GEOG 1350 Mass Wasting and flooding

What are the endogenic and exogenic processes that influence mass wasting

landform building and transforming processes creating relief

Modify landforms gradually by carving, shaping, and reducing the earth’s surface

The rock cycle

What are the two main classifications of mass wasting

1) physical/mechanical (breakdown of rock into smaller pieces by heat, water, ice, and pressure)

2) Chemical (decomposition of rock due to chemical reactions altering chemical structures (involved water)

What is mass wasting

is the slow to rapid downslope movement of rock and earth under the sole influence of gravity

What is the driving force behind mass wasting

shear stress (acts as a downslope “pull”)

What are the factors of driving force

• gravity

• weight of sediment and structure

• slope angle

• shape of surface material

What is the Resisting force behind mass wasting

Shear strength (resists the pull)

What are examples of resisting force

• friction

• cohesion

• vegetation

What are stable vs unstable slopes

what are stable slopes, when the strength exceeds the weathering and erosion processes

What are unstable slopes, the slope materials are weaker than the weathering and erosion processes

What is the equation of slope stability

balance of shear strength vs shear stress ratio

F = (resisting force)/(driving force)

What is the angle of repose

maximum angle loose material can remain in place without sliding represents a balance between driving and resisting forces

Why might the angle of repose change

moisture conditions and sediment shapes

How does the angle of repose influence mast wasting

The larger the slope angle, the higher the chance of the mass wasting

What is the angle of repose normally around

33-37

What are the triggers of mass wasting

Vibrations (earthquakes, volcanoes, blasting, fracking, construction, sound vibrations)
Over steepened slope (road cutes and coastlines)

Increased weight on slope (buildings, change in drainage patters, precipitation)

Removal of vegetation (roots hold ground together, removing trees, roots pull water)

How does water impact mass wasting

water shifts the balance of the forces

• weight of water: sedimentary rocks and soil have pores that fill with water, adding to their mass

• clay, water can make clay layers swell and become slippery

• Absorption

• Positive pore-water pressure

What is positive pore-water pressure

increase of water concentration that pushes particles in soil apart causing slope instability

Dry vs Damp vs Saturated Sand

Dry sand- angle related to friction between grains

Damp sand- surface tension of water helps to keep sand grains together

Saturated Sand- water between particles keeps them apart

• Lost all cohesion

• Creates mass wasting events

What are rock falls

• detachment of rock or block of sediment (due to weathering)

• falls straight down or bounce and hits things

What are the broken up rocks at the bottom of the slopes from rock falls

Talus slopes or cones (shattered rock)

Do Rock falls have water involved

some, but doesn’t require any to cause this movement

How fast or Rock falls

very fast with no warning

What are rock avalanches

starts as one block => breaks up as moving down slop

Forms when a massive rock falls and explodes apart on contact with a slope

It kind of acts like a fluid (fluidization) which means it moves faster because of less fricition

what is an example of rock avalanches

Banff, Bow Glacier Falls 2025

What are the two types of landslides

1. Transitional slides

2. Rotations slides/slumps

What are landslides

a mass of sediment/rock traveling downslope as a coherent block along a tilted plane or surface of weakness

What are translation landslides

slides downslope as a coherent block along a straight failure slop or slip face

What is the difference between translational and rotational slides

translational is along a linear plane whereas rotational has a slight bent in the path, almost a semi circle (curved) that causes a small amount of rotation

Why do Japan get landslides

road cuts increase the steepness of slope along with building along these slopes adding to the weight

What happened during the Frank Slide in Alberta

1903, largest mass wasting event in Canada

Peak of turtle mountain collapsed

1km wide block

70 people died

In 100 seconds 30 million m³ of rock fell

What are theories of why the debris traveled so far in the Frank Slide Alberta

Air cushion → pocket of air beneath is → removed friction

Fluidization

What were some of the triggers of the Frank slide

mountains unstable structure due to mining and antisycline structure. The weather the preceding day was warm and then very cold during the night

What are rotational slides/slumps

• downward at top and outward rotation at the bottom

• involve movement along one or more curved slip/failure surfaces

• Often due to clay layers

What are flows

movement of individual particles within the flow itself

• Steep slope

• Lots of water

• Fast and chaotic

What are the two types of flows

Debris and mudflows (also includes Lahars)

