40.430.020 Standards
A. General.
The following requirements for development activities in geologic hazard areas list prohibited activities, buffer requirements, and setback requirements. The following section describes required buffers and setbacks, and general requirements for development activities in geologic hazard areas.
1. Development on steep slope hazard areas is regulated to prevent potential landslide damage by placing improvements away from steep slopes and leaving steep slopes in natural vegetation.
2. Development in landslide hazard areas is generally not allowed, and requires buffers that keep vegetation in a natural state on and around the landslide hazard area.
3. Seismic hazards due to liquefaction, ground shaking amplification and landslides exist for large areas of the county. Only detailed site analysis can determine how soils and structures will respond at a particular site. Site investigation requirements of the International Building Code are used to ensure that structures are built to minimum safety standards based on existing knowledge of earthquake hazard. Section 40.430.020(F) provides additional guidelines describing where site investigations should be required for seismic design.
4. If an applicant wishes to perform development activities not allowed by Sections 40.430.020(D) and (E), a geologic hazard area study meeting the requirements of Section 40.430.030(C)(5) must be completed. The development proposal may be approved, approved with conditions, or denied based on the responsible official’s evaluation of the suitability of the mitigation measures proposed by the geologic hazard area study to protect life, safety, and slope stability on abutting properties.
(Amended: Ord. 2005-04-15)
B. Erosion Requirements.
All activities on hillsides subject to severe erosion hazard must minimize erosion by following management practices prescribed by the erosion control standards of Chapter 40.386.
(Amended: Ord. 2005-04-15; Ord. 2009-01-01; Ord. 2015-11-24)
C. Stormwater Requirements.
For projects within one hundred (100) feet of steep slope hazard areas or landslide hazard areas, runoff shall not be infiltrated into the ground. Runoff should be directed through a water-tight pipe beyond the base of the slope or landslide area and discharged to a suitable drainage way. An energy dissipating device shall be placed at the discharge point.
(Amended: Ord. 2005-04-15)
D. Steep Slope Hazard Areas.
1. Except for mineral extraction practices, development activity on or within one hundred (100) feet of slopes steeper than forty percent (40%) that do not have a mapped or designated landslide hazard shall comply with the requirements of this section.
2. Buffer and Setback Distances.
a. Activities at the base of ascending slopes (building at the bottom of a steep slope):
(1) For slopes greater than or equal to forty percent (40%) and less than one hundred percent (100%), buffers shall extend a distance away from the toe of the slope that is equal to the vertical height of the slope divided by two, but not to exceed fifteen (15) feet (Figure 40.430.020-1). For slopes less than one hundred percent (100%), the toe of the slope is defined as a distinct break in slope at the base of a steep slope.
(2) For slopes greater than one hundred percent (100%), the buffer shall extend a distance back from the toe of the slope equal to the height of the slope divided by two, not to exceed fifteen (15) feet. The buffer shall be measured horizontally from a plane, drawn tangent to the top of the slope at an angle of forty-five (45) degrees to the proposed structure (Figure 40.430.020-3).
(3) The setback shall be eight (8) feet beyond the buffer.
b. Activities at the tops of descending slopes (building at the top of a steep slope):
(1) For slopes greater than or equal to forty percent (40%) and less than one hundred percent (100%), buffers shall extend a distance back from the top of the slope equal to the vertical height of the slope divided by three (3), but not to exceed forty (40) feet. The top of the slope is defined as a distinct break in slope at the top of a steep slope (Figure 40.430.020-1).
(2) For slopes greater than one hundred percent (100%), the buffer shall extend a distance back from the top of the slope equal to the height of the slope divided by three (3), but not to exceed forty (40) feet. The buffer shall be measured horizontally from a plain drawn at forty-five (45) degrees (one hundred percent (100%) slope) from the toe of the slope to the proposed structure (Figure 40.430.020-2).
(3) The setback shall be eight (8) feet beyond the buffer.
c. For projects not required to have a landslide protection area under Section 40.430.030(B), the setback from the steep slope shall be equal to the buffer distance set in this subsection.
3. The responsible official may approve buffers and setbacks which differ from those required by Section 40.430.020(D)(1) if the applicant submits a geologic hazard area study described in Section 40.430.030(C), which technically demonstrates and illustrates that the alternative buffer provides protection which is greater than or equal to that provided by the buffer required in Section 40.430.020(D)(1).
4. The responsible official may increase buffers or setbacks where necessary to meet requirements of the International Building Code.
5. All portions of steep slope hazard areas and steep slope buffers on the site which are planned to be undisturbed by permitted development activities shall be designated as landslide protection areas in accordance with Section 40.430.030(B).
6. Other than for exemptions listed in Sections 40.430.010(B)(3) and 40.430.030(B), vegetation removal is not allowed on slopes over forty percent (40%) without an approved geologic hazard area study described in Section 40.430.030(C)(5).
7. Buffers, landslide protection areas and setbacks for steep slopes on projects having approved grading shall be based on regulated steep slopes that remain after that grading.
(Amended: Ord. 2005-04-15)
E. Landslide Hazard Areas.
1. A development proposal on a site containing a landslide hazard area shall meet the following requirements:
a. A minimum buffer of fifty (50) feet shall be established from all edges of the landslide hazard area. The buffer shall be extended as required to mitigate a steep slope or erosion hazard or as otherwise necessary to protect the public health, safety and welfare. In cases where the diameter of the landslide area is smaller than fifty (50) feet, the buffer width may be reduced to less than fifty (50) feet at the discretion of the department;
b. All portions of landslide hazard areas and buffers shall be designated as landslide protection areas in accordance with Section 40.430.030(B).
2. Other than exempt activities, clearing or alteration of a landslide is allowed only if the following are met:
a. A development proposal does not decrease slope stability on contiguous properties;
b. Mitigation is based on best available engineering and geological practice and is described in an approved geologic hazard area study as specified in Section 40.430.030(C)(5);
c. Such clearing or alteration of a landslide is certified safe as designed and under anticipated conditions by a registered geotechnical engineer or geologist licensed in the state of Washington.
3. Neither buffers nor a landslide protection area will be required if the activity meets the requirements of Section 40.430.020(E)(2).
(Amended: Ord. 2005-04-15)
F. Seismic Hazard Areas.
1. Development activity in a seismic hazard area shall meet all applicable provisions of the most recently adopted version of the International Building Code, as adopted by the county.
2. Buffers are not required for seismic hazard areas, except for fault rupture hazard areas where the buffer will be a minimum of fifty (50) feet and shall be one hundred (100) feet for critical facilities.
Figures 40.430.020-1 through Figure 40.430.020-3
(Amended: Ord. 2005-04-15)