Humboldt County Dept. of Public Works - Mattole Road P.M. 5.25 Storm Damage Repair


SCH Number
Lead Agency
Regional Water Quality Control Board, Region 1 (North Coast), Santa Rosa
Document Title
Humboldt County Dept. of Public Works - Mattole Road P.M. 5.25 Storm Damage Repair
Document Type
NOE - Notice of Exemption

Document Description
Winter storms in 2005-2006 resulted in a large landslide that caused 500 linear feet of the roadway surface to sink by up to 15 feet. An existing 24-inch diameter culvert at the north end of the landslide plugged. Flows diverted around the culvert and resulted in damage and erosion of approximately half the roadway width for a distance of 70 feet. Emergency repairs included fixing the culvert outlet and grading the roadway with additional fill material to restore traffic. This project involves reconstruction of the roadway including a stabilization trench with a perforated pipe underdrain. The slide area will be excavated to a depth of 20 feet below the roadway surface. A 3-foot wide and 10-foot deep trench will be excavated parallel to the base of the excavated cutbank for approximately 375 feet. Eight-inch diameter perforated pipe will be placed near the bottom of the stabilization trench and the trench will be backfilled with permeable material. Underdrain outlets will be installed at two locations along the underdrain to convey collected seepage to the hillside below the roadway. A new 18-inch diameter cross drain culvert will also be added under the restored roadway section to convey additional storm water runoff from the inboard ditch to the hillside below the roadway. The proposed project will result in permanent impacts to the roadside drainage ditch including 30sf of wetlands. A larger roadside drainage ditch will be created after the roadway is restored. The newly created roadside drainage area will be monitored by Humboldt County to ensure that impacted wetlands are restored onsite.

Contact Information
Dean Prat
Regional Water Quality Control Board, Region 1 (North Coast), Sa
5550 Skylane Boulevard, Suite A
Santa Rosa, CA 95403

Phone : (707) 576-2801


Other Location Info
West of Honeydew

Notice of Exemption

Exempt Status
Categorical Exemption
Type, Section or Code
Reasons for Exemption
The project consists of replacement of an existing roadway involving no expansion of use.

Disclaimer: The document was originally posted before CEQAnet had the capability to host attachments for the public. To obtain the original attachments for this document, please contact the lead agency at the contact information listed above. You may also contact the OPR via email at or via phone at (916) 445-0613.

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