What is the difference between debris and mudflows

Mudflows are dominated by small grain sizes

Slurry of mud (high water content and high velocity)

Debris flows start as mudflows but pick up debris as it flows, variety of grain sizes

Higher water content and high velocity

Where do debris flows evolve from

denuded slopes due to heavy rainfall evolves from large rock avalanches and fast moving slumps (mudflows)

What happened in Japan 2021 with mass wasting

mudflow => debris flow

Fast moving and very destructive

27 people died

131 courses destroyed

Massive loss in power

What triggers lahars

• eruptions

• post eruption rainfall

What are the traits of lahars

low-viscosity flows moves very rapidly down slope

What is creep

Slow movement down slope (cm/yr)

upper level soil movement

long term damage

What triggers creep

• freeze-thaw cycle

• wet-dry cycle

What is solifluction

the down slope movement of soil over a permanently frozen subsurface

Very slow movement of saturated particles

What does solifluction create

Forms lobes or terraces

How does solifluction occur

the active layer of permafrost melts, land become saturated and moves

What happened in Racho Palos Verdes, California 2024 with landslides (creep)

underground creep because of increased rainfall

• loss of power

• Damaged sewage system

No insurance => unable to move

How to minimize risks of mass wasting

• sites of previous mass movement with fail again

• identify hazardous slopes and vulnerable areas

What are soft solutions of mass wasting

• rezone as parks

• prohibit building

• education

• prediction

• monitoring

What are ways to monitor mass wasting

• rain gauges

• pore pressure

• slope movement sensor

• geophones

what are hard solutions towards mass wasting

• Drainage => drainage tiles, pipes that are perforated

• Plastic covering of slopes of short Crete => slopes water from saturating surfaces

• Reducing the slope angel

• Benching (stair step pattern)

• 'Rock bolts

• Blasting

• Channeling and containment

• dams and rock fall barriers (nets)

What are compound hazard associated with mass wasting

• earthquakes

• volcanoes

• wildfires

• atmospheric rivers

What is infiltration

water that enters the surface soil

What is subsurface and groundwater flow

underground water flows towards streams or the ocean

What is runoff

Water flowing across land surface to lower elevation, occurs when the amount of water at the ground’s surface is greater than the infiltration rate.

What increases runoff

• urban areas

• low vegetation

What decreases runoff

• vegetation

What are drainage basins and streamflow

where water drained from the surface and subsurface flow into river channels and eventually into the ocean.

What is the discharge of rivers

when a stream cannot accommodate increased discharge, it overflows its banks and flooding occurs

Discharge = velocity x depth x width

What are braided channels

• steep slopes

• multiple unstable channels with short-lived bars and islands (constant erosion and deposition)

• Variable discharge and excess sediment supply

• Coarse sand, gravel, and boulders

What are Meandering Streams

• Gradual Slopes

• Single sinuous channel

• Weaves back and forth across landscapes (carries small rocks, sand, and gravel)

• Asymmetrical channels form

What is a floodplain

• The flat, low-lying area adjacent to a channel of water and subjected to recurrent flooding.

• Built up by erosion and deposits

• Can be used as a prediction source, if a flood has occurred here before it there is a high change it will occur again.

When does flooding occur in rivers

when the stream’s normal capacity is exceeded and water overflows onto land

What are causes of flooding in rivers

extreme precipitation, storm surge, snow melt, frozen ground, damns, or levee failure, and ice jams

What happened in Spain 2025 related to flooding

excessive rainfall that cannot be absorbed, runoff into rivers; the rivers then rise with no warning flooding occurred, 50 mm of rain/hour

What is the most common disaster in Canada

Flooding

What is the main causes of flooding in Canada

too much water volume for the capacity of streams

• Coastal areas

• Outburst floods (glacier-dammed lakes)

• Dam Failure (triggered by natural events)

