River Road Over Gill Creek Bridge Replacement (Lake or Streambed Alteration Agreement No. EPIMS-SON-33058-R3)

River Road Over Gill Creek Bridge Replacement (Lake or Streambed Alteration Agreement No. EPIMS-SON-33058-R3)

The project is limited to removal of the existing 40-foot-long and 23-foot-wide bridge that is constricting the Gill Creek channel and replacement with a new free-span bridge measuring 112-feet-long and 36-feet-wide to alleviate channel constriction. New bridge abutments will be located at the top of bank and the new bridge will pass the 100- year storm flow with four feet of freeboard. The new bridge will cause approximately 4,000 square feet of permanent impacts including rock slope protection that will be placed underneath the bridge, which will not be interplanted because plantings would not survive due to lack of sunlight. A temporary detour consisting of a gravel road through the dry streambed, will be constructed just downstream of the existing bridge before the bridge is removed, to allow continued public access for residents living on River Road. An existing concrete weir associated with the current bridge and located downstream of it will also be removed as part of this project to improve fish passage and prevent scour. The weir will be removed prior to construction of the detour road. As part of the detour road construction, a temporary 50-foot-long, 18-inch-diameter corrugated metal pipe culvert will be placed in the dry stream channel before the detour road is constructed in case of summer rain events. Prior to removal of the existing bridge, a temporary gravel work pad will be constructed below the existing bridge and in the footprint of the new bridge. The temporary work pad will allow access for equipment to demolish the existing bridge and to capture debris as the bridge is removed. Once the temporary work pad is installed, the existing bridge and associated wingwalls, measuring in total 1,200 square feet, will be demolished and debris hauled away to proper disposal sites. With the existing bridge demolished, approximately 12,000 square feet (0.27 acres) of the channel will be graded to remove approximately 1,920 cubic yards of accumulated sediment to accommodate the new wider bridge and allow the channel to migrate more naturally. This area of grading and temporary impacts will contain the temporary detour road, the weir to be removed, the temporary culvert, and the temporary work pad. The banks of the channel will be graded at a 2:1 slope and approximately 10,500 square feet (0.24 acres) of rock slope protection will be placed and interplanted with native willow stakes. A portion of this rock slope protection will be placed at the confluence of an unnamed tributary to Gill Creek to prevent erosion on thenorthwest portion of the new bridge. Once earthwork is completed, the new bridge will be constructed onsite. Concrete foundations for the new bridge will be poured into formwork to prevent it from entering the creek channel. While some construction equipment may be working from the existing temporary gravel work pad, all concrete pumps will be staged outside of the channel. Once the foundation has set, the abutments and wing walls will be placed. The location of the abutments and wingwalls will be within the existing roadway, as the new bridge is longer than the existing bridge. The bridge superstructure will be constructed on-site with the use of cranes to lift materials into place. The bridge deck will be made of concrete and poured once the girders are in place. A debris tarp will be hung from the new bridge superstructure to prevent debris from entering the creek channel. This project will result in permanent impacts to riparian and open channel habitat of 2,800 square feet, with an additional permanent impact of 10,500 square feet (0.24 acres) from interplanted rock slope protection. The project will also result in temporary impacts of 12,000 square feet (0.27 acres) to channel habitat and ruderal grasses along the road shoulder. As part of this project, nine riparian trees will be removed. Access and staging will also occur in existing roads and pullouts.

Unincorporated area

Sonoma

Northern California, San Francisco Bay Area

Sonoma