Porter Creek Road Bridge Replacement Project

The project includes replacement of the existing Porter Creek Bridge over Porter Creek with a new wider bridge and construction of in-channel habitat enhancements. The Porter Creek Bridge was constructed in 1948 and does not meet current seismic standards, which makes it vulnerable to collapse in a large earthquake. Caltrans found cracks and fissures in some of the steel beams that help support the bridge. In an emergency action, county maintenance crews have temporarily re-supported the bridge using steel beams. The project will also construct safety improvements, including adequate shoulders and a turn lane on the upstream side of the bridge.

The replacement bridge is a single span concrete box girder approximately 160 feet long with 12-foot-wide traffic lanes in each direction of travel, a 12-foot-wide left turn channelization lane and two 10-foot-wide shoulders. The center left turn channelization lane would be used for vehicles accelerating onto Porter Creek Road from Franz Valley Road and residential driveways. In total, the traveled width of the bridge will be 56 feet. The bridge will be built in two stages. The first stage consists of one-lane bridges built upstream and downstream of the existing bridge. The second stage would be to remove the old bridge and construct a fill-in bridge between the two one-lane bridges to make a single 56-foot-wide bridge. Instream permit restrictions are in effect from October 15 – June 15, each construction season.

LOCATION: Sonoma County, CA
CLIENT: Sonoma County
PROJECT VALUE: $6 million


  • Bridge Replacement
  • Precast Girder Bridge
  • Creek Diversions
  • Waterline and Environmental Endangered Species
  • High Risk Utilities


  • Resident Engineering
  • Structure Representative
  • Inspection
  • Office Engineering