Jon Bevan

1
The Lee River Landslide

• Jon Bevan
• Meghan ODonnell
• December 10, 2003

2
Introduction and Objectives

• We examined an active landslide along side of the
  Lee River in Jericho, Vermont.
• The main slide occurred several years ago and
  recently there have been smaller slides.
• We wanted to investigate
• stratigraphy and cohesion of the sediment.
• vegetative cover on and around the slope.
• Infer geologic history of the sediment.
• Conclude why the slope continues to fail.

3
Location Map
N
37 meters in height 50 meters across
Scale 100 km
See Jon for Scale
N
N
Scale 0.5 km
4
Methods

• Dig eight, 1m deep soil pits vertically up the
  landslide to observe stratigraphy
• Orientation of bedding,
• Sediment grain size and graded bedding,
• Cross-bedding and Ripples.
• A handheld device to measure cohesion of
  particular sediment layers.
• Cohesion is the soils ability to stick to itself.

5
The Landslide Data

• The slope of the landslide is 28.
• The sediment continually increases in grain size
  with increased elevation, besides Pit 2.
• Alternating fine to coarse layers.
• All tilted layers, cross bedding, and ripples are
  oriented northeast.
• Finer grained sediment was more tightly packed
  and exhibited greatest cohesion.
• Vegetation covered the southern half of the slide
  was covered with vegetation.

6
Pit 2 Data
Unsorted fine to medium grained sediment, 7
meters from rivers edge
See tape measure for scale
Pockets of gravel within unsorted material
See tape measure for scale
Organic debris
See tape measure for scale
7
Vegetation and Ground Water
Vegetation covers approximately 50 of the slope.
The roots stabilize the slope laterally.
See Jon for scale
Outwash of subsurface groundwater at the base of
the slope.
Scale 6 in.
8
Geologic History

• 12-13ka the Laurentide Ice mass ablated from this
  region of Vermont.
• Served as an ice dam for northward flowing water
• Glacial Lake Vermont
• Creation of deltas into glacial lake

Note Photo from Benn and Evans, 1998. And please
disregard blank spots.
9
How the Slide has Changed (1962 and 1999)
1962
1999
1962 landslide is almost twice as larger than
present scarp. Vegetation on farmland terrace
has increased significantly.
10
Explanation of why the Slope Fails

• Slope of the slide is quite steep.
• Sub-surface flow of H2O
• Normal force decreases as pore pressure stays the
  same, decreasing cohesion.
• Undercutting of river
• Insufficient vegetation to completely stabilize
  the slope.

11
References

• Wright, Steven. 2002 and 2003. Personal
  Conversations and Class notes.
• Chapman DH. 1937. Late- and Post-Glacial
  Champlain Valley. In American Journal of
  Science. 3453.
• Bierman Paul. 2003. Class notes.
• Benn, D, Evans, D. 1998. Glaciers and
  Glaciation. Arnold Publishers. London, England.
• Bloom, A. 1991. Geomorphology A Systematic
  Analysis of Late Cenozoic Landforms. Prentice
  Hall. Englewood Cliffs, New Jersey, USA.
• Kochel R, Miller J, Ritter D. 1978. Process
  Geomorophology. WCB Publishers. Dubuque, IA.