The Central Street Culvert/Central Pond Project
By Gregory T. Federspiel
For the past few years, a considerable amount of study and analysis has been conducted of the dam/culvert that crosses over Sawmill Brook as in empties into the inner harbor and the upstream dynamics, including the tidal pond behind the Fire Station. The brook has caused considerable flood damage in past storms and the old dam and stone culvert (last reconstructed in 1900), which restricts water flow, is in poor condition. MADOT’s most recent inspections of the culvert in November 2018 and 2019 further classified it as having a “Severe/major deficiency” that should be addressed ASAP.
The dam and culvert on Central Street are critical pieces of infrastructure. We cannot afford an unplanned collapse or for MADOT to shut down the culvert for vehicular traffic. Emergency repairs were done four years ago but this was a temporary fix that has only gained us a few more years of use until a complete rebuild of the structures can be accomplished.
Engineering for the reconstruction is nearly done. A larger culvert with removal of the constricting tide gate (which only restricted flow out, not in as it is overtopped every high tide) will help prevent flooding upstream during significant rain events. The new culvert will be built to today’s engineering and construction standards. A stone façade will replicate the look of the original structure.
The construction of the new structures will be challenging given the small space to work in and the utility lines that cross at this same location. The major electrical feed for all of Cape Ann crosses at this point as does a water main, NGRID gas pipeline, and Comcast/Verizon ducts; the sewer main fortunately passes below Sawmill Brook and will not be impacted. Flow from Sawmill brook must be always maintained during construction, along with any increased flow from rain events and storm surges. The tides will also affect the project and may limit the contractor for some aspects of work during high tides. The contractor will need to have large pieces of equipment and materials onsite which further restricts the limited project area. Many of the necessary permits place restrictions on when and how the work can be performed. For example, the DEP 401 WQC permit stipulates we are not allowed to perform “in water work” (which is necessary for most of the project) between March 1 and June 30 due to rainbow smelt migration in Sawmill brook.
These complicating factors prevent the project from being done while maintaining one lane of traffic. Thus, during a six-month period there will be significant stretches of time when all vehicles will have to detour around the site. We hope to be able to maintain pedestrian access across the brook, however there are very limited locations out of the work zone for safe passage that could accommodate a pedestrian bridge and will require securing temporary easements from private landowners.
Our current projection is for the construction to begin a year from now. We are a contender for a large FEMA “BRIC” (Building Resilient Infrastructure and Communities) grant of approximately $4.5 million to pay for the nearly $6 million project. We will know about the grant award by the first of the year.
The project also includes work upstream. The retaining walls along the east side of Central Pond are in poor condition with stretches of the old wall having fallen into the brook. These walls will be rebuilt, and the brook restored to its natural state with salt marsh grasses reestablished in the current mudflats. The banks on the west side will be fortified with “living shorelines” to protect it from further erosion while maintaining the natural look. Other options for treating this stretch of the brook were the subject of workshops and presentations before the Board of Selectmen.
The brook serves as spawning habitat for endangered rainbow smelt and American eel. Removing the tide gate and restoring the brook to its natural state will have the added benefit of improving this habitat.
As the actual reconstruction start date gets closer, more details about project timing, how detours will be managed, and what communication strategies deployed will be further developed. The project, while very necessary, unfortunately will impose temporary hardships given its central location.