Instructor’s Notes: Earth’s External Processes and Chapter 9 Mass Wasting
La Conchita, CA 2005 (USGS)
The topographic features at the surface of the earth are the result of the earth’s internal processes (tectonics) and the earth’s external process.
External processes are the result of the earth’s surface interaction with the earth’s hydrosphere and atmosphere (including climate and weather).
They result in:
Weathering- soil and sediment
Mass Wasting- gravity
Erosion Transportation and Deposition of surface materials
The Four Agents of Erosion (Transportation, and Deposition!!!)
The four agents of erosion that shape the earth’s topography are:
Gravity
Water
Ice
Wind
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Gravity –Mass Wasting
Mass Wasting- down slope movement of rock, soil or regolith under the direct influence of gravity
The rate of down slope movement can be fast (slide or fall) or slow (creep)
The material can be consolidated bedrock or it can be unconsolidated sediments or soils
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3 Images Mass Wasting: La Conchita, California January 2005 – unconsolidated soils and regolith
La Conchita, CA January 2005 (USGS)
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NOAA Images
USGS Video La Conchita Landslide
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Southern California 2005
Stream Undercut (USGS)
Mass Wasting Classification
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Based on:
Rate of Movement
Slowest (Creep)
Fastest (Landslides)
Nature of Movements
Flow
Slide
Fall
Type of Material
Debris
Bedrock (rock)
Rate of Movement:
Rates of mass wasting range from creep, which is movement less than 1 cm per year, to slides and falls, which can be greater than 4 km/hour
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Two examples of rate of Movement Left is slow creep –gradual movement of sediment and soil down slope at a rate of 1-2 cm/year. Right: Landslide triggered by an earthquake blocked rover to form a lake.
Two examples of creep occurring over steepened hillsides on northern California. The areas have marine sandstone bedrock with a thin layer of soil formed on the surface. The soil is creeping downhill (notice
“wrinkly” pattern in grass).
Nature of Movement:
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Animations flow, slide fall: http://highered.mcgrawhill.com/sites/007252815x/student_view0/chapter9/animations_and_movies.html#
Flow- moving downhill as a viscous fluid
Slide- relatively coherent moving as a well defined surface
Translational slide- descending mass moving along a plane parallel to the slope of the surface
Rotational slide (slump)- movement along a curved surface-upper part moves down/ lower part moves out
Slides- massive sliding of rock or debris along an inclined surface
Debris slide- debris moving along a bedding plane or surface
Rockslide - bedrock- usually broken along bedding planes-progresses to rock avalanche
Fall- free fall or bouncing down cliff
Falls-
Debris fall - debris (soils, sediment and regolith) breaking loose and bouncing freely down a slope
Rock fall - bedrock breaking loose and bouncing freely down a cliff- associated with talus at the base of the cliff
Types of Material:
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Debris- Unconsolidated material at the earth’s surface- includes sediment (fine-grained
to boulder sizes, and soil)
Bed Rock- hard rock that has not been weathered
Consolidated (left) - unconsolidated/debris (right) bedrock sediment and soil
Mass Wasting Controlling Factors:
Over-steeping of slope increases slide potential
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Angle of Repose- maximum slope angle at which material such as soil or loose sediment remain stable when the angle of repose is exceeded mass movement occurs (generally between 45 and
60 degrees).
Climate Factors:
Precipitation- amount and frequency
Small amounts of water can stabilize debris-surface tension of water “holds” grains together
Large amounts of water destabilize debris- water reduces the sheer strength by increasing pore pressure
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A small amount of water is good it increase the surface tension between the grains of sediment and binds the material together, resulting in a higher angle of repose (think about a sand castle analog;, you always build a sand castle with wet sand for this reason)
What happens to the sand castle when a wave comes in? Too much water actually increases the pore pressure between the grains of sediment and pushes them apart. This describes perfect conditions for mass wasting.
Thickness of Debris:
Orientation of planes of weakness in bedrock:
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Bedding planes are planes of weakness, therefore, rocks are more likely to break and slide along a bedding plane.
Looking at the image on right: do you think this is the best place to position the road? Can you suggest a safer option?
Mass Wasting Controls and
Trigger:
Storms- large amount of water in short period of time
Earthquakes- liquefaction
Volcanic eruptions- lahars
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Removal of Vegetation
Natural – Fires
Man Made- deforestation, logging, and farming
Erosional Undercutting
Man Made Development!!
Examples of Fires in Southern California – 2003
The fires strip the surface of vegetation on the already over-steepened hillsides of the California Coastal Range. After heavy rains, the soil is more likely to slide, and with earthquakes to liquefy.
