Chapter 9
Debris and Mudflow.
A debris flow is a flow involving soil in which coarse material \(gravel, boulders) is predominant. A debris flow can be like an earth flow and travel relatively short distances to the base of a slope. Mudflow is a flowing mixture of soil and water, usually moving down a channel. It differs from debris flow in that fine grained (sand, silt, clay material is predominant. most solid particles are silt and clay. Most mudflows quickly become channeled into valleys. Most likely to occur in places where soil is not protected by vegetation.
Classification of mass wasting- based on the rate of movement, type of material, and nature of the movement.
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Mass Wasting is the movement of a mass of soil or bedrock toward the base of a slope.
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Rate of movement- movement of soil at a rate of less than a cm a year is slow movement.
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Solifluction is not the only hazard associated with permafrost. Areas become swampy in summer due to permafrost like in the arctic and subartic regions.
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Solifluction- special variety of earth flow, takes place in arctic, where ground is permanently frozen(permafrost) Debris Flow involves coarse material that is present in earthflow. Mudflow-slurry of mostly clay, silt and water.
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Type of flow - creep(soil), earthflow, debris flow, mudflow, debris avalanche, rock avalanche(bedrock) Slide- debris slid or earth slide/rockslide fall- landslides, and rockfall(bedrock).
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rockfall- the fall of broken rock down a vertical or ner vertical slope. rockslide- slab of rock sliding down a less than vertical surface.
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Controlling Factors in mass wasting- summarizes the factors that influence the likelihood and the rate of movement of mass wasting.
1. slope were exceptionally steep. 2the relief(the vertical distance between valley floor and men summit). thickness of soil over bedrock, orientation of planes of weakness in bedrock. Climatic factors- ice in gruond, water in soil, waer in soil or debris, vegetation. Triggers include earthquakes, weight added to upper part of slope, undercutting of bottom of slope. and heavy rainfall.Climate influence how much of what grows in an area.
A rock slides is the rapid sliding of a mass of bedrock along an inclined surface of weakness, such as a bedding plane, a major fracture int eh rock or a foliation plane.
A rock avalanche is a very rapidly moving turbulent mass of broken up bedrock. Flowage on a grand scale. Only difference between a rock avalanche and a debris avalanche is that a rock avalanche begins its journey as bedrock. Rockslide or rock As come to rest as the terrain becomes less steep.
type of material - mass wasting processare are usually distinguished on the basis of whether the descending mass started as bedrock.
Any unconsolidated or weakly consolidated material at which the earths surface- soil. (can be debris, earth, or mud). Debris implies that coarse grained fragements predominate in the soil. Fine grained (sand, silt, clay) is called earth Mud- high content of water, clay and silt.
Movement can take place as a flow, slide, or fall. Gravity is the driving force.
Components of gravitational force that propels mass wasting is the shear force, which occurs parallel to a slope. The resistance to that force is the shear strength of a rock or soil. If shear force exceeds shear strength, mass wasting takes place. water is usually an important factor in mass wasting
Preventing landslides- mass movement of soil usually can be prevented.
Construction generally makes a slope more susceptible to mass wasting of soil in several ways. -the base of the slope in undercut, removing the natural suppose for the upper part of the slope -vegetation is removed during construction -buildings constructed on the upper part f a slope add weight to the potential slide and -extra water may be allowed to seep into the soil **Preventive measures- retaining wall, drain pipes(these are put in so the water can percolate through and drain away rather than collecting soil behind the wall; avoid overstepping the sleep-hill can be cut back through a series of terraces
Common types of mass wasting
Creep(or soil creep) is a very slow downslope movement of soil. Shear forces over time, are only slightly greater than shear strengths. usually less of cm per year. Two factors that contribute to creep are water in the soil and daily cycles of freezing and thawing. Several processes contribute to soil creep- particles are displaces in cycles of wetting and drying. the soil tends to swell when wet and when dry o that movement takes places in a manner similar to that o a freeze thaw cycle. burrowing worms sand other creatures stir the soil and facilitate movement under gravity's influence. Creep is not as dramatic as landslidng. It can be costly nuisance.
Number of other factors determine whether movement will occur and what type.
Creep- occurs relatively gentle slopes, usually aided by water by soil. Colder climates- repeated thawing and freezing of water within the soil contributes to creep. Landslides- more rapid mass wasting of rock, soil, or both,.
Flow occurs when motions is taking place within a moving mass of unconsolidated or weakly consolidated material. Grains move relative to adjacent grains or motion takes place along closely spaces, discrete fracutrs. Varieties of flow- earth flow, debris flow, mudflow, and debris avalanche.
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Flows- earthflows, debris flow, mudflows, and debris avalanche.
Earth flows- finer grained material is predominant. debirs avalanche(more rapid)- turbulant masses of debris, water, and air.
