Sub Basin 42.1 and 42.1a Watercourse Stabilization
Project Status
To help keep area residents informed of our construction progress, we will post project updates to this site on a regular basis. Click here to see status updates.
Project Details/Neighborhood
- Watercourse east of East Mercer Way near 6500 block
Background
Temporary streambank erosion control measures (sandbags) installed more than 15 years ago are beginning to fail which poses risks to stream banks and increases potential for sedimentation and impacts to habitat in Lake Washington.
Description
Stabilize 1,200 linear feet of eroded watercourse east of East Mercer Way near 6500 block with a series of stream bed control structures that are made of logs and cobbles in the channel. Also install timber frame network on soft banks to reduce sediment transport. Where the channel has a steeper slope at the upstream reach, large stream rocks will be placed to protect the toe of the existing gabion baskets from undercutting.
Project Timeline
Construction in Q3 2021
Estimated Budget
$389,657
Contact
Fred Gu, City of Mercer Island Project Manager: (206)275-7803 or fred.gu@mercerisland.gov
www.mercerisland.gov/storm21-27
Status Updates
February 2022 Update
The contractor returned in late December and completed re-planting work along the stream banks and at timber frame structures with native plants and trees.
November 2021 Update
The contractor will return in late fall and winter to complete re-planting work along the stream banks and at timber frame structures with native plants and trees.
Update September 9, 2021
Photo 1. Completed timber frame structure
Photo 2. Completed streambed control structure
Photo 3. Downstream end of stream cobble lined channel off EMW
All in-water works were completed as of last Friday, right before the Labor Day long weekend. These works included finishing all fifteen streambed control structures, seven timber frame structures, and placing approximately 100 feet of stream cobble off East Mercer Way in the ravine where existing gabion baskets were being undermined.
To restore the site and prevent soil erosion from construction activities, a thick layer of mulch has been applied to the areas that were disturbed by machine access, while coir fabric is used to cover exposed steep slope.
Contractor will return in late Fall and Winter to complete re-planting work along the banks and at the timber frame structures with native plants and trees.
Update August 27, 2021
Lane closer
On Monday 8/30, the northbound lane of East Mercer Way by 6500 block will be closed from 8am to 7pm. This single-lane closer may take up to two days if the scheduled work cannot be completed on Monday.
Contractor for the Sub Basin 42 Watercourse Stabilization Project has scheduled a crane to be used when placing large stream cobbles down the steep ravine for approximately 100 LF off East Mercer Way.
Traffic control and flaggers will be on-site to direct traffic.
Update August 24, 2021
Photo 1. Streambed Control Structure (before completion)
Photo 2. Timber frame structure (mid construction)
Construction activities continue at the lower 1000 feet of stream. Last week the contractor has begun installing fifteen streambed control structures into the existing sandbag weirs to help hold the existing material in the streambed. The bed control structure is constructed with logs, slash, stakes, and streambed cobble. The result is expected to resemble log jam that would have naturally formed at the site, providing bed-stabilizing benefits like protecting the banks from erosion at the toe, adding hydraulic roughness to the channel bed, slowing flows to better capture and store sediment in-situ.
Timber frame structures are another major component of the project. Seven of them are being installed to stabilize the soft banks and to provide soil stability to support reestablishment of riparian plant communities. As the vegetation becomes more established, it will provide long-term stabilization of the banks as well.
The contractor hopes to have all in-water work completed by the first part of September.
Update August 11, 2021
Photo 1. Slash is used to protect access route along the banks
Photo 2. Tree protection along the access route
Last week, Accord Contractors, began to mobilize to the lower staging area in preparation for installing the series of bed control structures in the stream bed and timber frame structures on the soft banks.
This week, access route along the banks is being established, materials for the structures are being stockpiled, temporary stream by-pass pump and hoses are being installed as well. Since these works occur outside of major arterial, traffic disruption to the public is kept to minimal. However, when the contractor switches gears to the upper reach work, ie installing large stream rocks to the first 100 LF of stream off East Mercer Way, temporary one lane closure on East Mercer Way at around 6500 block is expected and it will likely last for 1-2 days.
Update June 28, 2021
Contract awarded to Accord Contractors LLC
21-27 location map (2 MB)