City will create wetland to mitigate undercrossing construction

November 10, 2009

By Warren Kagarise

Crews will build a manmade wetland in Emily Darst Park next year to replace wetlands destroyed when teams break ground on the Interstate 90 Undercrossing and a pedestrian overpass on state Route 900.City planners said the manmade wetland would offset wetlands lost when the city moves ahead with the undercrossing, a long-planned transportation link between north and south Issaquah. In addition to the undercrossing, the Darst Park project will replace sensitive areas removed by the construction of a pedestrian overpass above the westbound ramp to I-90 at state Route 900 and future development on U.S. Postal Service-owned property near the undercrossing.

City Public Works Engineering Director Bob Brock said the wetland project — as well as the first piece of undercrossing construction — would open for bids from contractors by the end of the month. The price tag for the project is $1.6 million.

The undercrossing would link Northwest Gilman Boulevard to Southeast 56th Street via a two-lane roadway built from the traffic signal at the post office. Crews plan to connect the road into the rail corridor behind Gilman Station. The roadway would continue beneath the existing I-90 overpass.

The road would be built within the former railroad right of way. North of I-90, the road would form a T-shaped intersection at Southeast 62nd Street, then continue north along 221st Place Southeast to Southeast 56th Street.

Planners want the undercrossing to alleviate traffic congestion at existing interchanges at Front Street North and I-90, and state Route 900 and the interstate. The planned roadway is scheduled to open by the end of 2010.

The other project included in the Darst Park wetland project, the I-90 pedestrian overpass, will extend the boardwalk trail from the I-90 eastbound off-ramp to Northwest Sammamish Road. Most of the $5.6 million project will be funded with federal and Sound Transit grants.

Darst Park, on the north side of the interstate, is not far from where the undercrossing will be built.

“The scale and type of work is not new to us,” Public Works Engineering Deputy Director Sheldon Lynne said.

Crews will grade land, and add native plants and tree debris to create the Darst Park wetland.

City Environmental Planner Peter Rosen said the effort would be “carefully done to protect the vegetation in Darst Park.”

However, he pointed out how the nature of the project would result in some habitat loss.

“There’s no doubt that there’s displacement of habitat when you’re clearing vegetation,” Rosen said.

The city and the postal service approved a development agreement in August to allow the city use of a right of way on land near the post office.

The agreement requires the city to build the undercrossing and install a traffic signal at the post office entrance. City crews are also responsible for street improvements at the site.

City Council members yielded to environmental concerns and delayed a vote on the agreement after officials and residents raised concerns about wetlands near the undercrossing site.

City planners identified several small wetlands on the land. But Rosen said the wetlands on the post office land were too small to sustain quality habitat and were overrun with invasive plants.

Adding wetlands to replace the small wetlands on the post office property is a process known as “advance mitigation,” because the process will be completed long before the land is developed.

Warren Kagarise: 392-6434, ext. 234, or Comment at

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