Big Gulch Sanitary Sewer Reconstruction Project
A Brief Historical Overview

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The Project

Five different scenarios were evaluated by the team of experts for reconstructing the sewer pipeline down the gulch. It was determined that replacing the pipeline down the gulch was the best option. To protect the new pipeline from the increased stormwater flows, the project also required the stormwater flows be reduced back down to a level equivalent to what was running down the gulch before the adjacent lands were developed.

To do this, a new $165,000 stormwater headworks structure was constructed by the District at the top of the gulch, on the south side of Harbour Pointe Blvd., across the street from the library. This device separates the stormwater flows during rainfall events and allows only a small portion of the stormwater flows to go down the creek. The remaining portion of water (what we call the excess stormwater flows resulting from the adjacent development) is diverted into a large bypass pipeline. This pipeline runs down the gulch, bypassing the sensitive stream environment above, and returns the stream flow back into Big Gulch Creek below the end of the new sewer pipeline. This bypass pipeline also captures the uncontrolled stormwater pipes from the neighborhood developments adjacent to the gulch.

The new sanitary sewer pipeline was installed down the gulch. This pipeline replaced the damaged pipeline that was originally constructed in 1969. Along with this pipeline, a third pipeline was also placed to accommodate the future needs for transporting fluids up and/or down the gulch to the wastewater treatment plant.

These pipelines are all buried several feet under the creek. To do this required the protection and extensive restoration of the creek environment. Resident fish populations were temporarily relocated, streambed gravels were protected, and the stream flows diverted around the construction zone.

The restoration of the stream and adjacent areas is a major expense to the project. The stream itself was reconstructed using proven techniques that incorporated root wads, tree segments, selected streambed gravels, and plant materials. All of this work was required and approved by the Washington State Dept. of Fish and Wildlife, the Department of Ecology and the City of Mukilteo, at a significant cost to the project.

An additional cost of the project is the establishment of maintenance access down into the gulch. This has been developed from two directions and will ensure MWWD is able to maintain the pipeline and protect the stream well into the future.

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