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New Storm Water Requirements

Published:  September 19, 2016

[Originally published December 10, 2015. Updated September 19, 2016.]

Sonoma County is now required to enforce new storm water requirements that were adopted by the North Coast Regional Water Quality Control Board.

Increased Wet Season Grading Restrictions

  • No grading of slopes greater than 10% (previously 20%) from October 1 to April 30th without an exception from Sonoma County. Exceptions require a review of a monitoring plan monitoring program and inspections showing no degradation of water quality.

  • All grading permits will require pre-project area slope on the cover sheet.

Slope Only Update on RB1 Storm Water Requirements for MS4 Permit 2016 to 2021 (PDF: 36 kB)

Low Impact Design Requirements for the North Coast Region

  • All new development and redevelopment projects creating or replacing 10,000 square feet or more of impervious surface require post-construction best management practices (BMP) implementation. BMPs must:

    • Treat following pollutants of concern including particulate metals, pathogens, nutrients, hydrocarbons, trash, fine sediment, and other debris.

    • Be sized to treat all of the runoff generated using the modified Rational Method with an intensity of 0.2 inches per hour and capture (infiltrate, evapotranspirate, and/or reuse) the increase in storm water runoff volume generated by the site due to the increase in impervious surface for a one inch rain event over a 24 hour period using the Curve Number Method.

    • Filter or treat the flow rate of runoff produced by the 24 hour 85th percentile rain event hourly rainfall intensity (for each hour of a storm event), as determined from the local historical rainfall record, multiplied by a factor of two.

    • Detention facilities which are integrated for hydraulic system design may be used to provide volume capture and/or treatment if the design meets the design criteria specified for LID.

    • Have a recorded maintenance declaration stating that the property owner will maintain the BMPs and the have the appropriate funding and technical ability to inspect, maintain, and provide BMP maintenance records.

For more information see BMP Inspection and Maintenance.