Maple Road and Ash Way Intersection and Drainage Improvements

Project Description

Snohomish County and the City of Lynnwood are working together to address flooding in the vicinity of the intersection of Maple Road and Ash Way. Frequent flooding of this intersection forces multiple road closures per year, negatively impacting access into and out of Lynnwood and emergency vehicle response times.

The project area is adjacent to a large, high quality wetland complex (approximately 300 acres) associated with Swamp Creek. The project area generally drains to the northeast and south toward Swamp Creek through ditches, stormwater pipes, culverts, and a Washington State Department of Transportation (WSDOT) stormwater pond.

During relatively moderate rain events Swamp Creek swells, increasing water runoff into storm pipes and culverts, reducing flow capacity. The intersection is the low point and during high flow events the existing culverts become plugged with sediment causing flooding on the roadway. There is minimal grade change between Swamp Creek and the roadway intersection which contributes to the frequent flooding.

The project area is also underlain with peat up to 30 feet thick. Peat is very compressible and the weight of the existing roads has compacted the peat, contributing to settlement of the intersection area.

Snohomish County and the City of Lynnwood propose to raise the elevation of the intersection and increase the capacity of the drainage system. Approximately 600 feet of Ash Way and 700 feet of Maple Road would be raised from 0’ to approximately 5’ above the current elevation using a combination of pin piles, pile caps, grade beams. A structural slab would be used to reconstruct the roadway. Pin piles would be driven 37 to 42 feet below the ground surface through the peat soils and down to a stable glacial till layer. The elevation of the new intersection will be a minimum of six inches above the existing 25-year flood elevation of Swamp Creek.

In addition to raising the road, it is also necessary to increase the capacity of the drainage system to reduce upstream flooding. This project will install new culverts on Maple and Ash Way.

Environmental Review - Public Notice

A SEPA Addendum was issued March 17, 2017

A State Environmental Policy Act (SEPA) Environmental Checklist was issued for this project on July 15, 2016. The drainage system has been modified from the original design to reduce impacts to the adjacent wetlands and streams. A SEPA Addendum has been issued due to the changes in the proposed project design.

An environmental review of the Maple Road and Ash Way Improvemenst has been completed in compliance with the State Environmental Policy Act (SEPA). SEPA requires that all state and local agencies identify and evaluate the environmental impacts of their land use decision. An Environmental Checklist has been prepared and a Determination of Non-Significance has been issued for this project. These documents are available by viewing the links below. The comment period for the SEPA Environmental Checklist ended August 8, 2016.