Stormwater Road Improvement Project – StormTank®

Low Impact Development

Background Information and Challenge

The Seattle Hill Road Stormwater Road Improvement Project was a 13.2 million dollar project for road improvements in Washington and was a collaboration between Snohomish County Public Works and the City of Mill Creek. The project required 1.6 miles of roadway to be widened from 2 lanes to 3, a center turn lane, resurfacing, fewer hills and sharp turns, a dedicated sidewalk and bike lane, planter strips, and tree box filters. It would also have grass-lined retention areas and porous concrete sidewalks. 

The existing road had 15,000 passenger vehicles and 67 buses per day using a 2-lane roadway through a medium-density residential district with ongoing development. The roadway had many issues, including no dedicated turn lane, sidewalks, or bike lanes. It also had many potholes brought on by frequent floods due to an insufficient stormwater system, resulting in more frequent collisions.


Layfield worked with the project partners to approve using the StormTank® system rather than a more traditional stormwater detention system. Layfield’s line of subsurface stormwater storage units can be flexibly designed to meet your specific site requirements and achieve compliance with strict runoff regulations. Layfield provided and installed 5,000 StormTank® units for the 1.6-mile road improvement project, which will provide retention of approximately 35,000cf of stormwater that can be pre-treated and metered into the local stormwater system.


The Brentwood systems will alleviate historic roadway flooding and soil saturation and allow pre-treatment before introducing it to the larger stormwater system.

The new stormwater road improvement project also provided space for porous sidewalks, bike lanes, and planter strips, replacing the current storm drain and dry well system, which gave a very narrow road shoulder. The customer is pleased with the installation to date, and Snohomish County intends to allow specification of the Brentwood StormTank® for future stormwater detention and retention applications.


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