Palmer Ave Detention Basin Retrofit

Project Goal:

Reduce stormwater pollution leaving the Palmer Ave neighborhood and improve downstream water quality in Dole Brook.

Overview:

The City continues to strive to improve water quality for its constituents and meet its Municipal Separate Stormwater Sewer System (MS4) permitting goals by reinvesting stormwater utility fees in capital improvement projects. This detention basin retrofit project will increase treatment of all stormwater runoff conveyed to the system from the Deering Run Subdivision (Allison Ave, W Lynne Ave, Deering Run Dr, Palmer Ave between Allison Ave and Continental Dr, and a small portion of Washington Ave). The runoff from this 34 acre drainage area is collected by a series of catch basins and storm drains throughout the neighborhood before discharging into the Palmer Ave detention basin and eventually a tributary branch of Dole Brook, which Maine DEP defines as an impaired stream. If you're interested in learning more about stormwater runoff and regulations, check out Think Blue Maine.

The existing Palmer Ave detention basin is located at 284 Palmer Ave and was constructed in the 1980s as part of the Deering Run Subdivision. The detention basin was designed to mitigate stormwater detention time between the pre- and post-development conditions and was not constructed to provide water quality treatment other than some limited settling capacity for settleable solids.

To meet current Maine DEP stormwater treatment standards to the extent practicable, this project will convert the detention basin to a gravel wetland system. A gravel wetland is an underground flow through treatment system where stormwater travels horizontally through a saturated gravel substrate with a microbe rich environment, removing phosphorus, nitrogen, bacteria, suspended solids, and metals from the stormwater. The design also includes a multi-staged outlet to control flows during various storm conditions. For more details on gravel wetlands, check out the Maine DEP Stormwater Best Management Practices Manual.

Scope:

• Approximately 3 feet of excavation depth from the bottom of the existing detention basin footprint and installation of a new impermeable liner.
• Installation of a concrete sediment forebay to provide pretreatment of the stormwater runoff before entering the gravel wetland.
• Installation of approximately 300 feet of storm drain/perforated pipe as well as 1,000 cubic yards of crushed stone and gravel to make up the gravel wetland system.
• Installation of a new outlet control structure, emergency spillway, and riprap plunge pool for conveying various flows.
• Final surface restoration with wetland soil and seed mix.
• The chain link fence will be removed and reset where necessary to complete the work.

Schedule:

• Bidding of this project is expected to take place this winter, with construction taking place over the 2027 construction season.
• In the meantime, please join us for a neighborhood meeting on 11/17 at 5pm to ask questions about the project and provide any feedback you may have. Please refer to the meeting details under "Important Dates" for additional information.

Contact:

Reed Allen, P.E.
Water Resources Project Engineer
Email: rhallen@portlandmaine.gov
Phone: (207) 874-8828