Cascade Drive Box Culvert and Retaining Wall Repair Project (Notification of Lake or Streambed Alteration, No. EPIMS-MAN-49628-R3)

Cascade Drive Box Culvert and Retaining Wall Repair Project (Notification of Lake or Streambed Alteration, No. EPIMS-MAN-49628-R3)

The California Department of Fish and Wildlife has executed Lake and Streambed Alteration Agreement EPIMS Notification No. EPIMS-MAN-49628-R3, pursuant to Section 1602 of the Fish and Game Code to the City of Mill Valley. The Project is limited to repairs to a retaining wall and a concrete box culvert. Repairs to the retaining wall will be made by removing the existing timber piles and replacing with steel soldier piles anchored by concrete. Repairs to the concrete box culvert will be made by injecting pressurized cement grout into the voids underneath the invert slab. Prior to Project implementation, the work area will be dewatered to ensure access to voids beneath the invert slab and proper curing time for injected cement. Dewatering will follow the guidelines provided in the National Marine Fisheries Service (NMFS) Programmatic Biological Opinion (2016), unless otherwise required by the NMFS Biological Opinion issued for the Project. Cofferdams will be placed upstream and downstream of the work area, and Old Mill Creek will be allowed to partially dewater naturally. Then for approximately 160 linear feet, the creek will be temporarily dewatered via a submersible pump that will pump from various deep pools into a banker tank on a flatbed truck. The water will be transported offsite and disposed of at a regulated sites by the contractor. The pumped water will not be released downstream, into any other stream, or into a storm drain system. The repairs to the retaining wall will begin by phasing the pile installation (pile numbers 1, 4 and 7, then piles 2, 5 and 8, lastly piles 3, 6, and 9), working with no more than three piles at a given time. The existing piles will be cut at the base, then the remnants and concrete will be drilled out 12 to 15 feet deep leaving a 16-inch diameter hole for the new steel piles to be installed. The unsupported wall lagging on each side of the existing pile will be laterally braced against the opposite creek bank. Bracing will be removed once the concrete at the base of the newly placed steel pile is cured. Since old timber piles may contain traces of preservatives in them, the sawdust from cutting the piles will be vacuumed and collected for appropriate disposal along with the piles themselves. The repairs to the box culvert will include removing the protruding one-foot cement slab at the inlet, and the two-foot extension at the downstream end of the culvert, then remove and/or tie new rebar in the invert slab, and lastly, inject cement grout into the void under the slab (Exhibit A). The project will result in temporary impacts to approximately 1,112 square feet and 160 linear feet of stream habitat caused by dewatering, installation and removal of cofferdams, replacing of concrete, and other project impacts. The Project will involve the potential removal of a coast redwood (Sequoia sempervirens) with two trunks between pile numbers 1 and 2. The Project will not result in permanent impacts other than the potential removal of this tree.

Marin

San Francisco Bay Area

Marin