GEOS 103

Fall 2024

Lab 2: Hillslopes

Background

Mass movements are hillslope processes that transport debris such as rocks, soil, and trees downslope by gravity. Three main types of mass movements are falls, slides, and flows. These mass movements can be further discriminated by the type of “debris” they carry. For example, rockslides are slides which primarily carry rocks, landslides carry dirt or soil, mudflows carry mud, and debris flows may carry a mixture of mud, trees, and rocks.

Figure 1: Three types of mass movements. Falls (a &d), flows (b & e), and slides (c & f). Rockfall on Mt. Webb near Chilliwack, BC (d). Debris flow on Mt. Joffre near Pemberton, BC (e). Historic 1965 ‘Hope Slide’ near Hope, BC (f).

Learning Objectives

In this lab we will hillslope processes and mass movements. After completing the lab you should be able to,

•    Differentiate between slides, flows, and falls

•   Apply conservation of energy to mass movement runout distances

•    Evaluate hillslope stability using force balances

•   Calculate the factor of safety and explain its relevance to evaluating hillslope stability

Required Files and Software

Only a calculator is needed for this activity.

Part I - Conservation of Energy and Mass Movement Runout

Mass movements convert gravitational energy into heat by moving material downslope. If material of mass M moves down a vertical height Has it runs out horizontally a distance L, conservation of energy provides the governing equation,

This equation says that the gravitational potential energy MgH is converted into heat over a runout distance L. The parameter Ris an energy dissipation rate per unit weight and distance. Ris a dimensionless parameter that characterizes how rapidly the mass movement converts gravitational energy to heat as it moves.

This equation allows us to calculate R by analyzing a mass movement’s source and deposit locations. Dividing the above equation by MgL provides,

If a mass movement has large R, its runout distance L will be small, but if it has small R, its runout distance will be large. This suggests that we can define the efficiency of a mass movement travelling downslope by,

In other words, the more efficient a mass movement is, the farther it will travel. In the next several questions, we will analyze the efficiency of Tahoma Watershed mass movements. Figures 2-4

Figure 3: Tahoma Creek Watershed mass movement sites explored in this lab.

Figure 2: Satellite images of the Tahoma Creek Watershed mass movements with the source (S - pink) and depositional (D - blue) zones outlined.

Question 1: Looking at figures 2-3 and using figure 1 as a reference, site A shows a ___________, Site B shows a _____________, Site C shows a _____________, and Site D shows a _______________. Options include rockfall, rockslide, & debris flow, and can be used more than once. (4 pts)

Figure 4: Tahoma Creek Watershed mass movement ‘long profiles,. Profiles measure elevation vs. downslope distance along mass movement centreline, starting from the source (S) and ending at the end of deposition (D). A-D corresponds with A-D in Figures 2-3.

Question 2: Using Figure 4, calculate the efficiency of mass movements A-D. Round your answer to 2 decimal places (4 pts)

Site A:

Site B:

Site C:

Site D:

Question 3: Based on your answers in Q2, order the efficiencies of slides, falls, and flows from smallest (1) to largest (3). (3 pts)

1.

2.

3.

Part II - Force Balances & Slope Stability

Slope stability results from a balance between driving and resisting forces. When driving forces exceed resisting forces, slopes can collapse. When resisting forces exceed driving forces, slopes are stable.

These statements are summarized by the factor of safety (FS), which is the ratio of resisting shear strength to driving shear stress. The following diagram describes the infinite slope model (ISM) for the factor of safety:

Figure 5: Infinite slope model for the factor of safety.

The factor of safety equation is,

Based on Figure 5 and the equation above, answer the following questions.

Question 4: Which values of the factor of safety characterize a stable slope? Circle all that apply. (1 pt)

A: FS > 1

B: FS < 1

C: FS = 0

D: FS ≥ 0.5

Question 5: Which would be the net effect on slope stability of an increased pore- water pressure if everything else stays the same? What could increase pore-water pressure? (1 pt)

Much more evidence of former mass movements can be found in the upper reaches of Tahoma Creek, just downstream of the South Tahoma glacier terminus. The area immediately in front of the glacier is termed a proglacial zone. In this area loosely packed glacially derived sediment (till) has become exposed as the Tahoma glacier retreated. River erosion and landslides from the banks of Tahoma Cr. leave scars or “scarps” in this loose sediment. Site (e) from figure 2 shows two scarps and one over steepened slope which may collapse in the future (Figure 6).

Sediment in the proglacial zone of South Tahoma Glacier (Figure 6) is loosely packed and sandy, so it is non-cohesive (C = 0) and quickly drains water, meaning it is usually unsaturated (m = 0). Under these conditions, the factor of safety equation can be simplified to.

Where Sm  is the maximum gradient that a stable hillslope takes on (tan ∅), and Sf  is the gradient of the potential failure plane (tan θ). For loose sand, ∅ ranges between 25˚ and 30˚.

Figure 6: Recent mass movements in till within Tahoma Creek Watershed proglacial zone. Corresponds to Site E from figure 2. Panel (a) shows an over steepened area which may collapse in the future (blue) and two former landslide scarps (II & III - orange). Panel (b) shows the longitudinal profiles from source to deposit along the black centrelines. Panel (c) shows a crescent-shaped landslide scarp traced in orange in the same area as panel (a) that led to a large debris flow in 1988.

Question 6: Estimate Sf for Slope I in Figure 6b. (1 pt)

Hint: Estimate Sf  by calculating the slope between distance 10 and 40 m (the steep part where a landslide is most likely to occur).

Sf:

Question 7: Using Sf from Q6, calculate the factor of safety for Slope I. Report your answer to 2 decimal places. (2 pts)

FS:

Question 8: Does the infinite slope model indicate that Slope I is stable or unstable? (1 pt)

Question 9: There are many assumptions used to derive the infinite slope model (ISM) that limit its application to natural hillslopes. By comparing the ISM definition  sketch above with the real Tahoma watershed landslide scarps in Fig. 6, identify and briefly explain (2-3 sentences) two major shortcomings of the ISM. (3 pts)