Part 9 RECOGNITION FEATURES of EARTHFLOWS. Topographic Expression of Earthflows.

Part 9Part 9

RECOGNITIONRECOGNITIONFEATURES FEATURES

ofofEARTHFLOWSEARTHFLOWS

Topographic Expression of Topographic Expression of EarthflowsEarthflows

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 PatternsDiagnostic 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 Topographic Keys to Identify EarthflowsEarthflows

• 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 old 100 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 As earthflows become less frequent their areal limits become increasingly sketchylimits become increasingly sketchy

PROFILES of COALESCING PROFILES of COALESCING EARTHFLOWSEARTHFLOWS

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 EarthflowsCrowley’s Ridge Earthflows