BIG ROCK TRAIL STREAM CROSSING REPLACEMENT PROJECT

Summary

SCH Number
2024101169
Public Agency
Marin County Open Space District (MCOSD)
Document Title
BIG ROCK TRAIL STREAM CROSSING REPLACEMENT PROJECT
Document Type
NOE - Notice of Exemption
Received
Posted
10/28/2024
Document Description
The proposed project consists of improving a failing trail stream crossing by removing a damaged culvert, failing retaining wall, and associated fill from an ephemeral stream crossing on the Big Rock Trail, and replacing the existing trail stream crossing with a free-spanning trail bridge. The purpose of the proposed project is to improve trail sustainability and visitor safety by removing the threat of a potential stream crossing failure and damage to downstream habitats, restoring habitat lost by the original placement of culvert and fill within the ephemeral stream channel, and continueing to provide public access with the least environmental impact. The damaged stream crossing is located approximately 4,150 feet from the Big Rock Trailhead on Lucas Valley Road. It consists of a 24-inch diameter by 16-foot lonog high density polyethylene (HDPE) pipe and a 6 foot tall by 40-foot long “Sutter” style retaining wall. The stream crossing culvert rests on bedrock at the natural stream grade and runs perpendicularly through the wall and associated backfill that comprises the trail tread. The wall and backfill extend 20-feet on either side of the culvert to span the steep side slopes flanking the stream. An extensive crack runs longitudinally through the entire length of the culvert. This has compromised the strength of the culvert and its ability to support the fill material above it. In addition, the culvert is no longer water tight. Stream flows leaking through the broken culvert may induce piping in the fill material and lead to further failure and collapse. The wall and backfill have also sustained damage, as they have been rotated approximately 10 degrees beyond vertical. Further roatation will eventually lead to failure of the wall and fill. The proposed project would remove the retaining wall, the stream crossing culvert, and all associated fill, and would then install a free-spanning bridge in its place. A SWECO trail dozer, mini excavator and power toter would be utilized to excavate and remove all associated fill, approximately 10 cubic yards. The fill material would be incorporated into the adjacent segments of the trail where it would not erode or deliver sediment to the stream. The retaining wall system and HDPE culvert would be completely dismantled and hauled off-site to a disposal facility. A 40-foot long Corten steel stiringer bridge with redwood decking would be installed to replace the culvert and retaining wall. The bridge would rest on and be affixed to concrete footers poured in-place, and located beyond the top-of-bank. Construction access and staging would be from the Big Rock Trailhead at Lucas Valley Road. Mini excavators, generators, power totes, cement mixers, electrical tools and hand tools would be utilized for bridge construction. Project implementation, including dismantling of the existing culvert and wall structure would take approximately 4 weeks. During this time, the Big Rock Trail would be closed to public access. Applicable Polices and Best Management Practices included in the MCOSD’s Road and Trail Management Plan would be implemented, including dry season construction, and implementation of erosion control and water pollution prevention measures, and avoidance/minimization measures associated with special status species. Additional measures that may be required as a result of regulatory permit authorizations would also be implemented. Regulatory permit authorizations would include a Lake and Streambed Alteration Agreement (LSAA, #1600 Permit), issued by the California Department of Fish and Game, and General 401 Water Quality Certification for Small Habitat Restoration Projects, issued by the San Francisco Bay (Region 2) Water Qualtiy Control Board. A Section 404 Clean Water Act Permit for the discharge of dredged or fill materials issued by the United States Army Corps of Engineers would not be required because the channel in the project area has been classified as an ephemeral stream, and ephemeral streams are exempt from 404 Permit regulation.

Contact Information

Name
Michelle Julene
Agency Name
Marin County Open Space District
Job Title
Senior Regulatory Planner
Contact Types
Lead/Public Agency

Location

Counties
Marin
Regions
Countywide
Cross Streets
Lucas Valley Road
Parcel #
164-310-14

Notice of Exemption

Exempt Status
Categorical Exemption
Type, Section or Code
Section 15302: Replacement or Reconstruction
Reasons for Exemption
The proposed project consists of improving a failing trail stream crossing by removing a damaged culvert, failing retaining wall, and associated fill from an ephemeral stream crossing on the Big Rock Trail, and repacing the existing trail stream crossing with a free-span trail bridge at the same site. The proposed trail bridge would have the same purpose and capacity as the existing culvert, retaining wall, and trail. The proposed project is located in an emphemeral stream environment however, all construction activity will be outside the stream and therefore, no impacts to the stream would result. The concrete footers for the proposed trail bridge would be constructed upslope and beyond the top of the streambank. The proposed project is not located on a hazardous waste site pursuant to Government Code Section 65962.5. The project area is not located within an officially designated state scenic highway and nonetheless would not affect scenic resources including trees, historical buildings, rock outcroppings, or similar resource. Implementation of the proposed project would not result in cumulative impacts; result in a significant effect to the environment due to unusual circumstances; or change the significance of a historical resource. Removal of the culvert and failing retaining wall is expected to result in beneficial effects to the ephemeral stream and Miller Creek by reducing existing and future sediment delivery and restoring continuity of the channel within the project area.

Exempt Status
Categorical Exemption
Type, Section or Code
Section 15333: Small Habitat Restoration
Reasons for Exemption
The proposed project consists of improving a failing trail stream crossing by removing a damaged culvert, failing retaining wall, and associated fill from an ephemeral stream crossing on the Big Rock Trail, and replacing the existing trail stream crossing with a free-spanning trail bridge. The purpose of the proposed project is to improve trail sustainability and visitor safety by removing the threat of a potential stream crossing failure and damage to downstream habitats, restoring habitat lost by the original placement of culvert and fill within the ephemeral stream channel, and continuing to provide public access with the least environmental impact. The total area would not exceed five acres in size. The proposed project would remove approximately 10 cubic feet of fill, consisting of the damaged culvert, failing retaining wall, and associated fill from the ephemeral stream environment and the proposed bridge would span the entire stream crossing. The concrete footers for the proposed trail bridge would be constructed upslope and beyond the top of the streambank. The ephemeral stream within the project area is a tributary to Miller Creek, which supports native salmonids. According to Acomb and Resnik (2009) , stream habitat that may support fish is located one mile downstream of the project site. The existing trail stream crossing has the potential to fail catastrophically and deliver sediment directly to the emphemeral tributary and to Miller Creek. Sediment is a pollutant that negatively impacts instream habitat utilized by endangered salmonids. The proposed project would result in beneficial effects to the ephemeral stream and Miller Creek by reducing existing and future sediment delivery and restoring continuity of the channel within the project area. Implementation of the proposed project would not result in significant adverse impacts on endangered, rare, or threatened species or their habitat pursuant to Section 15065. The proposed project is not located on a hazardous waste site pursuant to Government Code Section 65962.5. The project area is not located within an officially designated state scenic highway and nonetheless would not affect scenic resources including trees, historical buildings, rock outcroppings, or similar resource. Implementation of the proposed project would not result in cumulative impacts; result in a significant effect to the environment due to unusual circumstances; or change the significance of a historical resource.
County Clerk
Marin

Attachments

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