Homework #4 GEOL 4880 Humphrey fall 2022
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. And question 3 is specifically designed to force you to produce ‘nice’ computer plots, so give yourself time to access a computer!
1
Use Google Earth and look around the Laramie region. I have given you degrees, minutes, and secs
for locations. You need to identify the
geomorphic features at these locations, and answer any associated
question. You will have to zoom in close
to several of these features since some are small-ish. To change google earth to degrees minutes
sec, look under ‘tools’, ‘options’ ‘3D View’.
a)
40
36 45.46N, 105 55 35.68W, only 100m long, but a classic example of what? [as a bonus (no points off if you don’t get
it) what is the odd kilometer long feature (N-S orientation) just to the west
centered on 40 36 52.48N, 105 55 55.44W?]
b)
41
14 13.71N 106 07 09.59W, what type of ‘fan’ this?
c)
41
39 47.73N 106 29 58.48W, this fan has been dissected by more recent
erosion. What process made this fan?
d)
41
19 36.60 N, 106 07 48.75 W, the feature is about 1 km long in a N-S orientation. What is this feature?
e)
41
22 16.22N, 106 06 34.61W, although small, only a couple of 100 meters long,
this is a classic example of a geomorphic feature that can only exist in very
weak, clay rich materials.
f)
43
36 53.89N 110 32 57.94W, These features are very rare in old terrains, so the
ones around Laramie are very old and hard to identify. This is near Jackson Hole. Question, was water involved?
g)
(hard)
41 16 53.00N 106 25 24.70W, this is a
very odd feature, just try to come up with a name..
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 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 velocity as a function of depth
c) (a little hard) The slope parallel shear strain rate as a function of depth
