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Soil Erosion & Mass Wasting

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Page 1: Soil Erosion & Mass Wasting
Page 2: Soil Erosion & Mass Wasting

SOIL Just tiny fraction of earth materials Vital resources Very foundation of human life support

system Can be damaged by carelessness Basic role in providing food, fiber and other

basic materials Most abused resources

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SOIL EROSION Natural process Part of constant recycling of Earth materials

(Rock Cycle) The rates of soil erosion vary from one place

to another and depend on the soil’s characteristic

Incorporation and transportation of materials by a mobile agent usually water, wind and ice

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WIND Result of material movement by wind Generally occurs in areas with little or no

vegetation To reduce

erosion causes by wind, windbreaks (big trees) are often planted

A rock formation in the Altiplano, Bolivia sculpted by wind erosion.

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Two main effects: Deflation – wind causes small

particles to be lifted and therefore moved to another region

Abrasion – suspended particles may impact on solid objects causing erosion

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WATER/RAIN

Raindrops hit bare soil. Their kinetic energy is able to detach and move soil particles in a short distance

Each drop acts like a tiny bomb, blasting movable soil particles out of their position in the soil mass. Then water flowing across the surface carries away the dislodged soil particles

Rainsplash

Water and soil splashed following a single raindrop impact.

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Rill – small channel Gullies – larger channel That fraction of the rainfall which does not

infiltrate the soil will flow downhill under the action of gravity, it is then known as runoff or overland flow

Rill & Gully

Runoff may occur for two reasons:1. If rain arrives too quickly2. If the soil has already absorbed all the

water it can hold*Detention Storage/ Ponding – can protect the soil from

raindrop impact

Diffuse overland flow. Note the raindrop impacts.

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ICE Caused by movement of ice, typically as

glaciers Glaciers erode predominantly by two

different process:1. Abrasion – debris in the usual basal ice scrapes along

the bed, polishing the underlying rocks, similarto sandpaper on wood

2. Plucking – in this process, glaciers can also causepieces of bed rock to crack off

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Through those agents, it can result formation of:

Deep valley between cliff often carved from the landscape by a river

Most canyons were formed by a process of long time erosion

CANYON

Grand Canyon, Arizona

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Landform that extends above the surroundings terrain in a limited area, with a peak

MOUNTAINMount Olympus in Greece

CAVE

Natural undergroundvoid large enough for a human to enter

SPELEOLOGY – science of exploration and study of all aspects of caves

Canyon passage in Mammoth Cave, the world's longest cave.

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Underground layer of water-bearing permeable rock from which groundwater can be usefully extracted using a water well.

HYDROGEOLOGY – study of water flow in aquifers and the characteristics of aquifers

AQUIFER

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Caused by:

Results in:

EROSION

WIND WATER ICE

SURFACE UNDERGROUND

CANYON MOUNTAIN CAVE AQUIFERForms:

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MASS WASTING

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MASS WASTING - it refers to the down slope movement of rock, regolith, and soil under the direct

influence of gravity. - it does not require a transporting medium

such as water, wind or glaciers of ice.

GRAVITY - controlling force of mass wasting. - it causes material to move down slope.

MASS WASTING

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GENERAL MOVEMENTS OF MASS WASTING

SLIDE - Where the mass movement has a well-defined zone or plane of sliding.- is a movement of surface material down a slope.

- includes a wide range of ground movement.

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CREEP - is a long term process.-The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope.

FLOWS - involves continuous movement of material as a fluid behavior.

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FALLS - involves free fall of material.including rockfall, is where regolith cascades down a slope, but is not of

sufficient volume or viscosity to behave as a flow.

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LANDFORM DEVELOPMENT

STREAM VALLEY

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TRIGGERS OF MASS WASTING:

Trigger – it is an event that initiates down slope movement

Seismic shaking Increased overburden from

structures Increased soil moisture Reduction of roots holding the soil

to bedrock

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COMMON FACTORS THAT TRIGGERS MASS WASTING:

Saturation of material with water Over steepening of slopes Removal of anchoring vegetations Ground vibrations by earthquake

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Role of Water When heavy rains saturate surface

materials, pores in sediment become filled with water, the cohesion among particles is destroyed, allowing them to slide past one another with relative ease.

However, if enough water is added to fill the openings between the grains, the sand will ooze out in all directions.

Thus, saturation reduces the internal resistance of materials, which then are easily set in motion by the force of gravity.

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Mudflow

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Over-steepened slopes due to undercutting by waves,

streams, and human excavation, resulting in loss of support for materials higher up the slope

A slope whose material is stable at a fairly gentle slope angle may become unstable if its slope angle becomes steeper.   This can occur where a stream cuts into a valley slope, or where ocean waves remove the base (toe) of a slope.   Also, sometimes humans over-steepened slopes when constructing building sites, or roads in mountainous areas as shown in the image to the left.

