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Feb 20, 2024
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Lab 6: Mass Movement
GEOG 1113L – Introduction to Landforms Lab
Name: __Anora Oromoni ___________
I.
Erosion Due to Gravity
1.
What is mass movement and what is the ultimate purpose of it? Mass movement is the downward movement caused by gravity. It plays a major role in shaping the earth. Sand rocks and soil will move downhill until the slope becomes stable. 2.
What is the angle of repose? The angle of repose is the steepest angle at which loose material will not slide downward. 3.
Which of the following does not impact the angle of repose?
A.
Moisture level
B.
Weight
C.
Color
D.
Shape
E.
Size
4.
What factors increase the chance of a landslide? Heavy rains, deforestation, and earthquakes increase the chance of a landslide. 5.
__Creep____ is a slow form of mass movement that involves the downhill movement of soil. Bent _tree trunks______ may be evidence of this type of mass movement.
II.
Types of Mass Movement
Use your knowledge from the video you just watched and the provided definitions to determine which type of mass movement each of the 10 pictures below shows. Highlight the appropriate answer.
1. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
2. Which type of mass movement is shown in the picture above?
G.
Avalanche
H.
Mudflow
I.
Rockfall
J.
Lahar
K.
Creep
L.
Landslide
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3. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
4. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
5. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
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6. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
7. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
8. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
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9. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
10. Which type of mass movement is shown in the picture above?
A.
Avalanche
B.
Mudflow
C.
Rockfall
D.
Lahar
E.
Creep
F.
Landslide
III.
Angle of Repose
For this activity, you will be using images of sand and gravel to determine the relative angle of repose
for each material. Complete the questions that go along with the experiment, as well as the related exercises that follow. Read through the following experiment instructions on the maximum slope at which grains are stable (angle of repose) and utilize the provided images of the results to help you answer the questions. Step 1:
Slowly pour a stream of dry sand into the center of your pan or tray. Do not pack the sand with your hands. Only pour over the top of the pile using the scoop. Avoid disturbing your pile and notice the
maximum angle, or steepest slope, of the dry sand. This slope is called the angle of repose.
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Step 2:
Place a pile of damp sand in the center of your pan or tray. You will probably have to scoop the sand instead of pouring it. You can gently pack the pile together but use only gentle pressure. Notice the maximum angle of the damp sand.
Step 3:
With the cup, slowly pour water onto the pile of damp sand and observe what happens to the sand pile as the sand becomes saturated.
Step 4:
Slowly pour the rounded gravel into the center of your tray or pan. Make sure your pile is large enough to get the true angle of repose!
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Step 5:
Slowly pour the angular gravel into the center of your tray or pan. Notice the angle of repose.
1.
Is the dry sand’s angle of repose smaller or larger than the damp sand? Smaller
2.
Which sand – dry or damp – is more likely to move individually down the slope? Dry sand 3.
Which sand – dry or damp – is more likely to move in large groups down the slope? Damp sand
4.
Does the saturated sand maintain the damp sand’s angle of repose? No, because when the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid
.
5.
Which sediment condition – dry or damp – permits steeper slope angles? Damp sand permits steeper slope angles.
6.
Which sediment condition – dry or damp – lends itself to the most dramatic, quickest, and therefore most dangerous style of slope failure? Explain your reasoning. Damp sand is the most dangerous type of sand for slope failure because it has a lower angle of repose than dry sand. This means that damp sand is more likely to collapse and form a fluidized mass that flows down the slope like a liquid. This type of mass movement is known as a mudflow.
7.
Suppose a house is built on a slope made of sediment. The slope is slightly greater than the angle of repose. Is this house at risk from mass movement? Yes, a house built on a slope made of sediment that is slightly greater than the angle of repose is at risk from mass movement. Mass movement can be caused by several factors, including heavy rain, earthquakes, and human activities. When the slope is already unstable, even a small amount of additional stress
can cause it to fail.
8.
Is the rounded gravel’s angle of repose smaller or larger than the angular gravel? Smaller
9.
Compare the angle of repose for the dry sand and the dry gravel. What impact does grain size have on the angle of repose? Dry gravel has a steeper angle of repose than dry sand because its
grains are larger, heavier, and rougher. This makes it more difficult for gravel grains to move and slide past each other, which makes gravel more stable.
10.
Compare the angle of repose for the rounded gravel and the angular gravel. What impact does grain shape have on the angle of repose? Rounded gravel has a steeper angle of repose than angular gravel because its grains are smoother and interlock more easily. Angular gravel grains are rougher and interlock less easily. This is because the friction between two surfaces is greater when they are rougher.
Use the results of the previous experiment and the graphs below to answer the following questions. 11.
In the Seattle area, which four months experience the greatest amount of rainfall? November, December, January, and February
.
12.
In the Seattle area, which four months experience the highest number of landslides? December, January, February, and March. 13.
Thinking about the results of your experiments (especially in Step 3), explain why the peak landslide activity occurs later than the peak rainfall activity. The peak landslide activity occurs later than the peak rainfall activity because it takes time for the ground to become saturated with water. When water is added to damp sand, the sand grains become more slippery and less able to interlock. This makes the sand pile more unstable and more likely to slide.
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The USGS has recently developed and tested a model that establishes a precipitation threshold for the Puget Sound Area as it relates to landslides. In this area, landslides tend to occur if the precipitation amount for a 15-day period exceeds a specified level, and that 15-day period is followed by 3 days of rain
at certain levels. The graph below shows the Precipitation Threshold for Anticipating the Occurrence of Landslides.
14.
Use the “Draw” tab and red pen to place and label a point on the graph for the following days, and then fill in the remaining column in the table:
Date
15-day
cumulative
precipitation
3-day
cumulative
precipitati
on
Landslides
expected?
(Yes, no,
maybe)
1/5/56
4.73
3.38
Yes
1/6/96
1.27
0.55
No
1/7/96
1.33
1.27
No
2/11/96
4.94
0.07
Maybe
2/21/96
5.87
0.56
Yes
Above the line, Yes Below the line, No
Close to the line, Maybe