Dixon Mine Road Bridge Replacement Project (Streambed Alteration Agreement No. 1600-2016-0114-R2)

Dixon Mine Road Bridge Replacement Project (Streambed Alteration Agreement No. 1600-2016-0114-R2)

CDFW has executed Streambed Alteration Agreement No. 1600-2016-0114-R2, pursuant to section 1602 of the Fish and Game Code to the project Applicant, Alpine County (Permittee) as represented by Brian Peters. The proposed project consists of replacing the existing Dixon Mine Road bridge over Wolf Creek with a new cast-in-place concrete slab bridge. This new bridge would be located on the existing alignment with geometric enhancements that meet County roadway standards. The new bridge would be 25 feet wide by 65 feet long with two travel lanes and a roadway width of 18 feet. The bridge would be a single span structure on two 15-foot tall concrete abutments founded on driven steel piles. The abutments would be installed near the top of each bank of Wolf Creek, above the main creek channel. The foundations would be configured so that future scour would not threaten the bridge. The piles would be driven about 30 feet below the existing ground surface, which represents the maximum depth of ground disturbance during construction. Improvements to the roadway approaches and installation of the new bridge would require an estimated 400 cubic yards of imported material and excavation of an estimated 190 cubic yards of material. Temporary falsework would be installed across the creek at the location of the new bridge to help support the bridge structure as it is being constructed. The falsework would only be in place while the creek is dry and would be removed once the bridge is complete. Because Wolf Creek is a perennial stream, flows would need to be diverted around the in-stream work area. The diversion structure may consist of clean gravel, plastic visqueen, and plastic/metal culverts or pipes. Cofferdams consisting of clean gravel and lined with impervious plastic visqueen may be constructed up and downstream of the work area. Culverts or pipes would be installed through both cofferdams parallel to the creek to allow the water to bypass the work area and discharge into the creek downstream ofthe work area. Three or four plastic pipes would convey flow through the dewatered work area. The existing bridge would be demolished, removed fromt eh project area as construction progresses, and properly disposed of. Rock slope protection would be placed along the new abutments to protect them from scour due to creek flows. An estimated 510 cubic yards of 1-ton rock slope protection and backing are expected to be needed along the abutments. The area along the abutments would be excavated then back-filled with the rock slope protection. A 5-foot wide temporary path would be installed across the creek to provide limited public and construction access to the northeast side of the creek. It would be about 340 feet longa nd provide a detour around the work area. The path would be built along the upstream side (i.e., southeast) of Dixon Mine Road by contouring portions of the existing road embankment and importing fill material to create a relatively level bench. The path would require an estimated 320 cubic yards of imported material and excavation of an estimated 15 cubic yards of material form the embankment. The surface would be covered with Class 2 Aggregate Base. The path would allow construction workers and their equipment to cross the creek by foot, horseback, bicycle, and small state-registered or federally owned off-highway vehicles (e.g., quads, dirt bikes). After the new bridge is operational, the path would be dismantled by removing the temporary fill material and re-contouring the road embankment. The total estimated area of impact on habitats in the project area is less than 1 acre. Permanent impacts would result from the cuts and fills along the roadway and installation of the new bridge. These improvements would require the removal of vegetation and ground disturbance in Jeffrey pine forests along Dixon Mine Road and in riparian wetlands and the seasonal wet meadow south and east of Dixon Mine Road, resulting in total permanent impacts on approx. 0.1 acre of Jeffrey pine forests, 0.0006 acre of riparian habitat, 0.007 acre of riverine habitat, and 0.004 acre of seasonal wet meadow. No tree removal would be necessary in the project area. Within the Dept.'s jurisdiction, the bridge and adjacent road improvements would result in permanent impacts on 0.017 acre of Jeffrey pine forests, 0.006 acre of riparian habitat, and 0.007 acre of rivervine habitat in Wolf Creek. An estimated 270 cubic yards of soil and other material would be excavated where the existing abutments are removed and at the proposed rock slope protection (RSP) areas in Wolf Creek. the RSP would back fill the excavated area. Temporary impacts would result from construction of the temporary path across the creek from Dixon Mine Road south of the bridge to a point along the road northeast of the bridge. The path would result in temporary impacts on approx. 0.04 acre of Jeffrey pine forests, 0.004 acre of riparian habitat, 0.05 acre of riverine habitat in Wolf Creek, and 0.01 acre of seasonal wet meadow. This area would be restored following construction activities to pre-disturbance contours and with native vegetation. Temporarily fill used for the path would be removed from the project area. Some of the fill used to build the path would be incorporated into the new road bed of Dixon Mine Road; this fill is included in the permanent imapcts above. Within the Dept.'s jurisdiction, the temporary path and temporary construction activities along teh Creek for bridge installation would result in temporary impacts on 0.007 acre of Jeffrey pine forests, 0.004 acre of riparian habitat, and 0.053 acre of riverine habitat in Wolf Creek.

Alpine