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Removal of Vegetation anchors soil and regolith with plant

roots; removing plants makes the slope unstable and susceptible to failure A slope denuded of vegetation loses

surface protection from the impacts of raindrops, which can mobilize sediment grains with water flowing down slope.   The roots of plants on a slope can play a significant role in binding sediment together, reducing the likelihood of rapid or sudden mass wasting of a slope.   Removal of the vegetation, due to human cutting or harvesting, or due to fire, reduces strength of the slope.   The related photograph shows the effects of a fire that swept through the foothills of the San Bernardino Mountains, California in October of 2003.   Intense rainfall two months later produced a debris flow that killed 13 people in Waterman Canyon down slope.

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Earthquakes can dislodge rock and

unconsolidated material, resulting in landslides

The vigorous shaking of an already-unstable slope by seismic waves may cause it to fail.   Typically, the higher the magnitude of an earthquake, the more mass wasting will occur.   Click on the USGS image to the left to see the effects of slope failure due to the Northridge, California earthquake of 1994.

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CLASSIFICATION OF MASS-WASTING PROCESSES:

1. Slope Failures - a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.

A. Slump - form of mass wasting event that occurs when loosely consolidated materials or rock layers move a short distance down a slope. Slumps frequently form due to removal of a slope base, either from natural or manmade processes. Stream or wave erosion, as well as road constructions are common instigators for slumping. And Earthquakes also trigger massive slumps.

It divides into two broad categories and further subdivides these categories:

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The Thistle town in Utah was destroyed by a massive landslide (slump) around 1983

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B. Rock Falls and Debris FallsRock falls occur when a piece of rock on a

steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, and rocks. A rock fall may be a single rock, or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff.

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C. Rock Slides and Debris SlidesFrequently occur in high

mountain areas. It result when rocks or debris slide down a pre-existing surface, such as a bedding plane or joint surface.

A rock slide on Washington State highway 123 closed Cayuse Pass in Mount Rainier National Park.

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2. Sediment Flows - material flows down hill mixed with water or air. A sediment flow is a mixture of rock, regolith with some water. They can be broken into two types depending on the amount of water present.2 types of Sediment Flows:

1).Slurry Flows - are sediment flows that contain between about 20 and 40% water. As the water content increases above about 40% slurry flows grade into streams.

2).Granular Flows - are sediment flows that contain between 20 and 0% water. Note that granular flows are possible with little or no water. Fluid-like behavior is given these flows by mixing with air.

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Under the Slurry Flows are: Solifluction - produces distinctive lobes on hill slopes.

These occur in areas where the soil remains saturated with water for long periods of time.

Debris Flows- these occur at higher velocities, and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down hill forming lobes with an irregular surface consisting of ridges and furrows. debris flows are characterized by a mixture of sediment and water where the flow becomes slurry similar to wet concrete.

Mudflows- a highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus, after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous.

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This photo was taken ½ mile south of Genoa

This photo was taken in South of Genoa on Foothill Road.

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Under the Granular Flows are:

Earth flows - are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day. They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff.

Grain Flows - usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope.

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Creep - the very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles

Debris Avalanches - These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions.

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The toe of an avalanche in Alaska's Kenai Fjords.

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TYPES OF MOTION: Falls - are common on slopes that are too

steep for loose material to remain on the surface. Many falls result when freeze and thaw cycles or the action of plant roots loosen rock to the point that the gravity takes over.

Slides – many mass-wasting processes are slides, which occur whenever material remains fairly coherent and moves along a well-defined surface. Sometimes the word slide is used as a synonym for the word landslide.

Flow- occurs when material moves down slopes as a vicious fluid.

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RATE OF MOVEMENT:During events called Rock Avalanches, rock

and debris can hurtle down slope at speeds exceeding 200 kilometers (125miles) per hour. That is, high velocities result when air becomes trapped and compressed beneath the falling mass of debris.

But most mass movements, however, do not move with the speed of a rock avalanche. In fact, a great deal of mass wasting is imperceptibly slow like the creep. Its usually measured in millimeters or centimeter per year.

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Slump is a form of mass wasting event that

occurs when loosely consolidated materials or rock layers move a short distance

down a slope.

Slumped chalk slopes at Mupe Bay, Dorset

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Debris Flows are often referred to as mudslides,

mudflows, or debris avalanches. consist primarily of geological material

mixed with water. may also occur when strong rains on hill or

mountain slopes cause extensive erosion and/or what is known as "channel scour“.

can be as thick as wet mud mixed with rock, and can transport very large boulders.

may also eventually become thinner muddy flood waters as they deposit their heavier components.

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Scars formed by debris flow in great Los Angeles during the winter of 1968-1969. The photograph was taken within several months of the debris flows occurring.

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Lahars a type of mudflow composed of pyroclastic

material and water that flows down from a volcano, typically along a river valley

The term 'lahars' is originated in the Javanese language of Indonesia

can be extremely dangerous, because of their energy and speed

can flow several dozen meters per second and can flow for many kilometers, causing catastrophic destruction in their path

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Lahar from a March 1982 eruption of Mount St. Helens.

Lahar from June 1991 eruption of Mt. Pinatubo in Philippines

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Earth Flow is a down slope viscous flow of fine

grained materials that have been saturated with water, and moves under the pull of gravity.

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Downhill Creep is the slow downward progression of

rock and soil down a low grade slope it can also refer to

slow deformation of such materials as a result of prolonged pressure and stress.