35.35.120 Regulations – Geologically hazardous areas designation, classification, and mapping.
(1) Designation. Lands classified as landslide, erosion or seismic hazards are hereby designated as geologically hazardous areas and are subject to the procedures and standards of this chapter and Program.
(2) Classification – Landslide Hazard Areas. Lands potentially subject to mass movement because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors. The following classifications shall be designated as landslide hazards:
(a) Areas of historic failures, such as:
(i) Those areas delineated by the United States Department of Agriculture Natural Resources Conservation Service as having a significant limitation for building site development;
(ii) Those coastal areas mapped as class u (unstable), uos (unstable old slides), and urs (unstable recent slides) in the Department of Ecology Washington Coastal Atlas; or
(iii) Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the United States Geological Survey or Washington 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 groundwater seepage.
(c) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to the present) or which are underlain or covered by mass wastage debris of this 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 rockfall during seismic shaking.
(f) Feeder bluffs, feeder bluff exceptional, and feeder bluff talus described and mapped in the Clallam County Shoreline Inventory and Characterization Report or in the Washington Department of Ecology Washington Coastal Atlas.
(g) Areas unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action, including stream channel migration zones. Channel migration zones are areas within which the stream channel can reasonably be expected to migrate over time as a result of normally occurring hydrological and related processes when considered with the characteristics of the river and its surroundings. Such hazards are characterized by abandoned channels, ongoing sediment deposition and erosion, topographic position, and changes in the plant community, age, structure and composition. These areas do not include areas protected from channel movement due to the existence of permanent levees or infrastructure improvements such as roads and bridges constructed and maintained by public agencies.
(h) Areas that show evidence of, or are at risk from, snow avalanches.
(i) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding.
(j) Any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet except areas composed of exposed bedrock outcrop at the surface. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least 10 feet of vertical relief.
(3) Classification – Erosion Hazard Areas. Areas likely to become unstable, such as bluffs, steep slopes, and areas with unconsolidated soils. Erosion hazard areas may also include coastal and riverine erosion areas. Lands meeting the following classifications shall be designated as erosion hazard areas:
(a) Areas containing evidence of significant erosion activity caused either by natural or human-made factors that threatens public health, safety and welfare.
(b) Shoreline erosion hazard areas potentially subject to land regression or retreat due to a combination of geologic, seismic, tidally influenced, and/or hydrologic or human-made factors. Shoreline erosion hazard areas can be identified by the presence of any of the following indicators:
(i) Areas with active bluff retreat that exhibit continuing sloughing or calving of bluff sediments, resulting in vertical or steep bluff face with little or no vegetation. These areas include unstable slopes and feeder bluffs mapped in the Clallam County Shoreline Inventory and Characterization Report (ICR) and Washington State Coastal Atlas available from the Department of Ecology.
(ii) Areas with active land retreat as a result of wave action.
(c) Riverine erosion areas that are mapped subject to lateral erosion related to moving water, including the channel migration zone and the potential slope/bank failures resulting from river channel movement.
(d) Slopes 40 percent or steeper with a vertical relief of 10 or more feet, except areas composed of exposed bedrock outcrop at the surface.
(e) Soil erosion hazard areas are identified by the presence or absence of natural vegetation cover, soil texture condition, slope, and rainfall patterns, or human-induced changes to such characteristics that create site conditions which are vulnerable to erosion of the upper soil horizon. Soil erosion hazard areas include those areas with slopes of 15 percent or steeper and that are classified as having severe or very severe erosion potential by the U.S. Department of Agriculture Natural Resources Conservation Service (NRCS) in the most recent Soil Survey of Clallam County or the NRCS Web Soil Survey.
(4) Classification – Seismic Hazard Areas. Areas subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, surface faulting, or tsunamis. Lands meeting the following classifications shall be designated as seismic hazard areas:
(a) Landslide hazard areas and materials.
(b) Artificial fills especially on soils listed in subsection (4)(c) of this section and areas with perched water tables.
(c) Soil types described within the Clallam County Soil Survey as beaches, Mukilteo muck, Lummi silt loam, Sequim-McKenna-Mukilteo complex, and Tealwhit silt loam.
(d) Other areas as determined by the Clallam County Building Official pursuant to the Building and Construction Code, Chapter 21.01 CCC.
(5) Mapping. Geologically hazardous areas shall be mapped whenever possible. These maps shall be advisory and used by the Administrator to provide guidance in determining applicability of the standards to a property. These maps shall be updated periodically as new information becomes available.