Sahalee Way Stormwater Tightline

Sahalee Way Tightline

Summary

A portion of Sahalee Drive east of the Timberline Neighborhood in the City of Sammamish currently has a series of perforated underdrain piping, installed below the roadway in the 1980s, to drain groundwater from the area. Most of the underdrain pipes are connected directly to an existing, buried High-Density Polyethylene (HDPE) Pipe that conveys the emerging groundwater from the Sammamish Plateau northwards and downslope to a King County stormwater pond along NE 50th Street. Several underdrain pipes were not connected to the HDPE pipe and instead, discharge water at ground surface at the top of a slope. This has resulted in excessive erosion that, over time, may result in large-scale slope failures along the Sahalee Way hillside.

This project proposes to capture the emerging groundwater into a tightline system and convey the groundwater collected from these underdrains to the existing HDPE pipe conveyance system to the north. It is believed that due to the topography of the slope and existing vegetation present, a conveyance system consisting of a below-grade pipe installed through a traditional open-cut trench process is infeasible. The City is proposing a surface-staked pipe system to convey the water to the conveyance system.

Project Updates

PACE Engineers, Inc. is the City’s consultant for design on this project. We are currently (August 2017) performing a topographic survey of the site and performing a geotechnical site analysis.

Timeline

July 2017 – Project kick-off
August 2017 – Site survey and geotechnical and hydrogeologic site investigations
November 2017 – Final geotechnical and critical area reports. Apply for King County Permits
February/March 2018 – Construction bid opening
Summer 2018 – Construction of new pipe

Permits and Environmental Review

Coming soon, following City staff meeting with King County Department of Permitting and Environmental Review (September 2017)