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Natural Hazards 5

„Nature to be commanded, must be obeyed“ (Francis Bacon, 1561-1626)

W. Eberhard Falck eberhard.falck@uvsq.fr

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Exogenic processes involving the solid Earth

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Geomorphology •  the scientific study of landforms and the processes that shape them

•  Primary surface processes responsible for most features include wind, waves, weathering, mass wasting, groundwater, surface water, glaciers, tecto-nism, and volcanism.

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Geomorphological features: Desert

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Types of dunes

star dune

Seif / longitudinal dune

Barchan / crescent dune

transverse dune

migrating dune

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Migrating dunes •  Dune migrate by a wind-driven process called ‚saltation‘ = jumping

sand grains

•  Dune formation begins, when drifting sand is stopped by obstacles, e.g. vegetation

Sand dunes slowly bury a lighthouse in Jutland/DK

Dunes bury roads

... and

oases

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Geomorphological features: River System

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Geomorphological features: Sea Coast

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•  Landslides and rockfalls

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Examples

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Overview

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•  loss of slope stability due to destabilising porewater pressures following saturation by heavy rain, snowmelt etc.

•  loss of cohesion due to vegetation cover loss

•  loss of cohesion due to thawing

•  erosion of the toe of a slope by rivers or the sea

•  earthquake-caused liquefaction destabilising slopes

•  permafrost slide-plane

Causes of landslides

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Causes of land- and rock-slides

Undercutting by sea undercutting by road etc. frost-heave

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Mechanisms of landslides

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•  vibrations from machinery or traffic

•  blasting

•  earthwork that alters the shape of a slope, or places additional loads on an existing slope

•  in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock

•  changes to vegetation cover that changes the rainwater infiltration rate

•  frost-heave

Triggering of landslides

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Occurrences of landslides

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Classification of landslides

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Worldwide distribution

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Rockfalls

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Rockfalls

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•  loss of rock face stability due to destabilising porewater pressures following saturation by heavy rain, snowmelt etc.

•  loss of cohesion due to frost-thaw cyles - expanding ice widens fissures and cracks

•  undercutting by building activities

•  earthquakes

Causes of rock falls

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•  The biggest natural desaster in Switzerland in historical times

•  40-50 million m3 debris destroy two villages and kill almost 500 people

Rockfall above Goldau (CH) 02/09/1806

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•  Destruction or disruption of infrastructure, e.g. roads, service ducts

•  Destruction of human settlements

•  Blockage of rivers resulting in flooding upstream and flooding downstream when the river breaks through the fallen masses

•  Floodwaves, when falling into surface waters or occurring below the water table

Effects and impacts

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•  Methods consist mainly in reprofiling, increasing the cohesion, keying-in into underlying rocks and drainage to reduce porewater pressures

Mitigation of landslides

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•  Methods consist mainly in reprofiling, anchors, steel nets, gunnit cover, protective forests

Mitigation of rockfalls

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•  Some clay minerals can take up up to 1000% humidity •  Swelling of the clays is a consequence •  Swelling will cause

•  Differential movement in building foundation •  Cracking of buildings •  Buckling of pavements etc.

Expanding clays

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Erosion • occurs due to weathering and subsequent

– transport by wind, water, or ice; – down-slope creep of soil and other material under the force

of gravity; – solifluction – scraping ice – burrowing animals, in the case of bioerosion

• mud-slides or torrents are more dramatic forms of erosion

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Weathering

•  the decomposition of rocks, soils and minerals through direct contact with the atmosphere

•  Physical weathering through direct action of heat, water, ice and pressure

•  Chemical weathering by atmospheric chemicals or biologically produced chemicals

•  Weathering products and residues would remain in situ, if not removed by the action of wind, water, or gravity, i.e. by erosion

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Some examples of weathering

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Processes of erosion

•  loose material, i.e. clay, sand, gravel, is moved by the action of wind, water, ice, or gravity alone

•  the intensity of erosion depends on the energy transmitted, i.e. the velocity of the water etc.

•  after dissipation of the energy the material settles, i.e. sediments

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Forms of erosion

•  Splash erosion is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soil

•  Sheet erosion is the detachment of soil particles by raindrop impact and their removal downslope by water flowing overland as a sheet

•  Rill erosion refers to the development of small, ephemeral concentrated flow paths that function are both a sediment source and conduit

•  Gully erosion occurs when water flows in narrow channels during or immediately after heavy rains or melting snow!

