely deterred by the forklift prying the end of the pipe out from under the massive pile of sediment.
The scene this week at Popponesset Bay—the annual dredging of the outside channel leading into the bay, one of several dredging projects conducted regularly in Mashpee waters—is a marvel, even if also a common one. It is the latest chapter in a tale of man versus nature, of Mashpee’s efforts to maintain the bay and the spit, for navigation and water quality and recreation, as they are known today.
The pursuit is a costly one. Town Meeting appropriates between $50,000 and $75,000 a year on behalf of the Mashpee Waterways Commission for maintenance dredging in Popponesset and Waquoit bays. The town is in the process of an approximately $300,000 project to permit and re-dredge an old, largely filled in channel that runs up the center of the bay. The work is funded primarily through the cost of mooring fees and boat excise tax collections.
But, according to interviews with local officials and a review of several past reports about the spit and the bay, it is a necessary and worthwhile one, in large part to help sustain the Popponesset spit, the thin barrier beach that protects the bay, hosts thousands of beachgoers every summer, and provides habitat for endangered bird species.
On a mid-afternoon boat tour of the dredge on Monday, Harbormaster Perry F. Ellis and Wayne Jaedtke, supervisor of the county dredge operation, described the process and why it is needed.
With the tide low, a broad sandbar is visible outside the bay, curving off the tip of the spit into Nantucket Sound. Gulls stood in the water up to their ankles, just feet from the channel used to provide access to boats large and small. The sandbars both inside and outside the bay fill in with sand in part because the tides pick up sediment when passing through the opening, then drop the suspended material as the water spreads out and slows down in more open water, Mr. Jaedtke said.
For this job the dredge, working its way inbound, is scheduled to remove 5,000 cubic yards of material (enough sand to fill a box 51 feet high, by 51 feet long, by 51 feet high) from a section of the entrance channel. The new channel is meant to be 450 feet long and 90 feet wide.
The sand is to be piped to three locations on the spit, at the base of the stairs and at two other locations on the spit thought to be in jeopardy of breaching in a major storm.
Mr. Ellis said the outside dredging helps maintain access for larger-draft boats to get in and out of Nantucket Sound.
The outer channel is one of four main dredging projects on the radar. The town also dredges a short inner channel parallel to the spit and has plans to re-dredge a long channel through the center of the bay first cut in 1916 by the state. Save Popponesset Bay, a nonprofit funded by donations from residents, dredges a short channel between the southern tip of Popponesset Island and the inside shore of the spit.
What Dredging Does
Edward A. Baker, president of the Mashpee Environmental Coalition, said this week that each of the different projects has different consequences for the ecology of the bay.
The outer channel likely improves the tidal flushing by keeping the mouth more open, which in turn helps improve water quality inside the bay, he said. The more tidal exchange there is, the more bay water is flushed out and Nantucket Sound water is flushed in, the better for water quality, he said.
As evidence, Mr. Baker pointed to a shift in the line between fresh and salt water in the Mashpee River. The line has shifted up in the river since the outer channel dredging began in the late 1980s, which could be caused by an increase in the amount of salt water coming into the bay, he said.
Because of the benefits to water quality, the outside dredging likely improves shellfish habitat in the bay, he said.
The inside dredging, though, seems to be a different story, he said.
The dredging done by the nonprofit Save Popponesset Bay between the southern tip of Popponesset Island and the spit, an area dredged four times in the last 15 years, is the most controversial of all the regular projects in the bay.
Mr. Baker and other critics say the dredging pinches the spit in an area many think that it is most likely to breach and prevents the spit from naturally rebuilding. They also question whether the SPB dredging, and the town dredging of a short channel inside the bay along the spit, have harmed once highly productive shellfish beds in the area.
The increased tidal flow from dredging can prevent shellfish, including softshell clams, from reproducing because the juveniles can be washed away with fast-moving water.
“We should exercise caution in our battle with Mother Nature. Remember Wading Place Road appears to be named by visitors to the Popponesset Campground where families waded for Blue Crabs in the then-marshy area between the spit and the southern tip of Poppy Island,” Mr. Baker wrote in an e-mail to the Enterprise. “I am not suggesting a halt to the dredging at the southern tip of the island due to its potential negative impact on navigation. I would suggest, however, that navigation changes substituting an easily (comparatively speaking) maintained natural channel east of and parallel to Poppy Island leading to the 1916 channel would provide an acceptable route to the Sound opening while establishing an added much-needed protection for the spit.”
The SPB dredging, however, also has strong support from New Seabury Marina and homeowners in the area that use the channel regularly to get from Popponesset Creek out into the bay and Nantucket Sound.
Representatives of Save Popponesset Bay said alternatives have been studied in detail but found to be impractical.
“Water quality and safe navigation for all of Popponesset Creek depend upon the SPB Channel,” Gregory C. Smith, president of the SPB board of directors, wrote in an e-mail to the Enterprise.
While the dredging itself is something of a constant battle against the forces of Mother Nature, removing sediment that the tides, wind, and waves have brought in, only to do so again later, another aspect of the town’s current program seeks to work more in concert with her.
