16.50.400 Designation of Specific Hazard Areas

16.50.400 Designation of Specific Hazard Areas.

(1) Erosion Hazard Areas. Erosion hazard areas are those areas identified by the U.S. Department of Agriculture – Natural Resources Conservation Service (USDA-NRCS) as having a “moderate to severe,” “severe,” or “very severe” rill and inter-rill erosion hazard. Rill erosion tends to occur on slopes, particularly steep slopes with easily-erodable soils or poor vegetation. Erosion hazard areas also include those areas with a slope greater than 15 percent.

(2) Landslide Hazard Areas. Landslide hazard areas are areas potentially subject to landslides based on a combination of geologic, topographic, and hydrologic factors. They include areas susceptible because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors. Example of these may include, but are not limited to the following:

(a) areas of historic failures, such as:

(i) those areas delineated by the USDA-NRCS as having a “severe” limitation for building site development for factors other than slope for one or more types of building development;

(ii) those areas mapped by the 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); or

(iii) areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Department of Natural Resources;

(b) areas with all three of the following characteristics:

(i) slopes steeper than 15 percent;

(ii) hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and

(iii) springs or ground water seepage;

(c) areas that have shown movement during the Holocene epoch (from 10,000 years ago to the present) or that are underlain or covered by mass wastage debris of that epoch;

(d) slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials;

(e) slopes having gradients steeper than 80 percent subject to rock fall during seismic shaking;

(f) areas potentially unstable because of rapid stream incision, streambank erosion, and undercutting by wave action;

(g) areas that show evidence of, or are at risk from snow avalanches;

(h) areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; and

(i) any area with a slope of 40 percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock.

(3) Seismic Hazard Areas. 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 for future earthquake damage is a record of earthquake damage in the past. Ground shaking is the primary cause of earthquake damage in Washington. The strength of ground shaking is primarily affected by:

(a) the magnitude of an earthquake;

(b) the distance from the source of an earthquake;

(d) the type of subsurface geologic structure.

Settlement and soil liquefaction conditions occur in areas underlain by cohesionless, loose, or soft-saturated soils of low density, typically in association with a shallow ground water table.

(4) Mine Hazard Areas. Mine hazard areas are those areas underlain by, or affected by mine workings such as adits, gangways, tunnels, drifts, or airshafts, and those areas of probable sink holes, gas releases, or subsidence due to mine workings. Steep and unstable slopes may be created by open mines (e.g. open basalt rock pits, rock quarries, sand and gravel pits). Factors that should be considered include: proximity to development, depth from ground surface to the mine working, and geologic material.

(5) Volcanic Hazard Areas. Volcanic hazard areas are areas subject to pyroclastic flows, lava flows, debris avalanche, inundation by debris flows, lahars, mudflows, or related flooding resulting from volcanic activity.

(6) Other Hazard Areas. Geologically hazardous areas shall also include areas determined by the Director of Community Development to be susceptible to other geological events including mass wasting, debris flows, rock falls, and differential settlement. (Ord. 21-15 §6, 2021; Ord. 19-6 §23, 2019; Ord. 03-18 §41, 2003).