•  Shoreline erosion occurs through the action of currents and waves!•  Stream erosion occurs with continued water flow along a linear

feature

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Forms of erosion - Images

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Coastal erosion

•  can be very dramatic and threatens the very existence of e.g. islands

•  for instance, the land-loss at Sylt (Germany) can be 1.4 m per year

1648 today

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Coastal protection strategies

•  Do nothing - leading to abandonment •  Managed retreat - re-aligning coasta defences and relocation of

threatened infrastructure •  Hold the line - stabilisation by e.g. seawalls or beach

nourishment •  Move seaward - construction of breakwaters etc. seawards •  Limited intervention - protection of individual structures rather

than areas

•  Holding the line can be achieved by hard or soft techniques •  Hard techniques include seawalls, dikes, groins etc. •  Soft techniques consist in beach nourishment and managed

inundations

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Coastal protection: Hard defences

Groynes Seawall Rock armour

Revetments Gabions

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Coastal protection: Beach nourishment

•  Sand is taken from the seabed further out and pumped onto the shore to replenish sand lost by erosion and to create a reservoir for several years

•  The nourisment has to be repeated every few years, depending on the erosional losses

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Factors influencing the rate of erosion

•  frequency, intensity and duration of precipitation

•  frequency, intensity and duration of winds

•  drainage patterns

•  inclination of slope

•  vegetation cover

•  freezing-thawing cycles

•  presence or otherwise of glaciers

•  agricultural practices (crop rotation, direction of ploughing vs. slope)

•  disturbance by (burrowing) animals

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Factors enhancing erosion

•  deforestation due to e.g. logging or wildfires

•  slash-and-burn extensive agriculture

•  overgrazing resulting in loss of vegetation

•  ploughing furrows down-slope

•  melting of permafrost

•  river ‚corrections‘ enhancing flow velocities

•  increased rainfall rates - particularly in winter

•  uplift / tilting of slopes

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Impacts of erosion •  loss of fertility due to soil loss

•  silting-up of drainage conduits

•  silting-up of reservoirs

•  dust-storms

•  dust-bowl effects

•  reduced groundwater recharge due to faster run-off

•  The resulting socio-economic impacts can be dramatic, e.g. 5 million people emigrated from the ‚dust-bowl‘ region in the USA between 1940 and 1970.

•  Coastal erosion management consumes considerable resources to combat it.

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Monitoring erosion

• erosion can be a slow, but steady process that is difficult to observe directly

•  indirect monitoring techniques are required to quantify erosion and to give early warnings

– monitoring sediment loads in creeks and rivers – monitoring vegetation covers using multi-spectral satellite

images

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Mitigating erosion

•  addressing the factors promoting erosion, e.g. –  re-vegetation/re-forestation –  coastal defense measures –  improved agricultural practices –  re-naturation of rivers –  re-profiling of slopes

•  mitigation measures have to carefully analysed for their consequences, as they may simply displace the erosion problem

•  measures have to aim at dissipating the energy that causes erosion

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Wildfires

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Definition

•  A wildfire is any uncontrolled fire that occurs in the countryside or a wilderness area

•  Reflecting the type of vegetation or fuel, other names may be given, such as

–  brush fire - grass fire –  bushfire - hill fire –  forest fire - peat fire

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Causes of wildfires

•  wildfires can be triggered by inter alia –  lightening –  spontaneous combustion –  vulcanic eruptions –  sparks from rock falls –  electric arcs on power transmission lines –  human negligence: cigarette butts –  slash-and-burn cultivation –  arson

•  in Europe 99% of wildfires involve some human action

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Forms of wildfires

•  Crown, canopy, or aerial fires – burn suspended material at the canopy level, such as tall trees,

vines, and mosses

•  Ladder fires –  consume material between low-level vegetation and tree canopies,

such as small trees, fallen logs, and vines

•  Crawling or surface fires –  are fueled by low-lying vegetation such as leaf and timber litter,

debris, grass, and low-lying shrubbery

•  Ground fires –  are fed by subterranean roots, duff and other buried organic matter

