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Natural Disasters BYUI Nelson Fall 2011

What forces will effect mass wasting

Slope -stable vs. unstable
pore h2o -unsaturated, partially, saturated
material -igneous, layered, sediment
orientation-dips upslope, horizontal, dips downslope
vegetation -no development, moderate, extensive

what changes the resistant forces and driving forces as conditions/factors on the slope charge

Saturation of the material
Over-steepening of the slope
Slope Modification
Ground vibrations
Expansion/contraction cycles of soil/regolith

what processes/events enhance the likelihood of mass wasting


examples of how these factors change the resistant and driving forces and what impact this may have on a slope

saturation of the material-increase pore pressure (increase downslope force decrease cohesion force)
over steepening increases weight, decreasing the resisting force
slope modification -a decrease in cohesive force &/or increase downslope force due to added weight will occur
ground vibrations-decrease in cohesive force
expansion/contraction cycles of soil/regolith-

identify localities that may be vulnerable, what factors are influencing this risk


how are mass wasting events classified

type of material (bedrock, unconsolidated material--soil, regolith, sediment)
rate of movement (mm/yr, km/hr)
Water content (dry, wet)
Type of motion (fall, slide, flow)


free fall of detached particles, slope teep enough that material falls to base


material remains cohesive and moves along a well-defined surface


material moves downslope as a viscous flow/ most are saturated with water

two types of slides

translational and rotationals


type of slide-donward rotation of rock along a concave upward curved surface (block remains relatively undisturbed) leave scarps on hill slopes--generally as a result from human activity--heavy rain and earthquakes can also trigger slumps
DEBRIS FLOW often associated

why do rotational slumps occur?

water often squeezed out of top material and redistributed to lower portion of the slide causing it to flow and form in lobate structures

sediment flow

mixture of rock with some water or air that begin to flow down slope due to gravitational forces

slurry flow

material moves as a viscous fluid or diplsays plastic movement
common in arid and semi arid enviornments triggered by rain
generally around pre-existing channels


the very slow (mm/yr) usually continuous movement of regolith down slope
expansion/contraction cycles play a key role

evidence of creep

bent trees, offests in roads and fences and inclined utility poles

expansion/contraction cycles

movement of material due to wetting and drying cycles
freeze-thaw cycles


flowage of regolith where material remains saturated for very long periods of time
produces distinctive lobes on hill slopes. similar to creep
occurs mainly in regions underlain by permafrost (permanently frozen water bearing ground)
during warm periods top portion thaws and becomes saturated
saturated zone flows over frozen layers

debris flow

flowing regolith with velocities between 1m/yr and 100m/hr caused by saturation of the regolith with water
-occasionally start with slumps then flow down hill forming lobes with an irregular surface consisting of ridges and furrows
common after heavy rains
common in semi-arid regions and along volcanoes (some lahars)

what is in a debris flow

mixture of rocks, mud, and water

Aberfan Wales

debris flow--school crushed


highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like and wet concrete
result of heavy rains in areas were there is a lot of loose sediment
mudflows can travel for long distances over gently sloping stream beds
common near volcanoes

Snow avalanche

loose snow type=start at a point and gets wider
slab type--cohesive blocks move downhill

Slope mitigation-stop the flow

barrier walls
rock dams
wire mesh

slope mitigation-stabilize the slope


slope mitigation-divert the flow

diversion channels
rock ledges

slope mitigation-remove the hazard

change the slope
scaling and trimming

slope mitigation-remove the water

driving drainpipes into the hillside
-remove some sub-surface water and help stabilize a hillside

landslide control is all about....

drainage and slope angle
--diversion channels, benches, vegetation

slope mitigation-mapping hazards

slope stability map
combine a slope map, landslide deposit map, susceptible rock map

Indication of unstable ground conditions

-springs, seeps, or saturated ground in areas that have not typically been wet before
-new cracks or unusual bulges in ground, street pavements, or sidewalks
-soil moving away from foundations
-structures such as decks and patios that tilt or move relative to main house
-tilting or cracking of concrete floors or foundations
=broken water lines or other underground utilities
-leaning telephone poles, trees, retaining walls or fences
-offset fences
-sunken or down dropped road beds
-sticking doors and windows or visible open spaces indicating frames are out of plumb

gros ventre, montana

landslide-river--water saturated shale beds lubriacted base of slide--slide debris in valley

Thistle, Utah

landslide-began moving in the spring of 1983 in response to groundwater buildup from heavy rains the previous september and the melting of deep snowpack for the winter of 1982-83 within a few weeks the landslide dammed the Spanish Fork River obliterating US highway 6 and the main line of the denver and rio grande western railroad. the town of thistle was inundated under the floodwaters rising behind teh lanslide dam

Provo rock fall 2009

47 hours after a snowstorm--contraction/expansion cycle


the breakdown and alteration of rock

mass wasting

the down slope movement of weathered material (under the influence of gravity)


the physical removal of material (water, air, and ice)

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