Part 4 PHYSICAL ATTRIBUTES OF DEBRIS FLOWS DEBRIS

Part 4 PHYSICAL ATTRIBUTES OF DEBRIS FLOWS

DEBRIS FLOW LOBES Debris flows coalesce in firstorder and second order drainages. They usually deposit debris on slopes of around 10% grade. These attributes can be programmed into a topographic recognition model.

EROSIVE CYCLES • Colluvium is typically stored within bedrock ravines and depressions until external and environmental factors trigger erosive cycles, such as those shown here.

Repeated Cycles of Erosion and Filling • C 14 dating within colluvial filled hollows suggests that they periodically fill and develop soil horizons, then undergo erosive cycles.

• Physical factors diagnostic of hillsides experiencing clastic debris flows. Debris is usually deposited on a slopes between 11 o and 6 o, averaging about 10 o.

Traction Erosion of Channels are easily eroded by traction and buoyancy of debris flow mixtures in confined channels where the hydraulic grade exceeds 10%

Destructive Snouts Debris flows arrest themselves and cease moving when they roll onto a slopes less than 10 degrees and/or are allowed to disperse laterally. This dispersion bleeds off excess pore water pressure and engenders shear strength to the one fluid mass, allowing it to ‘set up. ’

Inverse Sorting Debris flows are easily recognized by matrix support and inverse sorting of coarse clastic fraction, as sketched at left, in Gypsum Canyon, UT, along the upper reaches of Lake Powell.

SIEVING OF FINES In semi-arid regions fines are gradually sieved from the parent debris mass by runoff between extreme events which trigger debris flows. This often results in the development of ‘desert pavement’.

• Topographic expression of a steep alluvial fan against a bedrock escarpment, showing recent debris lobes emanating from ephemeral first-order channels. These would be poor locations for development.