VII. GEOLOGIC HAZARD AREAS
20.25H.120 Designation of critical area and buffers.
A. Designation of Critical Areas. The following geologic hazard areas are hereby designated critical areas subject to the regulations of this part:
1. Landslide Hazards. Areas of slopes of 15 percent or more with more than 10 feet of rise, which also display any of the following characteristics:
a. Areas of historic failures, including those areas designated as quaternary slumps, earthflows, mudflows, or landslides.
b. Areas that have shown movement during the Holocene Epoch (past 13,500 years) or that are underlain by landslide deposits.
c. Slopes that are parallel or subparallel to planes of weakness in subsurface materials.
d. Slopes exhibiting geomorphological features indicative of past failures, such as hummocky ground and back-rotated benches on slopes.
e. Areas with seeps indicating a shallow ground water table on or adjacent to the slope face.
f. Areas of potential instability because of rapid stream incision, stream bank erosion, and undercutting by wave action.
2. Steep Slopes. Slopes of 40 percent or more that have a rise of at least 10 feet and exceed 1,000 square feet in area.
3. Coal Mine Hazards. Areas designated on the Coal Mine Area Maps or in the City’s coal mine area regulations, LUC 20.25H.130, as potentially affected by abandoned coal mines; provided, that compliance with the coal mine area regulations shall constitute compliance with the requirements of this chapter in regard to coal mines.
4. Seismic Hazards. Areas of known faults or Holocene displacement, based on the most up-to-date information, or areas mapped areas of “moderate to high” or “high” hazard liquefaction susceptibility by the Washington Department of Natural Resources Liquefaction Susceptibility Map of King County, Washington, 2004, as amended.
B. Geologic Hazard Area Buffers. The following critical area buffers are established:
1. General Geologic Hazard Critical Area Buffers.
a. Landslide hazards: Top-of-slope buffer of 50 feet.
b. Steep slopes: Top-of-slope buffer of 50 feet.
2. Existing Development. Where a primary structure legally established on a site prior to August 1, 2006, encroaches into the critical area buffer established in subsection B.1 of this section, the critical area buffer and structure setback shall be modified to exclude the footprint of the existing structure. Expansion of an existing structure into the critical area buffer shall be allowed only pursuant to the provisions of LUC 20.25H.065.
3. Buffer Modification. Modifications to the geologic hazard critical area buffer may be considered through a critical areas report, LUC 20.25H.230.
C. Structure Setbacks.
1. General. The requirements of this section apply along with any other dimensional requirements of the Land Use Code (see LUC 20.20.010, 20.20.130, 20.20.190 and Parts 20.25A through 20.25G). The most restrictive dimension controls. Structure setbacks are required in order to:
a. Minimize long-term impacts of development adjacent to critical areas and critical area buffers; and
b. Protect critical areas and critical area buffers from adverse impacts during construction.
2. Minimum Setback of Structures.
a. Landslide hazards: Determined based on site-specific geotechnical studies to reflect site characteristics, including site topography and conditions that may be conducive to fast moving, shallow debris slides and flows.
b. Steep slopes: Toe-of-slope setback of 75 feet.
3. Structure Setback Modification. Structure setbacks may be modified only through an approved critical areas report. (Ord. 6417, 5-21-18, § 39; Ord. 5680, 6-26-06, § 3)