Homework #4 GEOL 4880 Humphrey fall 2020, (Mainly debris flows)
Note there are only 3 questions, but in question 2, you need to get parts a and b to get c. Plus question 3 depends on question 2. Remember, there are debris flow equations on the web page.
1
Use Google Earth and look around the Laramie basin. You should not have to look more than 50
miles from Laramie. Find examples of
slope failures and report their Lat. and Long. (bottom right corner of screen).
a)
A
talus slope/cone
b)
A
landslide
c)
A
debris flow fan
d)
As
a bonus, there are several interesting slide features around Laramie, including
a large (kilometers long) rock mass that has slid towards the basin, at least
one very large (near kilometer scale) rotational slump, a ‘rock glacier’ and
several ‘Felsenmeer’ features which show evidence of downslope movement. See if
you can find anything odd? Try to find at least one of these.
2
(debris flow calculations)
Suppose you see a debris flow, moving down a small
gully. The flow is about 1 meter deep and a couple of meters wide. The slope of
the gully is 10o. A sample of debris material yields a bulk density
of about 1,800 kg/m3 ("wet"). Mud
on the front of a tree stump in the path of the flow is about 10cm above
the mud lines on the bank of the gulley. You got the impression that about half
of the depth of flow was moving as a "plug". Later
investigation shows that the flow came to a halt and fanned out on a 4 or 5 degree slope. Make
sure you look at the summary of debris flow equations.
a) Based on the above, give an estimate of the surface velocity? (hint: tree stump stopped the local flow, causing the energy cascade from kinetic to potential energy)
b) Estimate the critical shear stress ( tc).
c) Estimate the effective viscosity (hint, once you have the speed and the critical shear stress, you can back-calculate the required viscosity)
3. Plot (using a computer!) the following for the debris flow in question2. Title the graphs, label the axes, and include the dimensions (ie meters, sec, kg whatever). Plot depth on the vertical axis of your plots, and make sure the surface of the flow is at the top, and the bed is at the bottom. Include the full depth of the flow. To stress this: make depth the vertical axis, with the top of the flow at the top, and the horizontal axis will be shear stress, strain rate, and velocity. (a large part of this question is being able to follow the above directions)
a) The shear stress as a function of depth
b) The slope parallel shear strain rate as a function of depth
c) The slope parallel velocity as a function of depth
