Geological Hazards engineering geology 2

engineering geology 2

1. Geological Hazards

2. Hazard
•Hazard is the probability of occurrence of
potentially damaging phenomenon within a
specific period of time and within a given area.
Types of hazard
•Relative hazard which is assessed by assigning
ratings of different factors contributing to hazard
•Absolute hazard which is expressed
deterministically e.g. factor of safety
•Monitored hazard which is assessed by actual
measurements of the effects. e.g. rate of
movement etc.

3. •Hazard can be natural or artificial
•Natural hazards are spontaneous hazards which
occur in nature and reducing the damage caused
by it is only possible
•Artificial hazards occurs as a result of human
interaction with the environment e.g. chemical
contamination, destruction of ozone etc.
•Risk is the potential loss of life and property and
may be defined as the expected number of life
lost, person injured, damage to property or
disruption as economic activity due to particular
phenomenon.

5. •Hazard (H): is the probability of occurrence within a specified
period of time and within a given area of a potentially
damaging phenomena.
•Vulnerability (V): is the degree of loss to a given element or
set of elements at risk resulting from the occurrence of a
natural phenomenon of a given magnitude. It is which
increase the susceptibility to the impacts of hazards.It is
expressed on a scale from 0 (no damage) to 1 (total damage).
•Element at risk (E): is the population, properties, economic
activities, including public services etc. at risk in a given area.
•Specific risk (R
s): is the expected degree of loss due to a
particular natural phenomenon. It may be expressed by the
product of
Rs= H*V
•Total risk (R
t): is the number of lives that could be lost,
persons injured, damage to property, or disruption of
economic activity due to a particular natural phenomenon. It
is therefore the product of specific risk (Rs) and element at
risk (E).
R
t= R
s*E= H*V*E

7. Geological Hazards

8. •A geologic hazard is one of several types of adverse
geologic conditions capable of causing damage or loss of
property and life.
•These hazards consist of sudden phenomena and slow
phenomena:
Sudden phenomena
•Earthquake
•Landslide
•Flood
•Volcaniceruption
Gradual phenomena
•Erosion
•Settlement
•Liquefaction
•Sand dune migration

9. Major Geological Hazards
•Some the geological hazards that occur in
Nepal that have caused and have potential to
cause loss of life and property are
oFlood
oGlacial Lake Outburst Flood
oErosion
oMass movement
oEarthquake

12. Flood

13. •A flood is an unusually high stage of river that
overtops the natural and artificial banks in any
reach of the river
•Though there is no definite stage above which a
river can be said to be in flood and below which it
is not in flood, there is an arbitrary elevation of
water surface established locally or nationally
•The river is considered to be in flood as soon as
water attains this level
•Floods are produced when the capacity of the
river is inadequate to carry off the abnormal
quantity of water arising from heavy rainfall
causing the river to overflow the bank

16. Causes
•Heavy rainfall-When it rains heavily for a long time, the
surface cannot infiltrate all the water and the water flows
downslope.
•In urban areas, due to concrete pavements, rain water
cannot infiltrate the ground and flow over the pavements,
through the sewers to the river, causing the river to flood
immediately after flood.
•Glacial lake burst: Bursting of glacial lakes upstream will
also cause the flooding in downstream
•Snow avalanches in the mountain region also cause flood
•Landslide damming and flood: Landslide occurring in the
stream or rivers dam the river and as the water increases
on the upstream the blocked water breaches the barrier
suddenly to cause flood
•Sea-storm causes flood in the coastal areas
•Seismic sea waves or tsunamis also cause the flooding in
the coastal region

20. Factors affecting flood
•Rainfall characteristics
oVariation of rainfall intensity
oDuration of rainfall
oDistribution of rainfall
•Snow melting combined with heavy rainfall
•Catchment characteristics
oArea of basin
oShape of basin
oSlope of basin
oBank storage

21. Effects of flood
•It decreases the fertility of land by depositing
stones, sand and other sediments on fertile land
•It damages individual and public property
•It damages civil infrastructures e.g. buildings,
roads, bridges, water supply lines
•Road transportation is disturbed
•Causes landslides
•Destroys natural vegetation, wildlife and aquatic
life
•It causes erosion
•Causes epidemic diseases like dysentery,
diarrhoea, typhoid

22. Method of Flood control and mitigation
•Flood estimation and frequency analysis must be
carried out before construction of any structure
across or along the river
•Estimation of probable maximum flood is
necessary to design of bridges, dams, roads and
irrigation projects
•Warning system to inform people on downstream
about incoming flood.
•Dam-dam and their associated reservoirs are
designed for flood control
•River defence-river defences such as
embankments, levees, weir etc.

24. Glacial Lake Outburst Flood
•When the rising temperature causes the glacier
to retreat, the melt water of the glacier existing in
sufficient amount under, beside and in front of
glacier form a lake called the glacial lake
•These lakes are ice-dammed and moraine
dammed
•Ice dammed lakes are formed at the sides of
glacier with ice impounding the water
•Moraine dammed lakes are normally found in
front of the glacier with terminal or end moraine
damming the lake

27. •The increasing melt water enlarges the lake which is
further enhanced by interconnections with each
other.
•As moraine is a loosely consolidated debris deposit
of glacier, it cannot stop the increasing mass of
water.
•Any disturbance on the glacial lake can easily alter
the balance of storage, consequently a great amount
of storage water would be released and cause the
flood along the path downstream in the lower
regions.
•This phenomenon is called the Glacial Lake Outburst
Flood (GLOF)
•GLOF is dangerous as it produces flash floods in the
rivers and damage the life and infrastructure in the
lower reaches.

