SE 172nd Street Water Quality Retrofit Project

Design

The Surface Water Utility solicited proposals from consultants in accordance with City policy 252-02. Five proposals were received and evaluated, and two consultant firms were selected for interviews. Osborn Consulting, Inc. ranked the highest and was selected to perform the work.

Council awarded the professional services contract to Osborn Consulting, Inc., in the amount of $257,889, at its regularly scheduled meeting on April 15, 2019. This contract has since been amended in the amount of $360,165.

Project work under the contract agreement and amendment includes survey, geotechnical, and civil engineering design services.

Construction

The Surface Water Utility advertised the construction contact in accordance with city policy 252-02. Five bids were received and evaluated. OMA Construction, Inc. was identified as the lowest bidder. Council awarded the construction contract to OMA Construction, Inc., in the amount of $3,838,896.77, at its regularly scheduled meeting on April 21, 2025.

This project will improve the water quality of stormwater runoff from roadways before it enters downstream natural systems. Roadway runoff is currently discharged untreated to a wetland south of 127th Avenue SE and SE 172nd Street. This stormwater continues south until eventually discharging into Big Soos Creek, a stream with known water quality issues related to pH, dissolved oxygen, and ammonia levels. Using Ecology-approved biofiltration systems, this project will provide enhanced basic water quality treatment to stormwater runoff from approximately 14 acres of area containing approximately two acres of untreated roadway.

This project will construct new stormwater conveyance pipe and catch basins to increase conveyance capacity of the stormwater system and reduce the likelihood of flooding. In order to help mitigate risk to homeowners and prevent future intrusive maintenance work, upstream stormwater runoff will be rerouted away from existing stormwater pipe systems running through homeowner properties and into the new stormwater system.

New pervious concrete sidewalks will be constructed on one side of each street within the project area. In tandem with new curbs, gutters, and landscape strips, the pervious concrete sidewalks will improve pedestrian safety by providing a pedestrian path separated from traffic by a landscape strip. Additionally, the porous nature of the pervious concrete sidewalks increase pedestrian safety and provide runoff mitigation for the new sidewalk surfaces.

Completed project milestones are as follows:

• April 2019: awarded professional services contract to Osborn Consulting, Inc.
• July 2019: 30% design complete
• September 2020: 60% design complete
• August 2021: 90% design complete
• April 2024: 95% design complete
• July 2024: 100% design complete
• March 2025: construction contract bid advertisement
• April 2025: awarded construction contract to OMA Construction, Inc.

Construction expected to begin July 2025.