Islanders have designs on the Lagoon Bridge
After six years of discussion, studies, and public hearings, plans for a permanent Lagoon Pond drawbridge replacement finally became a reality. On October 1, Parsons Transportation Group, in conjunction with MassHighway, presented a proposed design during a public meeting at the Martha's Vineyard Commission offices.
"We want to get your input on the permanent bridge," MassHighway bridge project manager Steve McLaughlin told members of the Lagoon Pond drawbridge committee (LPDC), Martha's Vineyard Commission staff, and the public at the meeting. Then, as a next step, he said MassHighway will direct Parsons to complete a 25-percent design plan.
In 2003, MassHighway announced plans to replace the failing Lagoon Pond drawbridge, starting with a temporary bridge, now under construction, next to the existing bridge. Once the temporary bridge is built, the existing bridge will be demolished, and a permanent bridge built in its place. Parsons was chosen as the overall contractor in charge of design and most of the planning for the permanent bridge.
Illustrations by the Parsons Transportation Group, courtesy of the Martha's Vineyard Commission
The temporary bridge is slated for completion in September 2009 and the permanent bridge in 2013. Mr. McLaughlin, project manager for both bridges, said at last week's meeting that the cost of the temporary bridge is estimated at $9.3 million and the permanent bridge at $35.7 million.
At a public planning session for the permanent drawbridge about three years ago, Islanders said they would like a new bridge that allows for better boating access in and out of the Lagoon, straightens a "dog leg" peninsula that juts into the navigational channel, incorporates a multi-user pedestrian/bike path, and is unobtrusive.
Based on the results of several studies, including a marine clearance study that tracked the number and height of boats going in and out of the Lagoon, Parsons Engineering drafted a bridge type study report from which the recommended design was derived.
At last week's meeting, a team from Parsons Transportation Group presented Power Point illustrations of the proposed permanent drawbridge design, using a three-dimensional program that offered views from every angle.
Parsons project manager Leslie Haines said that replacing the deteriorating and unreliable existing bridge provides opportunities to improve sight distances and increase vertical clearance to minimize the drawbridge openings.
Coming up with a design to eliminate the "dog leg" peninsula proved challenging, as it required shifting the channel slightly, Ms. Haines said. Although the goal was to move the channel by 30 feet, the Parsons design team determined that with the bridge's current configuration, it would require an 80-foot bascule span to accomplish that, which would affect the bridge profile and cost more money.
Making the bridge higher also was not a good option, because the higher the bridge, the longer and steeper the approaches would have to be and the greater the possibility of encroachment on the entrance to Eastville Beach on the Oak Bluffs side.
Instead, the design team came up with the idea of using a 50-foot bascule span and moving it one span closer towards Tisbury, so that it is completely outside the existing channel. The resulting 48-foot shift allows for a second channel under the fixed part of the bridge, enabling more boats to go in and out of the Lagoon with fewer drawbridge openings.
The shift in the channel also will aid in the transition during the time the temporary and permanent bridges are in place side-by-side in improving navigability and sight lines for boaters. The clearance under the new bridge will be from 2 to 4 feet higher than it is now, depending on which channel is used.
The permanent bridge construction will take 12 to 14 months. Ms. Haines said the boating channel will have to be closed to marine traffic for a period of time at some point during construction in order to cure concrete. In a follow-up call this week, LPDC chairman Melinda Loberg said the closure is estimated to last 12 to 14 weeks, and all efforts will be made to avoid impacting boating season.
The proposed new drawbridge will be wider than the existing bridge, with two 11-foot wide traffic lanes, two 4-foot wide shoulders, a 6-foot wide sidewalk on the operator's side, and a 10-foot wide multi-user path for pedestrians and bicyclists, as suggested in public hearings.
Mechanical engineer Jeff Keyt designed a set of warning gates and barrier gates that will be installed at each end of the bridge. When not in use, the gates fold down and swing to the side of the guardrails. Mr. Keyt said they are based on similar ones used for the first time on the Woodrow Wilson Bridge that spans the Potomac River near Washington, D.C.