Sonoma County, CA Environmental Rules: Erosion Control (2026)

Chapter 11 of the Sonoma County Code (Construction Grading and Drainage) requires a construction-grading permit before any grading, land clearing, vegetation removal, or other ground disturbance not specifically exempted. Section 11.04.010 sets out permit thresholds; Permit Sonoma's GRD-011 Erosion Prevention and Sediment Control Notes are the standard plan-sheet requirement. During the County-defined wet season (October 15 through May 15) the area of erodible land exposed at any one time may not exceed 1 acre or 20% of the permitted work area, whichever is greater, and time of exposure must be minimized to the maximum extent practicable. Outside the wet season, BMPs (silt fence, fiber rolls, straw wattles, stabilized construction entrances, hydromulch or other surface protection) must be in place and functional on any day when the National Weather Service forecasts 30% or greater chance of rain within the next 24 hours. Chapter 11A (and where applicable Chapter 36, the Vineyard and Orchard Site Development Ordinance - VESCO) impose additional protections on hillside sites: any new vineyard or orchard on slopes 10-50% requires a VESCO erosion-control plan, water-quality and biotic-resource setbacks from streams (typically 25-200 ft depending on stream class), and post-construction vegetative cover. Slopes over 50% are generally prohibited from new vineyard/orchard development. The County coordinates inspections with the Regional Water Quality Control Boards under the MS4 permits and the State Construction General Permit (sites of 1 acre or greater also require a SWPPP filed with the State Water Board).