Stantec Hydraulic Report Draft

Stantec – Alewife Brook Hydraulic and Stream Improvement Study

Stantec was contracted by the Mystic River Watershed Association (MyRWA) in 2024 to conduct a Hydraulic and Stream Improvement Study, which included two-dimensional (2D) hydraulic modeling of Alewife Brook, field assessment of existing conditions, and conceptual development of three potential improvement projects. The study area included an approximately 8,000 linear foot portion of Alewife Brook, extending from CSO outfall CAM401A in Cambridge to the Mystic River confluence in Medford.

Stream Channel Characteristics
Based on field observations, the study area of Alewife Brook was broken into five segments (going downstream – south to north) with unique channel characteristics. The table below summarizes the extents of each stream segment evaluated in this study along with average water surface slopes. There is a lot of accumulated sediment in the concrete segments. Note that the slope of the second segment between the Concord Turnpike and Massachusetts Avenue is extremely low – and this is where we have seen most of the recent flooding. Numerous storm drains from Arlington, Cambridge and Somerville discharge into the brook being the main source of flood waters.

Stream Segment Summary
Segment
Extents
Average Water
Surface Slope
Channel Characteristic
1
CSO Outfall CAM401A to Concord Turnpike
0.40%
Natural and
Vegetated
2
Concord Turnpike to Massachusetts Avenue
0.01%
Trapezoidal
Concrete
3
Massachusetts Avenue to Henderson Bridge
0.03%
Trapezoidal
Concrete
4
Henderson Bridge to Broadway
0.06%
Vertical Walls
5
Broadway to Mystic River Confluence
0.03%
Natural and
Vegetated
Potential Improvement Projects – Conceptual
Four potential improvement projects were considered. While the primary goal is to protect infrastructure, many of these potential projects also seek to enhance ecological value of the resource. The four alternatives selected include the following:

1. Channel Clean-Out: Dredging and/or Debris Removal
2. Riparian Wetland Creation and Enhancement
3. Crossing Structure Replacement (Alewife and/or Mystic River)
4. Draw-downs – Dam Controls
   Potential Improvement Projects – Evaluated
   Five potential project alternatives evaluated are prioritized below with consideration for effect on flooding, feasibility, practicality, and cost. The Channel Clean-Out scenario has been separated into
   Stantec Summary – Draft 1.docx

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  8/28/2026two parts: debris removal only and full-clean-out with dredging and lining removal. While both are feasible, they require dramatically different levels of effort and may be implemented together or individually. Four of the five listed prioritization alternatives are feasible but have significantly different costs and potential challenges.

1. Debris Removal Only - Under existing/current conditions, debris removal alone may have minimal effects on flood elevation; however, if debris is left in place, progressive development of channel blockages, especially at road crossings, could result in substantial backwater effects and ultimately more severe flooding. For this reason, it is recommended to address woody debris and trash accumulations in the channel as early as possible.
2. Wetland Creation and Enhancement have the potential to reduce flooding by providing storage in targeted areas where residential and commercial properties are positioned close to the Alewife Brook floodplain. While this is difficult to quantify at the conceptual stage, larger wetland features would have a larger benefit, but they tend to be very expensive to implement, and space within the corridor could be limiting. Smaller detention basin-style wetlands or emergent pocket wetlands positioned toward the back of the Alewife Brook floodplain may be more feasible from a cost standpoint and could be implemented in phases. A raised boardwalk could also be constructed along the part of the path that floods frequently.
3. The ‘full clean-out” scenario constitutes stream restoration due to its scope, with concrete lining removal, sediment excavation, channel geometry improvements, and riparian buffer re-establishment. While this alternative increases channel capacity and would be designed to promote stability without the hardened concrete lining, modeling results show that – even if this work were to be accomplished – there would be no significant reduction in maximum flood water depth or flooding extents that would reach adjacent properties at the 5-year and 25-year recurrence intervals. Cost and complexity of this full clean- out alternative could be preclusive.
4. Structure Replacement - The only restricted stream crossing on Alewife Brook was at Massachusetts Avenue; however, its effect on flooding is minimal and it cannot be considered for this alternative. The crossing structure itself is a listed historical resource that must be preserved.
5. Floodwater Drawdown - Stantec investigated potential downstream controls at the Amelia Earhart Dam. This was found to be impossible due to the tidal influence on the dam’s downstream condition. Because it cannot be changed, discharge regulation is not a viable option for flood reduction in Alewife Brook.

Conclusions
Based on this study debris removal as well as some wetland creation seem the most feasible options at present, although the effects on flooding will be moderate. Even the much more expensive full restoration option would not eliminate flooding in a 5-year storm. Although elimination of Combined Sewer discharges would greatly reduce the health risks.
Stantec Summary – Draft 1.docx

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  8/28/2026