Sir Francis Drake MP 20.28 Retaining Wall (Notification of Lake or Streambed Alteration, No. EPIMS-MAN-32313-R3)

Sir Francis Drake MP 20.28 Retaining Wall (Notification of Lake or Streambed Alteration, No. EPIMS-MAN-32313-R3)

The California Department of Fish and Wildlife has executed Lake and Streambed Alteration Agreement EPIMS Notification No. EPIMS-MAN-32313-R3, pursuant to Section 1602 of the Fish and Game Code to Marin County Department of Public Works. The Project is limited to installing a Cast-In-Drilled-Hole (CIDH) retaining wall to restore the roadway shoulder that started to erode due to intense storm events in October 2021, backfilling behind the retaining wall, regrading a slope below the retaining wall after damage caused by erosion, replacing a portion of a culvert, and placing rip-rap at drain outfalls. Piers for the CIDH retaining wall will consist of two-foot diameter reinforced concrete cast in holes drilled to approximately 30 feet below ground level at the base of the wall. Concrete will be poured in molds constructed around the piers to create the remainder of the retaining wall. The wall will be approximately 72 feet long and 15 feet high and installed downslope of the road. A segment of 15-inch diameter High Density Polyethylene (HDPE) culvert that crosses the planned location of the retaining wall will be removed and replaced in kind, then joined to segments of the existing culvert on either side. Removing this section of the culvert is necessary to access the area and to build the retaining wall. When the wall is complete, the area between the wall and road will be backfilled and compacted. Three trenches nine feet long and one foot wide will be excavated, and an eightinch HDPE subdrain will be placed in each trench before they are backfilled. The pipes will reach the surface of the slope approximately 10 feet from the wall face, and a 3-foot by 3-foot area of rip-rap will be placed where each pipe surfaces to dissipate water leaving the wall from the three subdrains. The area immediately below the wall will then be graded to approximately a 4:1 slope until it meets the existing slope and revegetated with native plants. All equipment staging and access will occur in the road or road shoulder. The Project will result in permanent impacts to 27 square feet and 9 linear feet of riparian vegetation and permanent impacts to 144 square feet and 72 linear feet of ruderal vegetation, grasses, and unvegetated area above the topof-bank and outside of the riparian zone. The Project will result in temporary impacts to approximately 16 square feet and 2 linear feet of riparian vegetation. The Project will result in the creation of approximately 216 square feet and 72 linear feet of riparian vegetation. The Project will not remove trees. The Project will not dewater the stream and no work will take place in the wetted portion of the stream.

Marin

San Francisco Bay Area

Marin