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Progress of wildfires •  Four elements have to come together:

–  combustible material such as vegetation, –  that is subjected to sufficient heat and –  has an adequate supply of oxygen –  an ignition source

•  A wildfire front is, where active flames meet unburnt material, or the smoldering transition between unburnt and burnt material

•  As the front approaches, the fire heats both the surrounding air and woody material through convection and thermal radiation

–  First, wood is dried as water is vaporised at a temperature of 100°C –  Next, the pyrolysis of wood at 230°C releases flammable gases –  Finally, wood can smolder at 380°C, or –  when heated sufficiently, ignite at 590°C –  heat transfer (radiation/convection) can precede the flames, warming the

air to 800°C and drying and pre-heating flammable materials –  flashover or torching may occur: the drying of tree canopies and their

subsequent ignition from below

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Fire tornados

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Assessment of hazard

The fire hazard depends on the

•  IGNITION probability

• PROPAGATION probability

• VULNERABILITY

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Enhancing factors

Fire risks increase due to •  extended periods of dry and hot weather •  monocultures of high-resin trees, such as pines •  accumulation of undergrowth and plant litter due to the absence

of fires for long periods •  accumulation of fuel due to strict suppression of fires in areas

where they would occur naturally quite frequently

Propagation of fires is enhanced by •  hillsides •  development of firestorms, i.e. a stack effect over a fire •  airborne transport of glowing ash and embers - ‘jumping’ of fires •  strong and changing winds

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Ecological aspects

•  Fires in themselves are not necessarily something bad, they do occur naturally in many areas of the world

•  The ecological impact depends on fire frequency and intensity •  Some ecosystems are adapted to recurring fires •  Certain tree species (e.g. cork oaks) are adapted to fires of not too high

intensity •  Strict fire supression leads to accumulation of fuel that might lead to

stronger fires and therefore has counterproductive effects - the ‘extinction paradox’

•  Certain plants only germinate after a fire event •  There are certain plant successions that naturally recultivate areas affected

by fire

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Firefighting techniques •  Firefighting consist in measures to suppress the

actual fire and to prevent its further spreading •  The supression attempts to exclude oxygen and

to cool the fuel below the flame temperature •  Oxygen can be excluded by dowsing with water

or sand, or by beating the fire with flaps on sticks

•  Water evaporates on the fire an cools the fuel; fire retardants may be added to the water

•  Water may be deployed by on land tankers-pumpers, or airborne by plane or helicopter

•  The spreading of fires can be controlled by cutting vegetation-free zones - firebreaks into forests

•  Counter-fires - controlled burning of undergrowth and litter to remove fuel, can help to control large-scale fires

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Firefighting in remote areas •  In some countries with extensive pine forests

in remote areas, such as Alaska, Canada and Siberia special strategies and techniques for firefighting have been developed

•  Overland deployment of crews may be difficult and may take too long

•  Therefore, airborne crews are deployed by parachute (‘smoke jumpers’) or helicopter

•  These crews cut fire breaks and remove fuel, e.g. by counter-fires, or use fire-flaps to extinguish small fires

•  The areas at risk are monitored from fire towers, by aerial photography or - today - by IR satellite imagery

•  Early detection and attack allows suppression of fires while still small

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Socio-Economic impacts

•  Wildfires can cause serious losses in –  economically valuable timber resources –  wildlife in general –  game –  biodiversity

•  Wildfires at the urban-rural interface can cause loss of property and human life, and can damage or destroy infrastructure

•  Wildfires can increase the risk of (flash) floods due to diminished water retention capacity

•  Wildfire monitoring, supression and consequence mitigation diverts resources from other activities

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Environmental and health impacts

•  Wildfires can have a climatic and health impacts due to the release of fine air-borne particles - soot and flyash

•  Wildfires can promote the release of contaminants and toxic or carcinogenic chemicals from damaged buildings

•  Heavy metals from paints and consumer goods

•  PAHs, PCBs etc. from plastics

•  Asbestos from insulations

•  Wildfires can damage industrial and waste management facilities, leading to a releas of toxic materials

•  Wildfires can increase the risk of (flash) floods due to diminished water retention capacity

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Next Sequence

•  Extraterrestrial hazards •  Risk governance