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J.E. “Sam” Ryan, CBO

5280 Northwest Drive
Bellingham, Washington 98226
Telephone: (360) 676-6907

Critical Area Functions and Services Critical Areas Logo

Geologically Hazardous Areas (handout)

xGeologically hazardous areas are areas that, due to their susceptibility to erosion, sliding, earthquake, or other geological events, may expose development to risks that are inconsistent with the protection of public health and safety. These include landslide hazard areas, seismic hazards, mine hazards, alluvial fans, and erosion hazards areas.

Landslide Hazard Areas – Landslide hazard areas are areas throughout Whatcom County where there is potential for slope failure due to a combination of geologic, topographic, and hydrologic factors. These include any areas, including bordering uplands that are susceptible to landslides because of any combination of bedrock, soil, slope, slope aspect, structure, hydrology, or other physical factors.Seismic Hazard Areas –Seismic hazard areas are areas that are subject to a severe risk of earthquake damage as a result of seismically induced ground shaking, differential settlement, or soil liquefaction. These include areas where there are:

  • Surface deposits of man-made fill or partially decomposed organic materials at least 5-feet deep;
  • Filled wetlands; and
  • Areas of alluvial deposits that are subject to liquefaction.

Mine Hazard Areas – Mine hazard areas are lands located in proximity to abandoned coal mines and associated underground mine workings. Mine workings include mine entrances, tunnels, rooms and chutes (large voids), drifts (water level tunnels), pillars (coal left for support), and air shafts. Related hazards include:

  • Subsidence - uneven downward movement of the ground caused by cave in of underground workings;
  • Ground and surface water contamination from tailings and underground workings;
  • Concentrations of lethal or noxious gases; and
  • Underground mine fires.

Alluvial Fan Hazard Areas – An alluvial fan is a fan shaped deposit of sediment and organic debris that forms where a stream flows, or has flowed, out of a mountainous upland onto a level plain or valley floor.x As a stream flows out of a mountainous environment, it abruptly loses capacity to transport sediment and debris resulting in an accumulation of transported materials. As materials accumulate and build on top of older deposits, stream channels can shift and begin building in other areas.

Alluvial fan hazard areas are those areas on a fan that have the potential to damage or harm the health or welfare of the community. They generally correspond to the path of recent and potential future stream flooding, boulder flooding, and/or debristorrents.

Erosion Hazard Areas - Channel migration zones are one form of erosion hazard that may be found along a river or stream where a channel is susceptible to erosion and can be predicted (based on historic record, geologic character, and evidence of past migration) to have a high probability of movement over time. Channel migration is a natural and normally occurring hydrologic process that can cause significant adverse impacts to property or public improvements and can result in a net loss of ecological functions associated with that river or stream.