Sonoma County supervisors Tuesday approved a design contract to replace a historic steel-truss bridge near Sonoma over objections of preservationists who called the action premature.

Watmaugh Bridge was built in 1929 to span Sonoma Creek and connect what is today Arnold Drive with Highway 12 leading into Sonoma.

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. The span has the second-lowest rating for structural safety of any in Sonoma County, according to Caltrans data.

"If I can't, and this county can't, do something about the safety about that bridge ASAP, then I think the responsibility we are experiencing is overwhelming," Sonoma Valley Supervisor Valerie Brown said.

Supervisors voted unanimously to award the $500,000 design contract to Moffatt and Nichol, a global engineering firm based in Long Beach. The firm will study other alternatives to replacing the span, including a retrofit or building a parallel bridge downstream.

County transit planners prefer the option of replacing the bridge with one that is 32 feet wide -- 10 feet wider than the current span. They said Caltrans has agreed to restore and replace the bridge's steel trusses, which the county considers to be the bridge's defining historic trait.

But Supervisor David Rabbitt expressed reservations about that plan, saying restored trusses might not fit the bridge's new design, add to costs and "be ugly."

Winemaker Jim Bundschu, one of about a dozen preservationists and neighbors to address the board Tuesday, said a new span would detract from Sonoma Valley's "ambiance."

"It's the last historical bridge in Sonoma Valley that hasn't been bastardized by being modernized," he said.

Several speakers urged supervisors to delay awarding the contract until they had more information about the bridge's condition -- which they say is not as serious as what county staff has made it out to be -- and alternatives to replacing it.

"Approving a half-million-dollar design contract without understanding the bridge is incomprehensible," said Ken Niles, who lives near the bridge.

The county Landmarks Commission voted last week 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.

Nancy Simpson, Sonoma Valley's designee to the Landmarks Commission, noted that Board of Supervisors in 1998 designated Watmaugh and 11 other bridges as part of a countywide historic bridges district.

The bridge in 1981 was designated a county historic landmark after preservationists saved it from being torn down.

"This is an opportunity for the current board to respect past supervisors' efforts to preserve our historic bridges," Simpson said.

But Supervisor Efren Carrillo said "safety is the number one issue we should be concerned with here."

Gary Helfrich, executive director of the Sonoma County Bicycle Coalition, told supervisors that the Watmaugh bridge is "perhaps the most dangerous bridge for cyclists in Sonoma County.

The county's plans call for putting new bike and pedestrian lanes on a replacement bridge.

County staff say the total cost of replacing the bridge to be $4 million to $5 million, and that the state and federal funds available to do the work are tied to building a new span.

The project also will include an environmental review under the state California Environmental Quality Act.

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