GG231 Study Guide - Final Guide: Consumerism, Debris Flow, Anastomosis

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Published on 15 Apr 2013
School
WLU
Department
Geography
Course
GG231
Professor
Risks & Disasters Final Exam Notes
Landslides:
Three types of movement:
Fall
o Moves through air, lands at base of slope
o Triggered by vibration, ice or vegetation
o Rapid process
Slide
o Movement in contact with underlying surface
o Ex. Rockslide, translational slide slump, slip
Flow
o Plastic or liquid movement in water and sometimes air
o Mass breaks up and flows during movement
o Ex. Solifluction, mudflow, debris flow, debris, snow avalanche
Landslides are mainly a result of a change in the forces on the slope:
Driving forces
o Uplift and erosion
o Cause the rock material to move faster down slope
Resisting forces
o Acts as friction force that is against the motion of rock materials
Geographic Distribution: distribution of landslide-prone materials in Canada:
Landslides are most common in mountainous areas, the St. Lawrence Lowland, and river valleys on the
western interior plains (British Columbia, Yukon and Alberta)
Materials particular prone to land sliding include lacustrine and marine silts and clays, cretaceous shales,
Quaternary volcanic rocks.
Three factors are expected to increase the occurrence of landslides in some areas of the world in this century:
Urbanization and development will increasingly expand into landslide-prone areas
Tree cutting will continue in landslide-prone areas
Changing global climate patterns will result in increases in precipitation in some regions
Prediction and Warning of Landslides:
slopes and seismic activity
o consider over steepened slopes enhanced by seismic activity
geology and structure
o consider soluble rocks or those weakened by water, bedding planes
surface water build up
o consider the water conditions and springs, pools of standing water
topographic and vegetation features
o consider scarps, old landslides, hummocky and young vegetation
accelerated creep
o consider equipment embedded in slope, vegetation stems, and cracked road surfaces
Techniques for Preventing Landslides:
slope drainage
o interceptor drains, capture runoff and take away from slope, repair pipes and wells by draining
or pumping water out
slope reduction
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o reduce instability by introducing benches and terraces, remove falling material
engineering to resist mass movement
o armour surface with concrete, crushed rock to reduce erosion
engineering to mitigate damage
o netting or wire fencing, rock sheds or tunnels
Case Study: California
The 2005 landslide that took place in La Conchita, California started as a slump and then turned into a
flow that buried 30 homes and killed 10 people. The landslide took place above the sensors that were put
in place after the 1995 landslide; therefore a warning was not able to be put in place.
Social adjustment would be the best approach because even though sensors were put in place after the
1995 landslide, they still were unable to provide a warning to the residents. Also imputing engineered
techniques like netting or wired fences would cost a lot of money, and would not necessarily work due to
the fact of how steep the slope is and how weak the sedimentary rocks are.
La Conchita is a part of the “landslide capitals” in southern California and the sea cliff that is exposed
consists of older landslide deposits and weakly cemented sedimentary rocks with little shear strength.
With heavy rainfall, it is likely that a landslide may occur in the future.
more landslides are likely to occur as uplifts of 4 to 6 meters occur every thousand years, and these uplifts
occur when landslides take place. With the steep surface, low shear strength and large amounts of water,
it is likely that these landslides will continue.
Contributing factors:
o Presence of steep, high slopes
o Presence of weak rocks
o Presence of numerous historic and prehistoric landslides
o Periodic prolonged and intense rainfall
Solutions:
o Stabilize the slope
o Change the land use
o Install an effective warning system
Floods:
Drainage Basin:
Region drained by a single stream. Each stream thus has its own drainage basin that collects rain and
snow.
A large river basin, such as the Red river basin, is made up of hundreds of small watersheds drained by
smaller tributary streams.
Flash Floods:
Often associated with intense rainfall events such as those that occur with convective thunderstorms
Usually account for most flood-related deaths since they are highly unpredictable and little warning is
possible.
o Example: Big Thompson Canyon in Colorado
Regional Floods:
Often cover large areas in extensive river valleys with flat topography
Occur following periods of prolonged rainfall or large snowmelt
o Example: Mississippi River flood of 1993
o In Canada, southern Manitoba
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Discharge:
The volume of water that moves through a cross-section of a river per unit of time
It is calculated by multiplying the cross-sectional area of the water in the channel by the flow velocity, and
is commonly reported in units of cubic metres per second.
Three channel patterns:
Braided
o Large number of intersecting active channels
Anastomosing
o Two or more channels and intervening stable islands where sediment is temporarily stored
Meandering
o Single channel shaped like a snake
Use of hydrograph:
A hydrograph is made by installing a recording gauge to:
Obtain a continuous record of the water level (or stage)
This record is then used to produce a (b)stage-time graph
Field measurements at various flows provide a (c) stage-discharge graph
o Combining stage-time graph and stage-discharge graphs make the final hydrograph
Flood Prediction:
In managing the flood hazard it is important to determine the frequency of flooding. Planning and land
use are often dependent on the predicted interval of flood events.
Often, the bigger the flood the longer the return interval. Floodplains can be mapped based on a 20-year,
50-year, or 100-year flood.
Magnitude & Frequency of floods:
Smaller floods may be produced by less intense storms that occur more frequently, and larger floods
produced by intense storms may happen less frequently.
Recurrence interval:
Determined by the following equation and plotted to create a discharge-frequency curve:
o R = (N+1) M
Channelization:
The modification of the stream channel such as straightening or deepening. This often occurs in urban
areas where the stream is fitted to the built structures.
Channelization can have a number of impacts of the stream channel:
o Increase the possibility of flash floods
Which are shorter duration and larger in volume
o Increase in frequency
Wildfires:
Processes that initiate fire:
Natural
o Lightning strikes
Human
o Combination of intention and accident
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Document Summary

Fall: moves through air, lands at base of slope, triggered by vibration, ice or vegetation, rapid process. Slide: movement in contact with underlying surface, ex. Flow: plastic or liquid movement in water and sometimes air, mass breaks up and flows during movement, ex. Landslides are mainly a result of a change in the forces on the slope: Driving forces: uplift and erosion, cause the rock material to move faster down slope. Resisting forces: acts as friction force that is against the motion of rock materials. Geographic distribution: distribution of landslide-prone materials in canada: Landslides are most common in mountainous areas, the st. lawrence lowland, and river valleys on the western interior plains (british columbia, yukon and alberta) Materials particular prone to land sliding include lacustrine and marine silts and clays, cretaceous shales, Three factors are expected to increase the occurrence of landslides in some areas of the world in this century: Urbanization and development will increasingly expand into landslide-prone areas.

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