I'm a little mystified by "angle of repose" and the relationship to deposition and transport. In the following image, I detect 2 different angles of repose for the same material. How can this be?

enter image description here

Clearly, the alluvial fan has a very shallow angle to it, not to mention the much steeper alluvial fans to the left. However, an intermittent stream has cut erosional channels through it and exposed a much steeper "angle of repose", which, is likely more reflective of the true angle of repose for this material. So, how is the shallow angle of the alluvial fan possible?

  • $\begingroup$ Nice picture. Where is it from? $\endgroup$
    – Gimelist
    Jan 21 '17 at 7:28
  • $\begingroup$ The photo itself is from a University of Oregon website, but the location is somewhere in Death Valley, USA. Such fans are common in the west and many are angled much more acutely than the angle of repose would suggest for the native sediments...some barely 5 degrees! $\endgroup$ Jan 21 '17 at 16:41

Geological materials such as sediments are never totally homogeneous. There will always be softer and harder regions within sedimentary deposits.

The fact that erosion channels have been created is due to the force/energy of the water creating the channels, the extent of the force the water had when it created the channels, the softness of the material being gouged out and the hardness and cohesion of the material left behind.

The angle of the walls of the channels is not sustainable and over time the walls will slump (fail) to the lower angle of repose you see for the overall deposit. When this occurs the floors of the channels will be filled with the slump material.

In the meantime, when more rain falls, the runoff will naturally flow in the extant channels - the water taking the path of least resistance. The water will flush out the loose slump material and if the force of the water is great enough it may also widened the channel and eventually cause more material to slump into the channel.

A material's angle of repose is a function of its cohesion and it is the overall/average angle the material lies at over a long period of time.

  • $\begingroup$ "The angle of the walls of the channels is not sustainable and over time the walls will slump (fail) to the lower angle of repose you see for the overall deposit." You are suggesting that angle of repose is time-dependent. Are you saying that the cone at the bottom of an hourglass will eventually flatten? $\endgroup$ Jan 21 '17 at 16:34
  • $\begingroup$ @KnobScratcher: No! Just that the walls of the channels cut into the sediment will have short duration stand up time. The vast bulk of the material already at "repose" will remain so because it is at the angle of repose. $\endgroup$
    – Fred
    Jan 22 '17 at 1:26
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    $\begingroup$ The angle of repose is a bit of a theoretical concept. It is generally applied to the angle that the materials will maintain where gravity is the only force affecting them (except for addition of new material from above). In that sense I would say that the steepest slope seen approaches the angle of repose. The low slope fan is dominated by sediment moved by the stream so isn't at the angle of repose, but all the fans show that they have been affected by stream erosion during flash events. $\endgroup$
    – haresfur
    Jan 22 '17 at 20:54

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