19.70.415 Designation.
A. Areas susceptible to one or more of the following types of hazards are designated as a geologically hazardous area:
1. Erosion Hazard Areas. Erosion hazard areas include areas likely to become unstable, such as bluffs, steep slopes, and areas with unconsolidated soils. The following are considered known or suspected erosion hazards:
a. Areas located within the following soil map units: Nos. 46 and 47 Dystric Xerochrepts; No. 90 Lithic Haploxerolls-Rock outcrop complex (LH); or mapped severe erosion hazard, as identified in the U.S. Department of Agriculture Natural Resources Conservation Service Soil Survey of Skagit County Area, WA (1989)
.
b. Coastal, erosion-prone areas such as beaches or marine bluffs.
c. Areas susceptible to rapid stream incision and stream bank erosion.
2. Landslide Hazard Areas. Landslide hazard areas are those areas subject to landslide activity 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. The following are known or suspected landslide hazards:
a. Areas designated in the Washington State Department
of Ecology Coastal Zone Atlas, Washington, Volume Two Skagit County (1978) as U (unstable), UB (unstable bluff), URS (unstable recent slide), or UOS (unstable old slide).
b. Areas of previous failure such as earth slumps, earthflows, mudflows, lahars, debris flows, rock slides, landslides or other failures as observed in the field or as indicated on maps or in technical reports published by the U.S. Geological Survey, the Geology and Earth Resources Division of the Washington Department
of Natural Resources, or other documents authorized by government agencies.
c. Slopes having gradients of 15 percent or greater:
i. That intersect geologic contacts with permeable sediments overlying low-permeability sediment or bedrock and springs or groundwater seepage are present; or
ii. That are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials.
d. 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.
e. Slopes having gradients steeper than 80 percent subject to rock fall during seismic shaking.
f. Potentially unstable areas resulting from rapid stream incision, stream bank erosion, and undercutting by wave action.
g. Slopes with a gradient of 40 percent or more with a vertical relief of 10 feet or more, including marine bluffs, except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and is measured by averaging the inclination over at least 10 feet of vertical relief.
3. Seismic Hazard Areas. Seismic hazard areas are lands that, due to a combination of soil and groundwater conditions, are subject to risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement or subsidence, soil liquefaction, or surface faulting. The following are known or suspected seismic hazards:
a. Areas having “high” and “moderate to high” risk of liquefaction as mapped on the Liquefaction Susceptibility and Site Class Maps of Western Washington State by County
published by the Washington State Department
of Natural Resources. These are typically underlain by cohesionless soils of low density typically and must have a low groundwater table.
b. Areas located within one-quarter mile of an active fault as indicated on investigative maps or described in studies by the United States Geologic Survey, Geology and Earth Resources Division of the Washington Department
of Natural Resources, or other documents authorized by government agencies, or identified during site inspection.
c. Those known or suspected landslide hazards referenced in subsection (A)(2) of this section.
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 sinkholes, gas releases, or subsidence due to mine workings. Factors that should be considered include: proximity to development, depth from ground surface to mine working, and geologic material.
5. Volcanic Hazard Areas. Volcanic hazard areas are areas subject to pyroclastic flows, lava flows, debris avalanche, and inundation by debris flows, lahars, mudflows, or related flooding resulting from volcanic activity. Though there are no significant risks identified for the city and immediate surrounding area, other than airborne particulate impacts from an eruption.
6. Tsunami Hazard Areas. Tsunami hazard areas include coastal areas and lake shoreline areas susceptible to flooding, inundation, debris impact, and/or mass wasting as the result of coastal wave action generated by seismic events or other geologic events. Suspected tsunami hazard areas are indicated on the Tsunami Hazard Map of the Anacortes-Whidbey Island Area, Washington: Modeled Tsunami Inundation from a Cascadia Subduction Zone Earthquake.
7. Other Hazard Areas. Geologically hazardous areas may also include areas determined by the Director to be susceptible to other geological events including mass wasting, debris flows, rock falls, and differential settlement.
B. All areas within the city meeting one or more of these criteria, regardless of any formal identification, are hereby designated critical areas and are subject to the provisions of this chapter. (Ord. 3064 § 2 (Att. A), 2021)