Waves and the current move sand along the spit from the root to the tip, a phenomenon known as “littoral drift” or “longshore current,” according to “A Geologist’s View of Cape Cod,” by Arthur N. Strahler.
Placing the dredged material at the bottom of the spit therefore not only builds up the beach in the immediate area, but adds sand to the budget for the entire spit and helps rebuild the threatened barrier beach.
“Mother Nature is the moving company, isn’t she?” said Richard J. Bailey, executive director of Save Popponesset Bay.
Save Popponesset Bay contributed $20,000 to the cost of the dredging this year, part of a deal with the town in which half of the material would be deposited on the spit on property owned by the nonprofit, according to Mr. Smith. The payment by SPB secured the use of the secondary, booster pump, which is needed to get the material the full distance to the base of the spit.
The nourishment from dredging appears to be working, though there are still areas of concern.
Mr. Jaedtke said the spit looks “many times better” than it did a few years ago, before the spoils were pumped to the base of the spit.
“The beach is big, look at it,” Mr. Jaedtke said, gesturing to the sound side of the spit, with a bit of pride in his voice. He noted the line of debris at high tide, which in most places is a few feet below the knobby dunes that form the spine of the spit.
The spit also provides a nearby and relatively low-cost disposal site for the material, a convenience that the difficulties in obtaining a permit to dredge the Mashpee River prove the importance of.
A Different Bay
Asked to imagine the bay without dredging, James P. Hanks, a former chairman of the waterways commission who has closely studied the impacts of dredging in Popponesset Bay, said the loss of nourishment for the spit would be perhaps the most important change.
“If we don’t dredge and nourish the spit, the spit thins, gets lower, and overwashes are probable and potentially a breach,” he said. “And most likely the breach, if it was in the place before the 1960 dredging [near Popponesset Island], it might stay open but not a very big opening.”
Popponesset Creek would suffer with restricted tidal exchange fouling the water.
The horseshoe-shaped sandbar inside the bay would continue to grow, squeezing over into the channel in the middle of the bay.
He said it is difficult to predict what would happen to the overall tidal exchange in the bay: there are indications that the outside channel shoaling in restricts the flow, but the inlet would likely remain about the same size and cross-section.
Mr. Hanks said predicting how the bay would react to a halt in the dredging is guesswork at best, and many outcomes are possible. The spit could even elongate again, for instance, he said.
“Over a long period of time, the thing would probably fill in and become a marsh,” he said. “Anytime you’ve got a body of water, with water flowing and carrying sediment, gravity tends to fill in holes, so it gets shallower and shallower and vegetation moves in. The classic example is the way a freshwater pond turns into a marsh. That is the way geologic processes work.”
History And Future Of Dredging
The dredging of Popponesset Bay over time has profoundly affected the bay, and Mashpee itself.
Popponesset Bay is a shallow estuary, great for shellfishing but bad for boating access. Mashpee was one of the only towns on the Cape without a traditional harbor, a key to it remaining a rural, isolated Indian town during the 19th century.
Dredging has helped the bay become a host to recreational boaters who otherwise would have trouble getting in, out, and around the waters, part of the transition of the bay and the town to what they are today.
A major project in the early 1960s, for example, cutting a deep channel through Popponesset Creek, helped drive the initial development in New Seabury, north of the Popponesset neighborhood. The dredging, which moved 140,000 cubic yards of material out of the creek, according to a federal Army Corps of Engineers report, opened up boating access and created true waterfront homes in the Bright Coves neighborhood and on Popponesset Island.
The town petitioned the Army Corps of Engineers in 1972 to establish a federal navigation channel, but the proposal was turned down because the economic benefits did not justify the expense.
The first dredging project recorded in Popponesset Bay, in 1916, cut a channel up through the center of the bay, likely drastically improving navigational access to the upper reaches of the bay and increasing tidal flushing.
Yet the reason for that project—why the state division of public works would care enough about access to a small, shallow bay in a small, rural town, to oversee a large dredging project—remains an enigma. There was no port or marina to which the dredging provided access. It is a dead end. There were shellfishing and cranberry bog operations in and around the bay, but no large commercial fishing or other operations that would seem to justify dredging a channel up the bay. There was little residential development in Mashpee at the time.
“That is the mystery of the year,” Mr. Hanks said of the reasons for the initial dredging.
Local historians, including Joanne M. Ferragomo and Rosemary H. Burns, have reviewed town records and newspaper archives to try to locate some explanation for the project, but have found no clues.
Mr. Hanks and Kenneth H. Bates, chairman of the waterways commission, discovered state records establishing that the dredging was permitted and took place in 1916, and that it was redone in 1936, but nothing with clues about why.
“So who knows?” Mr. Hanks said.
The projects these days are maintenance dredging to sustain the channels that are already there. But there is still discussion of new projects beyond re-dredging the 1916 channel, like cutting a channel up through Ockway Bay, an idea mentioned by the waterways commission as the next prospective project.
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