Somerville Tide Gate Replacement

About this Project:

The Somerville MWR205 Tide Gate is located at the end of MWRA’s 84-inch outfall pipe that discharges treated combined sewer overflow (CS0) from the Somerville Marginal CSO Treatment Facility to MWRA’s permitted marine discharge downstream of the Amelia Earhart Dam.  The tide gate is located off Noonan Place in Somerville, between the Department of Conservation and Recreation parking lot and the Mystic River to the east.  The tide gate is housed within a below grade, reinforced concrete structure, whose top slab is flush with the surrounding ground surface.  The structure also includes a manually actuated sluice gate that has been left in the open position for approximately 10 years.  Water levels within the structure fluctuate approximately 12 feet each tidal cycle.  The structure is located within a gated chain-link fence to prevent unauthorized access.
An inspection of the 50-year-old six-foot by six-inch squared tide gate was conducted in June 2020 and revealed that one of the two pivot hinges is broken and needs to be replaced.  As a result, the gate does not fully seal with its frame, causing tidal waters to leak into the upstream system. MWRA has determined that this tidal leakage can fill a portion of the upstream CSO piping system, using available storage prior to a storm event, resulting in small increase to CSO discharge from the upstream CSO discharges to the upper Mystic River.  MWRA staff have concluded that the gate had reached the end of its useful life and recommended replacement.

Scope of the Project:

The construction contract includes fabrication of the new tide gate, fabrication of the stop logs to isolate the vault structure while dewatering, cleaning and removing the existing tide gate, prepare & coat the thimble surface and install the new tide gate.  The contractor will also provide the necessary safety equipment while working in the confined space during the work.