The Project is limited to bank stabilization, replacement of existing concrete bed with a series of boulder cascades, bridge replacement, and installation of a water diversion pipe. Project components are limited to the following activities:
1. Remove and replace an existing 8-foot-wide, 49-foot-long continuous span cement-topped bridge with a 16-foot-wide, 30-foot-long continuous span cement-topped bridge to service full sized emergency and service vehicles. To accommodate larger vehicles, the bridge and stream will be slightly re-aligned towards the right bank to provide a wider turning radius. The new bridge will result in an increase of 80 square feet of shaded stream.
2. Install a 30 linear foot wood lagging retaining wall, supported by six, 18-inch diameter concrete piers from the downstream left edge of the bridge and extending downstream to stabilize the eroding left bank. The retaining wall will be placed at the top of bank, backfilled with a layer of drainage rock and will be 2.8-feet tall.
3. Remove the majority of the concrete and grouted rock spillway (85 linear feet) starting 3 feet downstream of the flashboard assemblage. The existing spillway is undercut and deteriorating, and stream banks downstream have eroded significantly. A small amount of concrete may be left intact on the upper right bank just downstream from the bridge to protect existing riparian trees. The upstream most 20 linear feet of
the spillway will be replaced with two boulder cascades and the downstream 58 linear feet will be replaced with engineered boulder step-pool sequence with 8 boulder weirs. The final 11.7 linear feet will end with another boulder cascade that terminates at an existing scour pool. All existing concrete pieces will be removed from the scour pool then the pool will be rock armored. The edges of the boulder weirs along the
banks will be interplanted with willow (Salix sp.) posts. The step pool and boulder cascade design was informed by an in-depth analysis of site conditions and hydrology to determine appropriate weir spacing and rock configurations and sizing.
4. Install a 93-linear foot keyed rock toe, interplanted with willow posts, on the right bank of the scour pool to restore the severely eroded bank. Three rootwad and footer log complexes will be installed within the rock toe to improve pool habitat complexity. They will be angled in the upstream direction and anchored with boulders and bank backfill. Above the rock toe, a 6-foot-wide, 68-linear foot flood terrace will be constructed to engage flows above the 2-year flood level and will be planted with native riparian tree species. Landward from the flood terrace, vegetated soil lifts (VSLs) will be installed with live willow cuttings and a buried gravel back drain to provide a drainage path for bank seepage. The upper bank will then be graded back to match the VSLs stable 2:1 slope, pushing the top of bank approximately 8-feet landward from existing.
5. Install an 82-linear foot keyed rock toe, interplanted with willow poles, to support a 6-foot-wide floodplain terrace on the left bank slightly downstream of the scour pool. The terrace will be built to the same elevation as the right bank terrace. An existing maintenance building near the top of bank at this location will be removed and relocated at a later date further from the stream.
6. Install a 14-inch diameter pipe to divert pond water from Pond 4 to Pond 3, in accordance with the Marin Country Club Water Rights agreement. Pond 4 is upstream of the bridge and spillway replacement work and the pipe will extend 121 feet, starting 15.5 feet upstream of the replacement bridge and buried underneath the right side of the reconstructed spillway to eventually discharge within the newly built lower cascade. Approximately 324 cubic yards (CY) of sediment (0.2 acres) will be excavated from Pond 4 around the location of the new pipe installation to lower the grade approximately 4 feet and off-hauled to an off-stream storage facility for later reuse as alluvial bedding material to fill the interstitial voids in rock placement and other future upland maintenance applications on the golf course.
7. Stabilize the eroding left bank of Pond 4 for 89 linear feet from an upstream existing footbridge footing to the beginning of the spillway. Install a 3-foot tall rock toe and keyway to support a reconstructed 2:1 slope VSL bank interplanted with native dogwood (Cornus sp.) cuttings. At the top of the new VSL bank, an existing road will be replaced and slightly expanded to provide an adequate turning radius for large
emergency/service vehicles. Stormwater drainage will be directed from the upland portion of the road to a drop inlet in the middle of the replacement bridge.
The Project will result in 1,391 square feet and 55 linear feet of permanent impacts creating new impervious surface associated with bridge and retaining wall construction; 3,514 square feet and 264 linear feet of permanent impacts creating soil, rock planted with willows, or vegetated area associated with erosion control or floodplain terrace construction; and an additional 0.468 acres and 263 linear feet of temporary impacts caused by concrete removal, rock placement, sediment removal, bank stabilization, and temporary site access. To complete the Project, the channel will be dewatered and work will occur in the channel. Two coast live oaks (Quercus agrifolia) may be removed to complete the Project.