Supersize manholes to clean water before it runs into harbor
Stormwater cleaning project in design phase
BY PETER B. BRACE
High tide coincided with last Thursday morning's heavy rains and proved that Nantucket's downtown storm drains cannot handle water from above and below at the same time.

During the height of the deluge, the high tide waters of the harbor backed up into the town's storm drain system through harbor outflow pipes preventing the heavy rain from draining into the harbor.

As a result, Easy Street flooded and the intersection of Easton and North Beach and South Beach streets drowned under more than a foot of water. After the tide peaked, the water reversed direction, and churned steadily out into the harbor.

Standing on the north finger pier at the Children's Beach boat ramp, one could see what washes into the harbor during such downpours - sand, silt, leaves, paper, plastic litter, and oil and fuel plumes all came bubbling up from the outflow pipe beneath the pier. Sewage may likely have been part of this mixture as well.

Painfully aware of the pollutants entering the harbor during rain events, the town, with the help of Earth Tech, its wastewater consultant of Concord, Mass., is in the design phase of a storm water-cleansing project that will remove much of the trash headed for the harbor before it reaches the many outflow pipes opening into the harbor.

The adoption of Article 10, "General Fund Capital Expenditures," at the annual Town Meeting in April, appropriated $5,500,000 for this three-phase public works project to purify storm water running into the harbor through storm drains and outflow pipes. Department of Public Works Superintendent Jeff Willett said the work includes installation of siltation controls, tide gates to prevent sea water intrusion during high tides and storms, increased pipe diameters and the redirection and elimination of many of the existing outflow pipes, a project he hopes to complete by May 2008.

"We hope to do the project a year from now," said Willett. "We go out to bid in the spring and would expect the project to begin in the fall. Now, I don't say that with a hundred percent certainty, but that is our intent."

Willett added that Earth Tech engineer Tom Parece will outline the work for the Shellfish & Harbor Advisory Board, most likely at one of its two December meetings, and explain the project in detail. Parece said that the aim of the project is to use what he called best management practices.

"Best Management Practices (BMP) are techniques used to control stormwater runoff, sediment control and soil stabilization, as well as management decisions to prevent or reduce non-point source pollution," according to www.stormwaterauthority. org/bmp/. "The EPA [Environmental Protection Agency] defines a BMP as a 'technique, measure or structural control that is used for a given set of conditions to manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner."

What that means for Nantucket, said Parece, are multi-function stormwater cleansing structures just inland of outflow pipes at Brant Point, Children's Beach, Straight Wharf, Easy Street, Commercial Street, the town pier, the Francis Street Beach

and the Creeks that are designed to handle the first flush of a two-year storm.

"We're going to put in these oversize manholes, very large structures that we're going to put in before the outflows into the harbor, to capture all the floatable items - paper products, litter and plastic - and to reduce the sediments that also would be flushed out into the harbor," Parece said.

Inside each supersize manhole is essentially a whirlpool-inducing structure that uses the centrifugal force of water flowing into a giant funnel to whip out the trash and sediments in the water, collecting the waste inside the structure and letting the water out into the harbor. DPW crews would be responsible for regular cleaning and maintenance of these stormwater treatment manholes, especially after major rainstorms.

Parece added that the first phase of the harbor stormwater project will include increasing the size of drainage pipes where larger diameter pipes are needed for higher efficiency, and

diverting many of the existing drainage pipes to the new stormwater manholes. This will reduce the overall number of outflow pipes in the lower harbor, but will not include most of those on private property.

For example, three of the four outflow pipes between the town pier and the Great Harbor Yacht Club are to be removed.

"There's a lot of individual pipes going in there [into the harbor]. We're only dealing with the major publiclyowned discharge pipes," said Parece.

What could lighten the load of the new stormwater installations is a recommendation in the Nantucket and Madaket Harbors Action Plan to label all storm drains in the downtown area, saying that what is poured into these drains flows into the harbors and directly impacts water quality and marine life.

In the future, Parece said phases two and three could include enlarging catch basins further inland of the harbor and installing infiltrator pits in strategic locations.

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