Copy of Lab 08 Mass Movements
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106
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Geology
Date
Dec 6, 2023
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GEOSCI/ENVIR ST 106: Environmental Geology
Lab 8: Mass Movements
Assignment Overview:
In this lab you will investigate the hazards posed to several
communities in the Pacific Northwest by lahars through examination of lahar hazard maps.
Instructions:
Fill out each red highlighted field (_________) according to each question’s
instructions.
Submission:
To submit the assignment on Canvas, use the following steps:
1.
In Google Docs, generate a PDF: File → Download as → PDF Document
2.
In Google Docs, use Share → Get Shareable Link, and copy the link address
3.
In Canvas, upload your PDF to the assignment.
4.
In Canvas, paste the link address to your Google Doc in the assignment comments.
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Part 1: Lahar characteristics
1. Define a lahar. (1 point)
____Mixture of rock and water flowing down the side of a volcano. Mudflow_____
2. How does a lahar originate? (1 point)
____Volcanic eruptions/activity _____
3. Why do lahars tend to follow river beds? (1 point)
____Easier flow, brought down by gravity and rainfall._____
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Part 2: Mount Rainier lahar hazards
Complete the following questions using the following lahar hazard map for Mount Rainier.
Figure 1.
Map of lahar hazards around Mount Rainier for three different scenarios (Cases 1, 2, and
3). Case 1 is for big lahars that occur once every 500-1000 years. Case 2 is for medium-sized lahars
that occur once every 100-500 years. Case 3 is for small lahars that occur once every 1-100 years.
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Colors indicate the areas that would be inundated by each type of lahar. From Cakir and Walsh
(2012).
4. Based on the hazards map and the distribution of roads on the map, where would you expect
the largest amount of property damage from a Case I lahar? (2 points)
____The most damage would be around Carbonado and Wilkeson. There would also be a
lot of damage along Riffe lake and Du Pont._____
5. Using the hazards map and other data sources (e.g., Google Maps), what major interstate
highway would likely be affected by a Case I lahar, and what major cities would be affected by
this disruption? On this map, major cities are distinguished from minor cities by the size of the
font used for their names. For example, in the top left corner of the map, the font size for
Bremerton indicates a major city, while the font size for Port Orchard indicates a minor city. (2
points)
Highway: ___5, 410, 165, 12, 162 ______
Cities: ___Pierce, South of Auburn, Puyallup, Tacoma______
6. Using the scale at the bottom of the map, estimate the maximum distances traveled along the
riverbeds (in km) by: (3 points)
Class III Lahars: ____30_____
Class II Lahars: ____60_____
Class I Lahars:
____150_____
7. (a) Speeds of lahars differ as a function of slope and fluidity, but 40 kph is not unusual. At that
speed, approximately how long would it take for a lahar from Mount Rainier to reach the small
town of Ashford (WSW of Mt. Rainier)? Show your work. (1 point)
____30km/40mph= 0.75 hours_____
(b) How long would it take to reach Puyallup, on the outskirts of Tacoma? Show your work. (1
point)
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____60km/40 mph= 1.5 hours _____
Part 3: Glacier Peak lahar hazards
Now that you’re familiar with the idea of a lahar hazard map, you will try to make one of your
own using the following satellite image of the Skagit Valley and the Glacier Peak volcano in
Washington State. You may find it helpful (though it is not strictly necessary to answer the
following questions) to investigate this area manually through Google Earth to get a better
feeling for the topography around Glacier Peak.
Figure 2.
Satellite image of Glacier Peak (see yellow pin on the right half of the image) and
surrounding terrain. White lines show the approximate outline of Skagit County. This is the
same image that is available in the Canvas lab module.
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8. In the Canvas lab module, download the file “Glacier Peak satellite image.png”. This is the
same image as that in Figure 2. Based on the map from Mt Rainier, and assuming a Case I
lahar of roughly the same magnitude as those in Figure 1, sketch on the satellite image where
you think lahar inundation zones would be on the map. (You are welcome to outline the lahar
inundation zones with any image editing software you like. For example, you could do this in
this Google Doc through Insert > Drawing > New, then dropping the “Glacier Peak satellite
image.png” file into the drawing space, and editing it there.) Paste your modified image here.
(3 points)
____
_____
9. Though prone to an assortment of volcanic hazards, the Skagit Valley is an incredibly
beautiful place. If you wanted to move to a town in Skagit County (see white lines for
approximate outline of Skagit County in Figure 2), but take into account the prospect of lahars,
where would you move and why? (Note: Island life is not an option – too expensive, sorry!)
Explain your reasoning in 2-3 sentences. For example, you can discuss the warning time you
would have, topography, and other factors. (2 points)
____I would move to Bow, Sedro-Woodley, or Mt. Vernon because they would be outside of
the Lahars or would be barely touched by them, therefore you would have a longer warning
time. Since lahar can move at about 40 kph you would have about 3 hours before the lahar
reaches you, whereas if you moved to Concrete you would only have 2 hours to evacuate.
Living in Concrete would also mean that you could get case II and III lahars potentially.
Concrete and Darrington also are located along a river so the lahars will flow along the river
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and overflow into their town causing damage. Sedro-Woodley and Mt. Vernon are at a
higher elevation as the map shows because it is a lighter color, so they are less likely to be
touched by lahars. _____
References
Recep C, Walsh, T. J., 2012. Loss estimation pilot project for lahar hazards from Mount
Rainier, Washington: Washington Division of Geology and Earth Resources Information
Circular 113, 17 p.
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