Slip & Slide! EarthCache
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This is an earthcache, so you are not looking for a container, but you are at the site of something geologically interesting. Submit your answers to the following questions below to my e-mail profile.
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The object of this earthcache is to learn about the geology of mudslides & landslides. At this location in May of 2010 over a two day period there was a rainfall of between 15-18 inches!
Landslides & Mudslides
Debris flow is a generic term used to describe the movement of rocks, soil, water, and vegetation downhill. A debris flow could be a mudslide or a landslide, depending on the amount of water present.
Landslides- are at the drier end of the debris flow spetrum. Their smaller water content makes them more like oatmeal on a camping trip.
Mudslides- contain more water than landslides. Mudslide consistency is a lot like cake batter. Heavy rains can trigger mud flows on unstable slopes.
The Role of Water
Water plays a big role in triggering landslides, it acts like WD-40 on a rusty screw. Water lubricates the surface between the soil and the bedrock, and makes it easier for the soil to slide off the soild underlyiing hillside. If the soil is already saturated, a period of heavy rain can trigger a slide.
Water combined with gravity is a powerful force in moving earth. How heavy is water?
"When it rains two inches in 24 hours, you have about 10 pounds of water on every square foot of soil," explains Tom Horning, a geological hazard specialist at Horning Geosciences in Seaside, OR. "That is about 50,00 pounds of water on a 50-by-100 foot lot."
Soils
Soil composition also determines where debris flows occur. The looser the soil, the more likely it will move if nothing is there to hold it down.
Soils with greater clay content stick together better because the clay particles absorbe water slowly and stick together. Soils with more organic material-- such as forest soils-- act like sponges to soak up excess water.
Slope
The steeper the slope, the more likely it is to slide. Landslides can occur on very gentle slopes, but these usually take years. Wash gullies, which funnel eroded soil downhill, form more often on slopes greatrer than 22 degrees. Quicker debris flows take place on slopes greater than 27 degrees.
Vegetation
The effect that vegetation cover has on hillslopes and slide potential is important. Trees intercept rain, lessining the impact that individual raindrops have on soil. Just imagine what these raindrops do to dried-out exposed soil. Rain hitting the ground loosens soil. Plants absorb the force of the rain and take up water for their own growth.
Human
The most common cause of 'slip & slide' is the human factor as when road construction alters the slope, making it too steep.
LANDSLIDE!
So what actually happens during a slide? As rain continues to fall, the soil becomes saturated. When the soil is completely saturated it becomes a soupy, muddy mass, and mud on a hill moves to the lowest point it can reach!
This particular 'slip & slide' is geologically called a Block Slip which happens when a hillside is cut into, such as when this road was made. This slope was at a higher risk because the soil is composed of sand & clay. Shear stress, the force that wants to tear the soil apart, is usually balanced by resistance to downhill movement. Resistance is also provided by roots holding the soil to the slope, or soil particles clinging to one another.
The road-cut at this location upset this balance, and the heavy rains of early May 2010,(15-18 inches in 36 hours), broke down resistance to a point where shear stress took over. Thankfully the gully beneath was large enough to receive the muddy sand & clay and the clay chunks that broke off.
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Requirements:
1- Estimate the height of the top of the clay slope and the length of the slide?
2- Estimate the width of the slope that broke off.
3- What degree is the slope now?
4- Besides the excessive rainfall in May 2010, what is the other major cause of this slide?
FTF: SweetPea&Crew
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