Downslope Movement Activity
Activity I – Basic Principles of Mass Wasting
Figure 1 illustrates how earth materials on a sloping surface are subjected to some portion or
component of gravity (gs) that acts in a direction parallel to the slope. In order for material on a
slope to remain stationary, frictional forces must be greater than or equal to gs.
1) The component of gravity in the slope direction causes the shear stress (gs) to
increase/decrease (circle one) as the slope becomes steeper.
2) If you have two identical slopes, but earth materials are stable on one and not the other, what
can you infer about the frictional forces on the two slopes?
3) As indicated in Figure 2, the weight of water in saturated materials creates an outward acting
fluid pressure in pore spaces and fractures. This so called pore pressure becomes greater
whenever the overlying column of water increases.
Briefly explain how a heavy or prolonged rainfall could trigger a mass-wasting event (include the
frictional force and pore pressure in your answer).
Figure 2. Microscopic views of: (A) the pore space between sediment grains, and (B) a planar void
space, such as a fracture, fault, or bedding or foliation plane.
Activity II –La Conchita, California
In this section we will take a closer look at the 1995 and 2005 mass wasting events at La Conchita,
California. Here the steep coastal terrain combined with poorly consolidated sedimentary material
cause the slopes to be inherently unstable. Note the road that traverses the hill slope and the
avocado orchard at the top of the slope. Think about the building and operation of the orchard
would affect the slope.
The blue and yellow ovals on the topographic map in Figure 3 show the location of the 1995 and
2005 mass wasting events, respectively.
4) Notice the dirt road that traverses the hillside above the town. Explain (several reasons) how
building this road would have made the slope less stable.
5) Describe two ways in which heavy rains would cause the slope to become less stable. (What
does water do to a slope? You can include pore pressure in your answer.)
Figure 3 – Topographic map showing the town of La Conchita and general location of the
movement in 1995 (blue) and 2005 (yellow). Contour interval is 20 ft. (From U.S. Geological Survey
Open-File Report 2005-1067)
Take a blue-colored pencil or marker and carefully outline the area on the photo in Figure 4 where
the movement took place in 1995.
6) The 1995 movement at La Conchita has been classified by the USGS as a complex slump. List
the physical characteristics you see in the photo that support the interpretation that this is a
slump. Please be specific such that if I couldn’t see the photo I can tell you’re describing a slump
and not another type of downslope movement.
Take a red-colored pencil or marker and carefully trace the road as it comes down the hillside.
Note that in the 1995 photo the road can still be found in the middle of the slump.
Figure 4 – Oblique aerial view of La Conchita after the 1995 event (Courtesy U.S. Geological Survey,
Robert L. Schuster)
Figure 5 – Oblique aerial view of La Conchita after the 2005 event (Courtesy U.S. Geological Survey,
The freshly exposed material visible on the photo in Figure 5 (previous page) represents the area
where renewed movement occurred in 2005. Use a red-colored pencil or marker and carefully
outline the recent area of movement on the photo.
7) The USGS classified the 2005 event as a debris flow. As before, list the physical characteristics
you see in the photo that indicates this is a debris flow. Be specific and distinguish from the
other types of downslope movement.
8) Based on what you see in the two sets of photos, which event–the 1995 or 2005–was more
deeply seated and which was shallower? Explain how you can tell.
9) From a hazard perspective, which event–the 1995 slump or 2005 debris flow–would have
posed the greatest threat to people living at the base of the slope? Explain why.
In both the 1995 and 2005 events, increased pore pressure associated with heavy rains is believed
to have served as the triggering mechanisms. From Figure 6 notice the March 4, 1995, slide
occurred approximately one month after the period of heavy rainfall ended. In contrast, the January
10, 2005, event took place near the end of a period particularly heavy rainfall (Figure 7).
10) Based on the timing of the slides relatively to the rainfall pattern, which event–the 1995 or
2005–do you think the water would have infiltrated deeper into the subsurface?
11) Explain how this could account for the difference in volume of earth material that you
determined in question #8. (Refer to Figures 6 and 7).
Figure 6 – Precipitation record in La Conchita area from October 1994 to March 1995.
Figure 7 – Precipitation record in La Conchita area from October 2004 to January 2005.
12) Refer to your textbook. How many people died in the 1995 slump? _____________
13) How many people died in the 2005 debris flow? ___________________
14) List 3 steps that could have been taken that would help minimize the chance of mass wasting
and loss of life at La Conchita.
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