Friday, October 4, 2002
Greenough Dam repairs present unique challenges
An enthusiastic dam consultant from Stevens Associates Consulting Engineers unveiled preliminary plans for renovation of the deteriorating dam on the town's Greenough Conservation Land at the conservation commission's September 19 meeting. Repair of the 70-year-old, man-made structure was found to be necessary by experts from the state's Department of Environmental Management in October 2000, and the 2001 Town Meeting authorized hiring of a dam engineering specialist to come up with plans and cost estimates for the project.
Engineer Bob Stevens completed a thorough study of the hydrology of Pages Brook, the Greenough Pond that it feeds, and the dam that interrupts its flow to form Carlisle's scenic gem. The town does not have the funding, nor the time, to perform an exhaustive survey of the hydrology of the Concord River which has in the past backed up to the top of the dam on the downstream side. Said Stevens, "It's been fun! I've never worked on a project with this much potential reverse hydraulic head load." However, the upshot of his sketches and explanations seemed to be, "Not to worry!"
Anticipating flood conditions
As Stevens explained the situation, when there is a serious flood condition, Pages Brook will normally rise first and pressure will build on the upstream side of the dam. If flood conditions intensify and cause the river to back up into the area downstream of our dike, a reverse force will begin to be exerted which can increase until the pressure and counter pressure on the dam are equalized as river waters approach the top.
Turning to the more pedestrian aspects of the project, Stevens' plan called for adding a new 24-foot spillway consisting of three 8-foot spans. The access road that crosses the structure must be capable of accommodating a large fire truck.
The existing second spillway will remain in place, but its top slab will have to be removed in order to check on its internal health and fix anything that needs attention. That slab will then be replaced with one that can carry the same emergency load as its counterpart. As for the access roadway, Stevens recommended that it be paved to prevent erosion and rutting. While a gravel surface would be cheaper and would work, it would also carry a hefty annual maintenance cost. Once the spillways are in, stumps from the trees that were cut down along the top of the dam last spring will have to be removed and the resulting holes filled in.
Design satisfies requirements
The design as now proposed meets the state requirements for a 50-year storm, but Stevens told the board, with the flashboard capacity he is adding, it will actually be capable of handling a 100-year event. To questions about a possible state mandate to replicate any destroyed wetland areas, he pointed out that given the new spillway, there is a probability that new wetlands will be created downstream to balance those that will be lost.
Protecting fish during repairs
Commissioner Tom Brownrigg asked if there were any way to protect the fish as the water level is lowered. Stevens figured that the pond will be dropped by only about two feet, leaving some deeper depressions where aquatic wildlife can hide. Also, he expects to use a cofferdam, a water-tight structure that holds the water back during construction of the spillway, and could extend it further if the commission so desires.
Following a suggestion from commissioner Tricia Smith that he present and price a basic plan and then allow for alternatives based on price, on-going maintenance and aesthetics, Stevens responded that he is already close to finalizing just such a plan. Smith recommended strongly that he provide for a generous amount of riprap in the downstream channel to prevent any future erosion or undermining of the structure.
With a closing kudo from engineer Smith that, "I think it all looks good," the commission asked Stevens to present a detailed proposal at one of their two November meetings. He agreed.
© 2002 The Carlisle Mosquito