18E.80.040 Landslide and Erosion Hazard Area Standards.
A. Active Landslide Hazard Areas. Any development, encroachment, filling, clearing or grading, building structures, impervious surfaces, and vegetation removal shall be prohibited within active landslide hazard areas and associated buffers except as specified in the following standards:
1. Stormwater Conveyance. Stormwater conveyance shall be allowed when it is conveyed through a high-density polyethylene stormwater pipe with fuse-welded joints and when no other stormwater conveyance alternative is available. The pipe shall be located on the surface of the ground and be properly anchored so that it will continue to function in the event of an underlying slide.
2. Utility Lines. Utility lines will be permitted when no other conveyance alternative is available. The line shall be located above ground and properly anchored and/or designed so that it will continue to function in the event of an underlying slide. Utility lines may be permitted when it can be shown that no other route alternative is available. It must be demonstrated by the applicant that a utility line within a landslide hazard area has been designed in a manner that: does not impact the stability of the slope, minimizes or eliminates the potential for rupture or failure, and assures that in the event of failure there will not be a life/safety risk. Appropriate design features such as above ground installation, restrained joint ductile pipe, welded steel pipe, pile supports, the use of high density polyethylene pipe with fuse welded joints, increased wall thickness, special coating, or other measures shall be employed. Automatic shutoff valves shall be provided on fluid or gas transmission lines.
3. Trails. Trails shall be allowed when all of the following conditions have been met:
a. The removal or disturbance of vegetation, clearing or grading shall be prohibited during the wet season (November 1 to May 1).
b. The proposed trail shall not decrease the existing factor of safety within the active landslide hazard area, or any required buffer.
c. The proposed trail shall not create the need for larger landslide hazard area buffers and setbacks on neighboring properties unless approved through a notarized written and recorded agreement with the affected property owners.
d. The proposed trail cannot be located outside the active landslide hazard area or its associated buffer due to topographic constraints of the parcel or size and/or location of the parcel in relation to the limits of the active landslide hazard area and /or its associated buffer.
e. The proposed trail is for non-vehicular use only, and is a maximum of 4 feet in width.
f. Trails shall not be sited within active landslide hazards or their associated buffers when there is such a high risk of landslide activity that the use of the trail would be hazardous.
g. Trails shall be designed and constructed using an engineered drainage system or other methods to prevent the trail surface from becoming a drainage course.
4. Lots may be created that contain an active landslide hazard area as long as the lot is designed in such a way that future development of the lot will not impact the active landslide hazard area or its associated buffer. The created lot(s) shall be designed in such a manner that a sufficient buildable area is provided after all setbacks, pertinent critical area standards, critical area protection measures, and other County regulations are applied.
B. Landslide Hazard Management Areas. All regulated activities may be allowed in areas located within 300 feet of an active landslide hazard area subject to the following standards:
1. The Department reviews and approves a Geological Assessment – geotechnical report and the Department's evaluation indicates that the potential landslide hazard area is stable.
2. The proposed development is located outside of an active landslide hazard area and any required buffer, as set forth in 18E.80.050.
3. The proposed recommendations and mitigation measures contained within the geotechnical report are adequate to reduce or mitigate risks to health and safety.
4. The proposed development shall not cause a decrease in the existing factor of safety within the neighboring active landslide hazard areas or associated buffers. The proposed development shall not decrease the factor of safety within the Landslide Hazard Management area below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the current version of the Washington State Building Code.
5. The removal and disturbance of vegetation, clearing or grading shall be limited to the area of the approved development and shall not be allowed during the wet season (November 1 through May 1) unless adequate provisions for wet season erosion have been addressed in the Geotechnical Report and approved by the Department.
6. Surface drainage from developed areas, including downspouts and runoff from paved or unpaved surfaces up slope, shall not be directed through an active landslide hazard area or its associated buffer unless it is conveyed in conformance with the provisions in 18E.80.040 A.1. above.
7. Stormwater retention facilities, including infiltration systems utilizing perforated pipe, are prohibited unless the slope stability impacts of such systems have been analyzed and mitigated by a geotechnical professional and appropriate analysis indicates that the impacts are negligible.
8. The proposed development shall not create a need for larger landslide hazard area buffers and setbacks on neighboring properties unless approved through a notarized written agreement with the affected property owner(s).
9. The proposed development shall be sited far enough from regressing slope faces to ensure 120 years of useful life for the proposed structure(s) or infrastructure.
10. Lots may be created that are located within or contain a landslide hazard management area as long as the lot is designed in such a way that future development of the lot will not impact the active landslide hazard area or its associated buffer. The created lot shall contain a sufficient buildable area after all setbacks, pertinent critical area standards, critical area protection measures, and other County regulations are applied.
11. Sites that are directly adjacent to any riparian area, wetlands, tidal marshes, and estuaries may be subject to additional buffer requirements and standards as set forth in Chapter 18E.40 PCC, Critical Fish and Wildlife Habitat Areas, or wetlands as set forth in Chapter 18E.30 PCC, Wetlands.