Triggers- a sudden event may trigger mass wasting of a hillside that is unstable. Movement would occur without the trigger if conditions slowly becomes more unstable
Earthquakes-commonly trigger landslides. Landslides often are triggered by heavy rainfall.
type of movement
Flow implies that the descending mass is moving downslope as a viscous fluid. Slide means the descending mass remains relatively intact, moving along one or more well defined surfaces. A fall occurs when material free falls or bounces down a cliff. In a transitional slide- descending mass moves along a plane approx parallel to the slope of the surface. A rotational slide(slump) involves the movement along a curved surface, the upper part moving downward while the lower part moves outward.
Earthflow, earth moves downslide as a viscous fluid the process can be slow or rapid. Usually occur on hillsides.
Flowing mass remains covered by a blanket of vegetation, with a scarp(steep cut) developing where the moving debris has pulled away from the upper slope. Rotational sliding(slumping) takes place about the earth flow(soil particles moving past one another roughy parallel to the slope). Rotational slide(upper part) and an earth flow(lower part). A hummocky(characterized by mounds and depression) lobe usually forms at the toe or front of the earthflo where soil has accumulated. Earthflows can be triggered by undercutting at the base of slope. bulldozers can do this.
Water is a critical factor in mass wasting. When soil is saturated with water, it becomes less viscous and more likely to fowl downslope. Added gravitation shear force from increased weight is usually less important than the reduction in shear strength. This is due to pore pressure in which water pores soil grains apart.
Loose grains adhere to one another because of the surface tension created by the film of water(added there when water doesn't completely fill the pore spaces between the grains of soil), is what allows you to build a sand castle. IF sand is too wet, it means the water completely occupies the pore space between sand grains, forcing them apart and allowing them to slide easily past one another.
Mass Wasting includes movement so slow that its called creep, as well as landslides(term for slow very rapid descent of rock or soil).
Mass Wasting effect people in many ways. Anything from devastating effects to a fence being pulled apart by a soil creep. More people died from landslides in 3 months of 1985 than in the past 20 years from other geologic hazards.
Soil is unconsolidated or weakly consolidated material regardless of particle size.
Soil=predominantly fine material; its earth. Predominately coarse; its debris
Debris Avalanche
The fastest variety of debris flow is this; its rapidly moving, turbulent mass of debris, air, water.
One variety of earth flow is usually associated with colder climates. Solifluction is the flow of water saturated soil over impermeable material. Because the impermeable material beneath the soil prevents water from draining freely, the soil between the veg cover and the impermeable material becomes saturated.
The impermeable material beneath the saturated soil can be either impenetrable bedrock- permafrost, ground that remains frozen for many years. Permafrost is a zone that if the soil is saturated is frozen during the winter and indistinguishable from the underlying permafrost. A solifluctoin movemtn is not rapid enought to break up the overlying blanket of vegetation along with it and forming a wrinkled surface. the soil collects at the base of the slope where he vegetated surface bulges into a hummocky lobe.
Mass Wasting
The movement in which bedrock, rock debris, or soil moves downslope in bulk, or as a mass, because of the pull of gravity.
Underwater landslides- the steeper parts of the ocean floors sometimes have landslides.
Underwater debris avalanche- turbidity current.
Rockfalls and Rockslides.
When a block of bedrock breaks off and falls freely or bounces down a cliff- rockfall. bedrock-crakcs(joints) and other planes of weakness such as foliations(met rock), or sedimentary bedding planes. Commonly oan apron of fallen rock fragments called talus accumulate at the base of a cliff.
How does construction destabilize a slope
addes weight to the top of the slope
What is the slowest type of mass wasting process? debris flow, rockslide, creep, rockfall, avalanche
creep
a descending mass moving downslope as a viscous fluid is referred to as
flow
the driving force behind all mass wasting processes
gravity
A flowing mix of soil and water, usually moving down a channel
mudflow
Gravity- the driving force for mass wasting is gravity.
normal force is perpendicular to the slope and is the component of gravity that tends to hold the block in place. Shear force- is parallel to the slope and indicates the blocks ability to move. Shear resistance is the force tht would be needed to move the block. Shear strength is the resistance to movement or deformation of that soil(controlled by pore pressure of water, and the anchoring effect of plant roots. Shear strength is also related to the normal force. If shear strength is les than shear force- soil will flow.
the largest landslide has taken place
on the sea floor
Preventing Rockfalls and Rockslides on Highways
planes of weakness are inclined into the hill, there is no chance of a rockslide. rockslide may occur where the planes of weakness are approx parallel to the slope of the hillside. Various techniques are used to prevent rockslides- detailed geological study before road is built, builders will be able to avoid hazards. Rock sliding prone to this can be "stitched." Radio transmitted, real time monitoring of areas whree mass wasting is active is valuable for predicting when mass wasting is about to speed up/dangerous.
How can landslides be prevented during construction
retaining walls, vegetations, install water drainage pipes.
the resistance to movement or deformation of soil is its
shear strength
Flow of water saturated soil over impermeable material is called
solifluction
an apron of fallen rock fragments that accumulates at the base of a cliff is
talus
