Central Street Culvert Replacement

Project Summary/Description/Background

Project Summary:
Replace the Central Street Culvert; remove the tide gate structure; restore and upgrade the stream banks and surrounding wetland area, including construction of a living shoreline, channel improvements, salt marsh restoration; and to conduct project monitoring.

Description:
The project includes three phases: 

1. Removal of the tide gate structure, demolition of the existing Central Street Culvert, and reconstruction with a concrete arch culvert with an approximate span of 20 feet. 
2. Streambank stabilization along Central Pond/Sawmill Brook. The proposed restoration design includes improvements to the existing retaining walls along the eastern shore and implementing nature-based techniques to stabilize the western shoreline. 
   1.  Construct living shoreline along Sawmill Brook. The proposed bank treatment on the west shoreline consists of living and nonliving plant materials together with natural materials to reduce erosion, establish vegetation that will create stabilization of the shoreline through various runoff and tidal regimes. Native species will be specified providing naturally occurring vegetation while providing access to the resource. 
   2. Improvements to the stone walls. The failing slopes along the fringes of Central Pond will be improved through installation of a more resilient “flexible” gravity wall along the eastern shoreline and drainage improvements behind the wall. The upgraded sections of wall will have a stone appearance consistent with the aesthetic of the adjacent walls to the extent possible. 
3. Channel improvements and salt marsh restoration. This final phase includes reestablishment of a channel through the sediments in Central Pond by reintroducing natural in-stream processes and restoring salt marsh within the interior sections of the mudflats. This process is currently in progress with the partial restoration of tidal flows and will continue with the removal of the tide gate structure and bridge replacement. This area will be allowed to self-seed with salt marsh grasses as regular tidal flow returns and augmented with planting of suitable species as appropriate. A period of monitoring will follow for adaptive management as needed to restore coastal salt marsh and eradicate invasive species. Given project constraints and existing infrastructure, Sawmill Brook will not be able fully return to a pre-settlement condition; however, the stream will be guided towards a more natural salt marsh condition. The design strives to decrease width/depth ratio within the low flow channel, improve instream cover and spawning habitat, reduce bank erosion, and improve sediment transport.

Background:
Manchester-by-the Sea experiences a high frequency of flooding in the Sawmill Brook Watershed, particularly in the downtown area. Sawmill Brook is also experiencing water quality degradation (Chase, 2002). These deficiencies are a result of hydraulic restrictions within the Brook, (e.g., culverts and the Central Street tide gate), increased stormwater runoff from developed areas, a highly channelized stream system, and poor infiltration conditions. Flooding within the watershed and hydraulic restrictions of the storm drainage system has resulted in sedimentation of suitable spawning areas and restrictions for fish passage. These problems will be magnified in coming years due to climate change impacts on inland and coastal flooding. 

The Brook contains numerous fish species including American eel and rainbow smelt. Sawmill Brook has been identified by the Massachusetts Division of Marine Fisheries as one of the region’s only suitable spawning grounds for rainbow smelt. Smelt populations are dependent on high quality water and a specific gravelly substrate for spawning (Chase, 2006). Increased riverine flooding coupled with greater storm surge has the potential to adversely impact the existing smelt spawning areas by altering salinity, filling in gravelly substrate with fine sediment, and undermining structures leading to a collapse of the stream channel. Evaluation of the tide gate has identified design deficiencies that may also be linked to the decline in observed smelt egg masses. Central Pond bank erosion has increased due to winter floods in 2016 and 2017. 

The Town completed a two-year feasibility study of Sawmill Brook management strategies and identified a plan of action to mitigate flooding and improve one of the few remaining rainbow smelt habitat areas in the northeast

Timeline (as of July 2024)

• Summer 2024 to Fall 2024: Final Permitting and bid documents are being completed 
• Late Fall 2024: The project is put out to bid per MGL Chapter 30 Section 39m
• Early Winter 2024 to Winter 2025: Project is awarded to contractor. Contractor starts due diligence and project submittals
• Winter 2025 to Spring 2025: Project submittals are reviewed and approved. Contractor may perform limited construction prep work onsite
• Spring 2025 to Summer 2025: Contractor prepares for full mobilization on Central St Culvert
• Summer 2025: Contractor may choose to work on portions of the Central Pond Restoration
• Labor Day 2025 to Spring 2026: Contractor is given full access to Central St Culvert and is allowed to shut down road to enable construction to be completed as expeditiously as possible
• Spring 2026 to Summer 2026: Road is re-opened to thru traffic. Contractor is finishing Central St Culvert Replacement. Contractor starts/continues work on the Central Pond Restoration Project 
• Summer 2026 to Fall 2026: Contractor continues work on the Central Pond Restoration Project
• Fall 2026 to Fall 2028: Monitoring of Central Pond 

Funding:
The estimated cost of construction for the Central Street Culvert Replacement and Central Pond Restoration Project is approximately $6-7,000,000. The Town has secured a total of $6,478,300 in grant funding spread across 3 grants for the project. The grants and their amounts are detailed below:

FEMA BRIC Grant = $4,484,673
MassDOT Small Bridge Grant = $500,000
NOAA/CZM Grant = $1,493,627

The Town has previously appropriated an additional ~$1,000,000 to fund the design and construction of the project. Of that $1,000,000, approximately $700,000 remains to be used for construction, if the grant funding is not adequate.