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Bridge-tunnel inspires transit planners

Scandinavian project studied for 520 ideas

Whatever is built above or below Lake Washington to disentangle the state Route 520 mess might have a Scandinavian influence.
The opening of the 9.5-mile Oresund bridge and tunnel between Denmark and Sweden Saturday not only advanced the science of crossing complicated bodies of water but got others thinking about the possibilities.

Among those are the Seattle-based Discovery Institute. The public-policy think tank sees the Interstate 5 corridor from British Columbia to Oregon, and the rail and highway systems within, as the catalyst for economic growth in the Pacific Northwest.

It is particularly concerned about the overcrowded 12.5 miles of Route 520, including the aging Evergreen Point Bridge.

There are 120,000 jobs in the two-mile-wide corridor around the limited-access highway.

Replacing the floating bridge and its approaches has long been the subject of regional studies and forums. The latest conference — “State Route 520, a Corridor in Crisis” — was held last week in Kirkland and was sponsored by the institute.

Among the agenda items: the $3.5 billion Oresund Link, connecting Malmo, Sweden, and Copenhagen, Denmark, by rail and highway. The link, across a strait leading into the Baltic Sea, consists of an artificial island with a tunnel on one side and a long bridge on the other.

“We are going to have to look at the world’s technologies and how it is done,” Bruce Agnew, who heads the institute’s transportation project, said this week.

The institute has shown an interest in an underwater alternative to bridging Lake Washington, as well as a “cut-and-cover” tunnel replacing the nearly 50-year-old Alaskan Way Viaduct along Seattle’s central waterfront.

The 47-member Trans-Lake Washington Study Committee, created by the Legislature in 1998 to look for a better way of connecting Seattle and the Eastside, last year discarded the idea of tunneling under the lake, which is more than 200 feet deep, saying it would be too costly and impractical.

However, a combination bridge-tunnel that involves tubes suspended above the bottom remains on the table, as does the possibility of tunneling under Montlake.

“We didn’t get into any level of detail,” said Melissa Loomis, a spokeswoman for the cross-lake study, which is being conducted through the state Department of Transportation’s Urban Mobility Office in Seattle.

The suspended tube and Montlake tunnel concepts do not appear in the recommended alternatives the cross-lake study group submitted for public comment last month. Loomis said this is because they need more study.

The alternatives to date are:

* Replacing the bridge with the same number of lanes — four — but with shoulders and bike-pedestrian lanes.
* A “minimal footprint,” four-lane alternative, plus better accommodation for transit and high-occupancy vehicles.
* Different combinations of HOV lanes, transit and general lanes, plus environmental and neighborhood improvements. This alternative could include two bridges, as well as incentives for car pooling and transit use.

The public comment period ends Aug. 3. After that, the recommendations, plus others proposals raised, will undergo further study.

Agnew will submit a report of last week’s conference as part of the public-comment process.

He said the institute’s interest in the Oresund Link centers on building a span for severe current and weather conditions, as well as “minimizing environmental impacts” in laying precast sections of tunnel in a trench on the bottom of the Oresund strait.

The three sections of the east-west Oresund Link include the 2.2-mile tunnel with two rail tubes and two highway tubes, a 2.5-mile artificial island and the 4.8-mile bridge in three sections.

About 12,000 vehicles a day paying a minimum of $25 each way are expected to use the link — far fewer than the 120,000 vehicles that cross the Evergreen Point Bridge daily. Passengers on trains using the link pay $7.50.

A major criteria for the link’s design and construction was that it not affect the flow of salt and oxygenated water through the Oresund east to the Baltic Sea, or affect the eelgrass on the seabed.

Spillage of sediment from dredging was controlled to avoid harm to herring migration, birds searching for food in shallow waters and mussel banks.

Agnew said the methods used to build the bridge-tunnel could also offer environmental advantages here, where Lake Washington becomes shallow in Union Bay and flows through the estuaries surrounding Foster Island below the current route of 520.

“If you look at the Montlake area,” Agnew added, “you are going through a very sensitive marine environment” — one argument for tunneling under it.

In fact, Montlake residents have been the staunchest opponents of expanding 520 through their neighborhood — at least above ground.

The institute is also looking at methods to pay for building under or over the lake.

Options include a toll similar to that proposed for a new 520 bridge in the early 1990s but pushed aside by 1994 for lack of political support.