Part 9 RECOGNITION FEATURES of EARTHFLOWS

Topographic Expression of Earthflows

Earthflows are statistically the most common form of landslippage They tend to form in the axes of colluvial-filled ravines or areas of thick residual soils One flow tends to pile upon upon another, building large “debris fans”

Diagnostic Topographic Patterns Use drainage and topographic keys to recognize anomalous site characteristics typical of landslides Divergent contours Crenulated contours Arcuate headscarp evacuation areas Isolated topographic benches

Topographic Keys to Identify Earthflows Opposing contours Headscarp evacuation areas Necking down at transition between deflation/ inflation zones

Earthflow features become increasingly mollified with the passage of time, making them difficult to discern 10 to 100 years old100 to 500 years old More than 500 years old

Earthflows tend to form coalescing complexes, with one or two lobes active at any given time, separated by long periods of dormancy

As earthflows become less frequent their areal limits become increasingly sketchyAs earthflows become less frequent their areal limits become increasingly sketchy

PROFILES of COALESCING EARTHFLOWS Each successive earthflow tends to truncate and mask the previous event

Examples of earthflows in the Appalachian Highlands mapped by the USGS

Crowley’s Ridge Earthflows