Article 5. Geologically Hazardous Areas
15.18.510 Designation.
Areas susceptible to one or more of the following types of hazards are hereby designated geologically hazardous areas, in accordance with WAC 365-190-080(4)(a).
A. Erosion hazard areas are areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a moderate-to-severe, severe, or very severe rill and interrill (sheet wash) erosion hazard.
B. Landslide hazard areas are areas subject to landslides based on geology, soils, topography, and hydrology, including:
1. Areas delineated by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a severe limitation for building site development;
2. Areas mapped by the Washington Department of Ecology (Coastal Zone Atlas) or the Washington State Department of Natural Resources (slope stability mapping) as unstable (U or Class 3), unstable old slides (UOS or Class 4), or unstable recent slides (URS or Class 5);
3. 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;
4. Areas where the following coincide: slopes steeper than 15 percent, a relatively permeable sediment overlying a relatively impermeable sediment or bedrock, and ground water seepage;
5. Areas that have shown movement in the past 10,000 years or that are underlain or covered by mass wastage debris of that time frame;
6. Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials;
7. Slopes steeper than 80 percent subject to rock fall during seismic shaking;
8. Areas potentially unstable because of rapid stream incision, stream bank erosion, and undercutting by wave action;
9. Areas at risk from snow avalanches;
10. Canyons or active alluvial fans subject to debris flows or catastrophic flooding; and
11. Slopes of 30 percent or steeper with a vertical relief of 10 or more feet except areas composed of consolidated rock.
C. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting. One indicator of potential earthquake damage is a record of past earthquake damage. Settlement and soil liquefaction occur in areas underlain by cohesionless, loose, or soft-saturated soils of low density, typically in association with a shallow ground water table.
D. Mine hazard, volcanic, and tsunami hazard areas (none known to be present; see WAC 365-190-080).
E. Other hazard areas include areas susceptible to mass wasting, debris flows, rock falls, and differential settlement. [Ord. 492 § 3 (Exh. B § 161), 2007.]