Washington State Department of Ecology
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landslide hazard area

Signs of landslide hazards
Signs of movement
Where landslides occur
 
signs of movement

Landscape
Head scarps or steep cliffs at the top of a slope
Benches, scarps, and large cracks
Exposed clays uplifted on the beach
Hummocky and uneven terrain
Trees or large blocks of clay partially
buried in beach, not just drift logs
landslide scarp
Roads, utilities, and buildings
Sagging or taut utility lines
Separation of foundation from sill plate
Growing cracks in walls and window corners
Broken or leaking water or sewer lines
Doors not closing properly
Significant cracking of concrete slabs and pavement
Carlyon beach area cracked driveway
Vegetation
Tilted trees
Curved trees
Split trunks and stretched roots
Large clusters of trees of similar age
(often alder)
bending trees
Water

Small ponds on otherwise sloping terrain

Disrupted natural drainage

Unusually heavy or muddy seepage

Unusual increase or decrease in flow from springs
sag pond
 
where slides occur

Landslides occur where they have occurred before
Large, deep-seated slides tend to be a reactivation of existing landslide complexes. Slope stability maps can provide an excellent indication of unstable areas. A competent geological analysis can usually provide an estimate of stability of problem areas on a site. It cannot reliably provide a probability of failure or an exact map of the area to be affected.
slope stability maps
slope stability maps

Landslides occur on steep slopes
Steep slopes are typically found along shorelines where centuries of wave or river currents have eroded the toe of the slope. Most steep slopes around Puget Sound have experienced sliding in the past one or two hundred years.

Landslides occur on benches
Relatively level benches on an otherwise steep slope often indicate areas of past slope movement.
bench slides
bench slides

Landslides occur where drainage is causing a problem
Landslides are often triggered by the failure of drainage systems. Large amounts of water flowing from driveways, roof areas, roads and other impermeable surfaces can cause slides.
drainage
drainage

Landslides occur where certain geologic conditions exist
Landslides occur where certain combinations of soils are present. When layers of sand and gravel lie above less permeable silt and clay layers, groundwater can accumulate and zones of weakness can develop. In Puget Sound, this combination is common and widespread. Glacial outwash, often Esperance Sand or gravel overlies the fine-grained Lawton Clay or Whidbey formation.
geology
geology
 
links

What to look for during and after heavy rains, USGS.
Where landslides occur, warning signs, and what to do.
     
ip
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