The Carlisle Mosquito Online

Friday, January 31, 2003

News

The conservation commission received nearly complete construction plans and a cost estimate of up to $100,000 for repair of the Greenough Dam at its January 23 meeting. Consulting engineer Robert Stephens, who was hired following the 2001 Town Meeting to develop engineering specifications for the project, arrived with an armful of rolled-up drawings and construction specs, and a still-unfolding narrative of the challenges encountered in dealing with a deteriorating 70-year-old earthen dam.

The importance of repairing the structure that interrupts the flow of Pages Brook to form the open-water pond on the Greenough conservation land became clear in October 2000 when inspectors from the department of environmental management declared the earthen dam, and particularly its spillway, to be in poor condition and in need of timely attention. In September 2002 Stephens completed a thorough study of the hydrology of the brook as it makes its way to and through the pond and on toward the Concord River. At that time he told the commission his design would meet state requirements for a 50-year storm, but he believed the repaired dam would actually be capable of handling a 100-year event.

An aerial view of the Greenough Conservation Land shows the pond and the Concord River in the distance. The arrow shows the approximate location of the Greenough Dam, to the right of the house and barn. (Photo provided by the Carlisle Conservation Commission)

Dam design

In last Thursday's report Stephens showed detailed drawings for a new 27-foot triple concrete spillway and the bridge that will span it. The second, smaller existing outlet will require internal repairs and a new top slab to enable it, like its larger counter-part, to bear the weight of a 32-ton fire truck. He explained that his plan is only 90% complete because he has recently learned that he must construct a permanent rather than a removable cofferdam (a watertight structure protecting the construction area). He designed the bridge over the main spillway to meet Massachusetts Highway Department standards for a secondary road, which means the cofferdam must become an integral part of the final structure. It is also possible that one or more temporary coffers will be needed in areas of the earthen dam itself where the stumps of trees cut down last summer must be pulled out and the resulting cavities filled. The water level in the pond may need to be lowered by five feet, which causes concern for the safety of fish and other wild inhabitants. Stephens hopes that deeper holes within the total body of water will be sufficient to provide refuge.

The engineer was also concerned about finding an accessible area for storage of construction materials, some of which will require silt fencing and haybales. Abutter Harvey Nosowitz was asked if some stockpiling might be located on nearby land on the eastern side that belongs to a trust of which he is a member. He said he would check and appeared sanguine about obtaining permission.

Getting down to smaller details, Stephens recommended inclusion of wooden hand rails on the bridge over the spillway, mainly for safety reasons. He also noted that the Corps of Engineers requires use of large riprap (flat stones that help prevent washout) in the downstream channel to prevent youngsters from pulling the stones out.

And the cost...

Finally, arriving at the moment everyone had been awaiting with trepidation, the engineer gave his cost estimate for repair of the dam at $80,000 with a 20% slippage either up or down (but more likely up), depending on the bidding contractors' perceptions of the uncertainties.

A fisherman hopes for a bite on the pond side of the Greenough Dam, after trees growing on the earthen structure were cut down last summer. (Photo by Sylvia Willard)
With the matter of funding on the table, commissioner Roy Watson asked Stephens about the practicality of some private funding. "I would like to see us design tiles or 'bricks' that could be engraved with a donor's name and integrated into the basic structure. Would that be possible?" Stephens quickly eliminated the suggestion of a tiled wall as "an invitation to grafitti," but felt that embedding bricks of some durable composition in sand on the deck of the spillway would be "just fine." He estimated that the contractor could probably fit 400 bricks onto the span. Watson's brainstorm caught on with both the commission and audience, leading to exuberant speculation about the amount that might be garnered for naming the bridge itself.

Returning to more sober matters, Stephens closed his presentation with a request for input from the commission once they have had a chance to study the plans in detail. He also sought recommendations for suitable plantings to help stabilize the earthen face of the dam after the tree stumps have been removed. With their comments and suggestions in hand, he said he should be able to "wrap up" the project plans within two weeks.


2003 The Carlisle Mosquito