Article 11. Geologically Hazardous Areas
14.100.400 Geologically Hazardous Areas—Designation.
A. Geologically hazardous areas include areas susceptible to erosion, sliding, earthquake, or other geological events. The following regulations, in combination with the performance standards for development, will guide development in these critical areas. The purpose of these regulations is to maintain the natural integrity of hazardous areas and their buffers in order to protect adjacent lands from the impacts of landslides, mudslides, subsidence, excessive erosion and seismic events, and to safeguard the public from these threats to life or property. Geologically hazardous areas are designated as those areas that are susceptible to one of more of the following types of hazards:
1. Erosion hazard;
2. Landslide hazard;
3. Seismic hazard;
4. Other geological events including tsunamis, mass wasting, debris flows, rock falls, and differential settlement.
B. Some geological hazards can be reduced or mitigated by engineering, design or modified construction or mining practices so that risks to public health and safety are minimized. When technology cannot reduce risks to acceptable levels, building in geologically hazardous areas must be avoided.
C. Erosion Hazard Areas. Erosion hazard areas include areas likely to become unstable, such as bluffs, steep slopes and areas with unconsolidated soils. Erosion hazard areas are those areas of Aberdeen which:
1. Contain soils or soils complexes identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service or the Soil Survey for Grays Harbor County as having “moderate to severe,” “severe” or “very severe” erosion hazard potential;
2. Are impacted by shore land and/or stream bank erosion; or
3. Areas with a slope greater than fifteen (15) percent.
D. Landslide Hazard Areas. Landslide hazard areas shall include areas subject to landslides based on a combination of geologic, topographic and hydrologic factors. They include any areas susceptible to landslide because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology or other factors, and include, at a minimum, the following:
1. Sensitive Sloped Areas. Slopes exceeding forty (40) percent with a vertical relief of ten (10) or more feet except areas composed of bedrock and properly engineered slopes designed and approved by a geotechnical engineer licensed in the state of Washington and experienced with the site. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least ten (10) feet of vertical relief;
2. Areas mapped by the Washington State Department of Ecology (Coastal Atlas) or the Washington State Department of Natural Resources (slope stability mapping) as unstable (“U”), unstable old slides (“UOS”), or unstable recent slides (“URS”);
3. Areas designated by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having “severe” limitation for building site development;
4. Areas that have shown evidence of historic failure or instability, including but not limited to, back-rotated benches on slopes; areas with structures that exhibit structural damage such as settling and racking of building foundations; and areas that have toppling, leaning, or bowed trees caused by ground surface movement;
5. Slopes greater than fifteen (15) percent that have a relatively permeable geologic unit overlying a relatively impermeable unit and having springs or ground water seepage;
6. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action, including channel migration zones;
7. Areas located in a canyon or active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding;
8. Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Washington State Department of Natural Resources;
9. Areas that are at risk of mass wasting due to seismic forces; and
10. Slopes having gradients steeper than eighty (80) percent subject to rock fall during seismic shaking.
E. Seismic Hazard Areas. Seismic hazard areas must include areas subject to severe risk of damage as a result of earthquake-induced ground shaking, settlement or subsidence, slope failure, soil liquefaction, surface faulting, or tsunamis. Settlement and soil liquefaction conditions occur in areas underlain by cohesionless soils of low density, typically in association with a shallow ground water table. One indicator of potential for future earthquake damage is a record of earthquake damage in the past. Ground shaking is the primary cause of earthquake damage in Washington, and ground settlement may occur with shaking and include:
1. Areas subject to surface faulting during a seismic event;
2. Areas with underlying deposits indicative of a risk of liquefaction during a seismic event, including those areas mapped as “moderate,” “moderate to high,” and “high” by the Washington State Department of Natural Resources;
3. Areas subject to slope failure during a seismic event;
4. Areas that are at risk of mass wasting due to seismic forces.
Seismic hazards shall be as identified in Washington State Department of Natural Resources seismic hazard and liquefaction susceptibility maps for Western Washington and other geologic resources.
F. Tsunami Hazard Areas. Tsunami hazard areas are coastal areas susceptible to flooding and inundation as the result of excessive wave action derived from seismic or other geologic events, and include those areas mapped within the Tsunami Hazard Map of the Southern Washington Coast by the Washington Department of Natural Resources.
G. Other Hazard Areas. Geologically hazardous areas must include areas determined by the director as those areas subject to severe risk of damage as a result of other geological events including volcanic hazard areas subject to pyroclastic flows, lava flows, debris avalanche, or inundation by debris flows, lahars, mudflows, or related flooding resulting from volcanic activity, mass wasting, debris flows, rock falls and differential settlement.