What factors increase flooding in Canada

• amount and type of precipitation

• Characteristics of the drainage basin

• Characteristics of floodplain

• urban stormwater runoff

• Climate

Why are flood trends increasing in Canada by year

• more records/better records

• climate change

• urbanization

• loss of wetlands

• development on flood plains

What months are floods in Canada most common

• Apr/May - snow melt

• Jan - ice jams

What is a hydrograph

a graph of a specific stream discharge over time for a specific place

What do hydrography show

the relationship between precipitation and water flow in stream channels

How long it takes to rise from base flow to maximum and back

What are the different aspects of the hydrographic

• rising limb (surface runoff, ground and soil water reach the river)

• Crest (peak discharge)

• Lag time (time between peak precipitation and peak discharge)

• Falling limb (water is still reaching the river, but in decreasing amounts)

• Baseflow (stream discharge before a rainfall event)

What is each box pointing too that is covered

bottom left (lag time)

Top Left (rising limb)

Top right (peak discharge)

Bottom right (falling limb)

What controls the drainage basins

• size, shape, and relief

• Steep slope = faster water runoff = higher peak flow

How does urbanization change the shape of the hydrograph

• less infiltration = higher runoff (because of paved over ground)

• Storm sewers

• Urban floods only last 20% as long, but are four times higher

What is the frequency of floods

larger floods = longer recurrence intervals

• Flood frequency curves differ for different streams

• can be used to estimate return time of given flood size

What are the different flood types

• Local thunderstorms (flash flooding)

• extreme rainfall over days (atmospheric river, regional floods)

• Extreme rainfall over hours (regional or local floods)

• hurricane storm surges (coastal areas)

• broken ice on rivers, ice jams

• short lived natural dams

• Failure of human-built levees or dams

What are flash floods

thunderstorms can release heavy rainfall, creating flash floods, that have steep topography which causes rapid run off

What are majority of flash flood deaths from

50% of deaths are vehicle related (trapped in cars)

What are regional floods

inundation of an area with floods for weeks

Low sustained rainfall leads to saturated soils which then leads to runoff

Slower water rise - therefore evacuations can be applied

• few deaths

• extensive damage

• large river valleys with low topography

• widespread system → prolonged heavy rains

What causes regional floods

• atmospheric river

• La Niña

What is the red river attributions

a) unusual northward flow (spring flows)

• as winter snow melts in the south the water moves northwards, where the ground is still frozen therefore no infiltration = flooding.

b) geologically young

c) very low gradient of riverbed - slow flowing tends to pool

What are natural levees

deposits of past floods leads to natural levees of sand and rock at the side of a river. creates natural flood protection

What are mitigations of floods

structural

• Constructions of dams, seawalls, evacuation routes

• building levees

• channel modification

Non structural

• Flood forecasting

• evacuation planning

• zoning and land-use policies

• managed retreat

• education

• insurance

What is the best flood mitigation for long term and why

managed retreat

• flooding

• sea level rise

Strategic relocation, moving people, buildings, and assets and infrastructure away from areas at risk of foods

Reduces risk permanently

What are levees prone to failure

cost of building may be more than value of structure intended to protect (often maintenances fall behind) sense of security.W

What are the 4 ways levees can fail

• Wave attack

• Overtopping water

• Slumping

• Sand volcanoes

Explain the New Orleans Levees and Flood walls

• transformed the Mississippi into restricted ribbons of water, cutting off flood plains

• Flooded channels cannot spread so they are forced to rise and the levees become overtopped

What happened with Hurricane Katrine (2005) and New Orleans

Cat 3 hurricane

Lots of rainfall lead to levee failure (50 location overtopped - 80% of the city was flooded)

125 billion in damages

Why is New Orleans sinking

• built on silt, clay, and sand which comes from river deposits

• without these deposits from flooding there is no fresh silt to replenish, so the material there becomes compacted + there is sea level rise

• ~1.8 m below sea level

What are temporary Levees

Sandbagging (easily breached, more therapeutic)

Dams (they are limited by construction material, the type, and the rate at which sediments fills reservoirs

What happened during the Saguenay flood 1996

19-21 July 279mm of rainfall

Dam overtopped

flooding

How has Winnipeg mitigated against the red river

• The Floodway

Diverts the water around the city to a river

Used every 2-3 years

What is the Red River Floodway built to withstand

1 in 700 year event