Sonoma County transit planners want to begin work on replacing a historic steel-truss bridge near Sonoma over the objections of preservationists and neighbors.

County supervisors are to decide Tuesday whether to award a $500,009 design contract to a Long Beach firm to replace Watmaugh Bridge, which was built in 1929 to span Sonoma Creek and connect what is today Arnold Drive with Highway 12 leading into Sonoma.

The bridge is one of two steel truss spans in Sonoma Valley and in 1981 was designated as a county historic landmark after preservationists saved it from being torn down.

Transit planners and preservationists are at odds now over whether the bridge needs to be replaced or whether it can be retrofitted.

County officials say the bridge is at risk of collapse during an earthquake or major flood because of erosion around the piers that support it. But preservationists contend officials are inflating the risks and the span can be brought to safety standards without having to replace it.

County staff say it would cost as much to retrofit the bridge as it would to replace it and the state and federal funds available to do the work are tied to building a new span.

If supervisors don't authorize the design work, the project can't proceed and the county will have to reimburse Caltrans $100,000 in grant funds, said Tom O'Kane, the county deputy director of public works.

He said the state Transportation Commission has been "very firm that they want these seismic projects to be finished."

Moffatt and Nichol, a global engineering firm based in Long Beach, is poised to do the design work. O'Kane said no preference was given to local firms because the Watmaugh project involves no county funds.

He estimated the cost of replacing the bridge to be $4 million to $5 million and the current timetable for preparing an environmental review and completing the work is early 2015.

The county Landmarks Commission voted Tuesday to recommend a replacement bridge not be built because of the potential effects on the span's historical character and because alternatives had not been fully explored.

The commission has the authority to review changes to the bridge under a 1998 Board of Supervisors resolution that designated Watmaugh and 11 other bridges as part of a countywide historic bridges district. The ultimate decision rests with county supervisors.

County planners have made changes to their original proposal to alleviate concerns expressed by commissioners and by people who live near the current span. That includes scrapping a plan to build an entirely new overpass near where the current bridge stands.

The county's current plans call for replacing the bridge with one that is 32 feet wide. It would include bike and pedestrian lanes and be eight feet narrower than initially proposed. The current span is 22 feet wide.

The county also is offering to rehabilitate the trusses and attach them to the replacement bridge.

You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com