Part 4Part 4

PHYSICALPHYSICALATTRIBUTESATTRIBUTES

OFOFDEBRIS FLOWSDEBRIS FLOWS

DEBRIS FLOW DEBRIS FLOW LOBESLOBES

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

• Colluvium is Colluvium is typically stored typically stored within bedrock within bedrock ravines and ravines and depressions until depressions until externalexternal and and environmental environmental factorsfactors trigger trigger erosive cycles, erosive cycles, such as those such as those shown here.shown here.

EROSIVE EROSIVE CYCLESCYCLES

Repeated Repeated Cycles of Cycles of

Erosion and Erosion and FillingFilling

• C14 dating C14 dating within colluvial within colluvial filled hollows filled hollows suggests that suggests that they periodically they periodically fill and develop fill and develop soil horizons, soil horizons, then undergo then undergo erosive cycles. erosive cycles.

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

Traction Traction Erosion of Erosion of ChannelChannel

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

Destructive Destructive SnoutsSnouts

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

Inverse SortingInverse Sorting

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

SIEVING OF FINESSIEVING OF FINES

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

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