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 Thurston County
 Code of Ordinances
 Title 24. CRITICAL AREAS
 Chapter 24.20. FREQUENTLY FLOODED AREAS

  § 24.20.055. Channel migration hazards areas—Delineation—Unmapped hazard areas.  

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  • If the approval authority determines that a proposed use along a Type S or F stream is within a historic channel migration zone, based on field conditions, historic information, LIDAR imagery or aerial photography, and the one-hundred-year channel migration hazard area has not been mapped, the approval authority shall require the applicant to determine if a one-hundred-year channel migration hazard area is present on the site and, if so, delineate its location and extent.

    A.

    The determination as to whether the one-hundred-year channel migration hazard area affects the subject property shall be based on the findings of a qualified professional proficient in fluvial geomorphology using a reliable methodology to determine channel migration accepted by the department (e.g., as described in the Washington Department of Natural Resources' Forest Practices Board Manual, Standard Methods for identifying Channel Migration Zones and Bankfull Channel Features, dated 8/2001, as amended; or in "A Framework for Delineating Channel Migration Zones," Washington Department of Ecology, 2003, as amended). Maps delineating the one-hundred-year channel migration hazard area shall be of a scale and format specified by the department.

    B.

    The following areas shall be considered outside of the one-hundred-year channel migration hazard area:

    1.

    Areas separated from the stream channel by a legally established structure that the approval authority, in consultation with a qualified professional, determines will block channel migration. This may include, but is not limited to, dikes, levees and public roads that extend above the one-hundred-year flood elevation that are constructed to remain intact through a one-hundred-year flood. Constraints to channel migration that do not extend above the one-hundred-year flood elevation shall not be considered to limit channel migration unless demonstrated otherwise based on technical information; and

    2.

    Areas separated from the stream channel by a geologic feature, such as a rock outcrop, that the approval authority determines, in consultation with a qualified professional, will stop channel migration.

(Ord. No. 14773, § 3(Att. B), 7-24-2012)