28. •Nepal is considered one of the major GLOF prone areas
•Recent studies show that there are 20 potentially
dangerous glacial lake in Nepal namely, Dig Tsho, Imja,
Lower Barun, TshoRolpa, and Thulagi, all lying above
4100 m
•In 10 of them GLOF events have occurred in the past
few years and some have been regenerated after the
event
•Additional dangerous glacial lakes may exist in parts of
Tibet that are drained by streams crossing into Nepal,
raising the possibility of outburst incidents in Tibet
causing downstream damage in Nepal.
•The GandakiRiver basin is reported to contain 1025
glaciers and 338 lakes.

30. •The magnitude of GLOF and the damage inflicted by
it depends on the following parameters
•Surface area of the lake
•Volume of stored water
•Releasing rate of water
•Natural features of the river channel
•Status of infrastructure and habitation in the
downstream area

32. Causes of GLOF
•The moraine dam break due to external triggers
•Overtopping of water caused due to big waves
formed by huge mass of ice falling into the lake
•Continued melting of ice increases the volume of
the lake
•Snow avalanche or landslide disturb the dam
•Earthquake may also cause the dam to break
•Self destruction of the dam due to aging
•Ice-dammed glacier may burst duet to flotation of
ice dam, melting of tunnel through the ice etc.

34. Mitigation of GLOF impact
•Reducing the volume of water by controlled breaching of
the dam using explosives
•Construction of outlet control structure and reinforcing the
moraine dam
•Pumping of water using siphon
•Tunnelling through the moraine dam to release the water.
•Protecting the infrastructure downstream by making
diversion weirs, specially designed embankments
•Development of early warning system
•Monitoring of potentially dangerous Glacial lakes in the
mountainous regions during and after construction of
infrastructures using remote sensing
•Detailed study of snow hydrology in field
•Perfect engineering techniques to avoid damage during
peak discharges
•Public awareness.

35. Erosion
•Soil erosion is the process of destruction of
soil and the removal of the destroyed soil
material
•Vastly accelerated process of soil removal
brought about by the human interference,
with the normal disequilibrium between soil
building and soil removal is designated as soil
erosion

37. Factors affecting soil erosion
•Operating media –water and wind
•Erodability–resistance to the detachment
and transportation of the soil particles
•Steepness of the slope
•Vegetation–slows down the erosion
Effectiveness depend upon the height and
distribution of the leaves, the soil coverage
and roots density

38. Effect
•Loss of fertile soil
•Prolonged and extreme erosion may expose
the foundation of civil structures thus
rendering them vulnerable
•Erosion causes enlarging of gulleysand valleys
damaging bridges
•Bank erosion affects the infrastructures across
and along the river

40. Mitigation Measures
•Use of plants
•Crop rotation
•Reforestation
•Grazing control
•Use of bio-engineering
•Construction of weirs to prevent river erosion
•Soil treatment of increase infiltration
•Contouring of slopes

41. Mass Movement
•Mass wasting / mass movement is defined as
the downslope movement of material under the
influence of gravity.
•When the driving force acting on a slope
exceeds the resisting force, slope failure / mass
wasting takes place.
•The resisting force is due to shear strength of
the slope material. The driving force is the
component of the gravitational force acting
parallel to the slope.
•Influencingfactors:Saturationofmaterialwith
water,vibrationfromearthquakes,over-
steepeningofslopes,alternatingfreezingand
thawing,geology,&vegetationcover.

43. Falls are abrupt movements of
masses of geologic materials
that become detached from
steep slopes of cliffs

46. Debris Fall

47. Toppling Failure

48. •Rotational slide is one in which the surface of rupture is
curved concavely upward and movement is more or less
rotational about an axis above the slope slide.

54. Translational slide is where the detachment of
mass is along a planar surface.

56. Debris Flow

58. Creep
•Creep is the slow down-slope migration of soils and
loose rock fragments resulting from a variety of
processes, including frost heaving
•It is indicated by curved tree-trunks, bent fences or
retaining walls, tilted poles and small soil ripples.

59. Creep, Jharkotvillage, near Muktinath

63. Causes of Landslide
•Weak and sensitive geological material
•Weathered and sheared material
•Jointed and fissured
•Adversely oriented discontinuity
•Contrast in permeability
•Contrast in stiffness
•Fluvial erosion
•Deposition of load on the slope
•Vegetation removal
•Intense rainfall, prolonged precipitation
•Rapid snow melt
•Earthquake and artificial vibration
•Freeze and thaw weathering
•Volcanic eruption
•Mining

64. •Landslide triggering mechanism means event
that makes landslide happens.
•An external impetus such as intense rainfall,
earthquake, volcanic eruption, storm waves
and rapid stream erosion, human activities are
main triggering mechanism

76. Mitigation of Mass Movement
•Surface water drainage
•Groundwater drainage
•Rock bolt
•Pile works
•Retaining structures, and
•Bioengineering

77. Surface Drainage

78. Horizontal drainage to stabilize
the slope

79. Retaining structure with a provision of groundwater drainage

80. Afforestation
……means of landslide control

81. Tree roots anchors the soil mass
Tree roots

82. Before After