Solufluction and Permafrost:
Solufluction - the flow of water saturated material over impermeable material generally permafrost soils- Alaska and Canada
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Examples of Slides:
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Lafayette, California
Ventura, California (image USGS) All the areas that are light in color are recent slides. If you look closely you can identify areas that are covered with new vegetation; these would be associated with zones that had active slides in the past 2-5 years. I approximate that 60-70% of these hillsides have experienced landslide in the past 3-5 years. It is well know that mass wasting occurs continuously in this tectonic setting which is only problem when we decide to build structures in active areas like this, as shown with the subdivision in the lower left of the image. Also notice the early stage stream development, very straight stream courses with V shaped valleys- another indication of over-steepening.
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Ventura oblique air photography ( USGS )
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San Bernardino, CA- Fires during El Nino-
Debris Basin
One of the approaches taken to avoid mudflows and land slide in these hillside communities is to build Debris
Basins as show above. In this case, a large Basin is dug at the mouth of a steep canyon to catch all the “debris” during a heavy rain. Look closely and you will notice that in this neighborhood the debris actually breached the catchment basin and flowed into the neighborhood anyways! (image USGS)
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Monterey, California
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Marin County California Map of Landslide Hazards
USGS map of Landsides Hazards in Marin County, CA
Red= mostly slide
Orange= many slides
Yellow= flat
Gray = flat
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USGS Mudslide Hazard Map Half Moon Bay California
Could Be Anywhere!
Don’t Make The Same Mistake:
What to look out for:
1) Build on bedrock
2) Check state geologic maps for landslide hazard
3) Look at hillside around the sight- look for signs of creep and erosional scarps from past slides as well as mounds of slide material
4) Realtors may not know or may not be able to tell you
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Evidence of over-steepened hillsides- If you dug a hole around the roots of this tree you would notice that the roots extend uphill. This tree on an over-steepened hillside is being pulled by gravity downhill while at the same time growing towards the sun. This “curved” geometry is a classic response to over steepening and should be a signal to a property owner to be aware of the possibility of mass wasting.
Preventing Mass Wasting:
This model represents a stable hillside. The angle of repose is within the range for a stable hillside, since there is vegetation cover that “anchors” the sediment and soil.
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This is the worst case scenario for modification to an otherwise stable hillside: the slope of the hill has been undercut, the vegetation has been stripped off, additional fill had been added increasing weight and supplying additional slide material, weight has been added by building a structure and watering the entire slope! It is a relatively common occurrence in places that are prone to mass wasting for homeowners to go away for the weekend forgetting to turn off the sprinkler systems and returning home only to find their yard and homes at the bottom of the hill.
Cynthia Mitchell Pavilion:
Not only are areas with steep mountains and hills are subjected to mass wasting here are a series of photographs taken at Cynthia Mitchell Pavilion in the Woodlands. In this case, a little over 8 years ago “The Pavilion” decided to increase the lawn space and they did so by increasing the size of the hill. The problem was that Lake Robbins Road could not be moved, so they steepened the lawn but did not account for a stable angle of repose on the Lake Robbins
Road side. This would be a fundamental principle that any civil engineer should understand!
Here are a series of photographs showing the change of the hillside over time.
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(North Gate 2006)
North Gate Entrance evidence of creep (2006)
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Left: the retaining wall on Lake Robbins Road Right: water trapped behind the retaining wall adds weight and could eventually push the wall over (see model below). Notice the slide in the center of the image on the right exposing the foundation of the upper retaining wall.
Right water trapped behind a retaining wall, adding weight and collapsing the wall. B) The solution to add drains to release excess water
November 2006 Cynthia Mitchell Pavilion
Brick structure around drain collapsing from the pressure of the hillside behind it. Note the exposure of the foundation of the upper retaining wall.
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Green plastic mesh commonly employed to stabilize vegetation on over-steepened hillsides
Major slide after a storm in 2006
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After Ike 2008 (note the slide shown in the above picture is located between the two areas that slid during Ike.
During the 2 year interim, the 2006 slide had been further stabilized by vegetation (see below).
After Ike Slide (close-up)
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2006 slide (vegetation covered) in 2008
Cynthia Mitchell Pavilion January –February 2009
(resurfaced and new sod)
New sod and rebuilt of brick drainage retaining wall
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March 2009 First Rain after New Landscaping
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The very first rain fall produced sliding of the landscaping on the hillside. The two bottom close-up images show the exposed green “plastic chicken wire mush” utilized to help stabilize the sod. This has already been repaired, but you can imaging the cost over time to continuously rebuild and landscape this hillside simply because the stable angle of repose of loose unconsolidated material was ignored!
Standing Water Behind the Retaining Wall, March (2009)
The boundary between the new sod and the older landscaping is evident and corresponds to where there is standing water trapped behind the retaining wall.
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