Transcript
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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
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