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February 9, 2026

Best in State - Gold Award: Uniqueness and/or Innovative Application of New or Existing Techniques

Photo courtesy of ACEC
The system includes a multi-layered sediment and pollutant removal ‘treatment train’ engineered to preserve the infiltration capacity of the native soils for up to 100 years.

Otak
Monroe Avenue NE Storm System Improvements
City of Renton

The Monroe Avenue Stormwater Treatment and Infiltration Facility represents a breakthrough in urban stormwater management and a model for engineering innovation solving long-standing environmental, flooding, and community space challenges within highly developed basins.

Located in the 260-acre Renton Highlands watershed, the project replaced an outdated infiltration pit with a resilient and highly engineered stormwater treatment and infiltration system. The facility was designed to manage storm events up to the 100-year recurrence interval, improve water quality and deliver lasting public benefits.

One of the project’s most innovative elements is its multi-layered sediment and pollutant removal ‘treatment train’ engineered to preserve the infiltration capacity of the native soils for up to 100 years. This advanced system includes a hydrodynamic separator, a treatment vault with a proprietary BioPod with StormMix media, multiple rows of Stormtech isolator chambers lined with geotextile fabric for sediment capture, and the novel use of geotextile seagull layers installed in two-foot lifts throughout 25 feet of imported infiltration gravel.

Developed collaboratively by civil and geotechnical engineers, this technique provides a final line of sediment defense while allowing future maintenance crews to selectively remove or refurbish portions of the gravel or chambers at low cost, extending the life of the facility and reducing long-term operations needs.

Because clean infiltration gravel meeting strict fines criteria was difficult and costly to source in the needed quantities, Otak engineered additional sediment removal redundancies so the facility could safely use more readily available AASHTO #3 gravel. The surface of the facility was designed as a community asset — an open green space with gentle slopes, trees, shrubs, and turf suitable for neighborhood gatherings and informal recreation. Perimeter fencing and planted berms enhance safety, comfort, and aesthetics while maintaining access for maintenance personnel.

Extensive hydraulic and hydrogeologic modeling was performed to optimize flow control, distribution and infiltration. Multiple alternatives — including open ponds, buried vaults, and direct discharge to the Cedar River — were evaluated for performance, maintenance, environmental impact, constructability and cost.

The project site was privately owned by Segale Properties, which sought to develop the surrounding land. Otak and the city of Renton created a solution allowing the city to acquire the site, restore it, and create an amenity that benefits both the new development and the broader community. The project was completed four months earlier than planned, under budget, and with minimal construction changes — a testament to effective planning, coordination and engineering.


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