City Seeks Gallagher Hill Watercourse Erosion Fix

Mercer Island City Hall is looking to hire contractors for a new stormwater project on Gallagher Hill to protect Lake Washington from siltation and water quality problems from a hillside basin there suffering from erosion.

A request for bids was issued Aug. 12 for the "channel and bank stabilization in the Sub-Basin 4 ravine" within the Gallagher Hill Open Space. Repairs to the watercourse basin is one of many repair projects on the Island's ravines and basins which have watercourses, which are causing "downcutting" and erosion in various locations.

Project limits include Sub-Basins 4.2, 4.3c and 4.4. Construction will utilize sustainable practices through minimizing trucking of materials to/from the site, minimize use of mechanically manufactured quarry rock, and use large woody debris and other natural practices where possible. Disturbed areas will be replanted with native vegetation.

Bids will be inspected and publicly read on Aug. 23
 
Spoecifically, the project will stabilize and restore a 600-foot reach in Sub-Basin 4 that is experiencing active channel down cutting and sloughing and approximately 200 ft of tributary stabilization. Work includes but not limited to channel dewatering, channel grading and reconstruction with streambed material, woody debris and boulders, removal of invasive vegetation, replanting of disturbed banks and slopes with native plants, erosion control, and minor repair and installation of additional support for undermined side slope.

The total cost of the project is estimated at $327,000.