Overlake Station Infiltration Vault: Regional Stormwater Flow Control and Runoff Treatment Adaptive Management at Project and Program Scale

The City of Redmond’s regional facilities program provides flow control and runoff treatment to reduce stream flooding, improve water quality, and facilitate dense urban development. Partnering with redevelopment projects through a connection fee, the program maximizes urban density, collocates stormwater infrastructure with Parks and Transportation infrastructure and implements LID principles to fully retrofit a 400-acre basin. Construction of a 4.5 acre-foot infiltration vault followed construction of a 20 acre-foot detention vault modeled to work together as the first two of three projects to meet the City’s vision. Design/Build Partnership: The City’s goal of constructing the infiltration vault was complicated by existing commercial development and plans for a new light rail station at the same site. A partnership with the City, Sound Transit and the design/build team completed the design in a way that met the goals of both agencies for efficient construction of the vault without premature impacts to the existing development or disruption to the construction of the light rail station. Design and construction flexibility were critical to project success. Adaptive Management: Constructing the regional infiltration vault required excavation through 27 feet of low permeability glacial till. A full understanding of the soil conditions was impossible during preliminary design due to the existing buildings at the site. Testing to the extent possible was conducted to build a picture of the conditions, but only full excavation would tell the whole story and allow for final design. The design-build plan allowed for soils testing once the full extent of the infiltration interface was excavated. Follow-up performance testing is informing design for the next regional vault. This adaptive management was only possible through acceptance of some risk — that was mitigated through flexible project and program goals.

Learning Objectives:

1. Implement a regional stormwater facilities program that meets regulatory requirements, improves water quality, reduces flooding, and supports urban redevelopment.

2. Leverage design/build principles to meet multiple project goals through inter-agency partnership, careful planning, risk management, and flexible design.

3. Plan for unknown site conditions and conduct field investigations during construction to support modeling and design to adaptively manage and keep a project on schedule while meeting project objectives.

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