Natural hazards 4-2013
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Transcript of Natural hazards 4-2013
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Natural Hazards 4
„Nature to be commanded, must be obeyed“ (Francis Bacon, 1561-1626)
W. Eberhard Falck [email protected]
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Exogenic Hazards continued
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• Ice surges • Ice falls • Melt-water surges • Glacier-generated earthquakes • Calving-generated tsunamis • Turning icebergs • Risks to overland travel - crevices
Glacier-related hazards
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• are short-lived events where a glacier can move up to velocities 100 times faster than normal (300 meters per day), and advance substantially.
• Surging glaciers are clustered around a few areas. • High concentrations of surging glaciers can be found in Svalbard, Canadian
Arctic islands, Alaska and Iceland. • Glacial surges can take place
at regular, periodic intervals. • In some glaciers, surges can
occur in fairly regular cycles with 15 to 100 or more surge events per year.
• In other glaciers, surging is unpredictable.
• Mechanisms are not yet very well understood
Glacier surges
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• are outbursts of melt-water from underneath a glacier • causes can be the built-up of melt-water lakes or a heat-source
underneath, namely volcanos • they are frequent e.g. in Iceland, where they are called jökulhlaup
Melt-water surges
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•
‚Tsunami‘ generated by rotating icebergs
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•
‚Tsunami‘ generated by calving glacier
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Arctic travel hazards: glacier crevasses
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• is ice that floats on the surface of the water in cold regions, as opposed to fast ice that is attached (‚fastened‘) to a shore.
• Drift ice is carried along by winds and sea currents. • When the drift ice is driven together into a large single mass, it is
called pack ice. • Wind and currents can pile up ice to form ridges three to four
metres high. • Typically areas of pack ice are identified by high percentage of
surface coverage by ice: e.g., 80-100%. • An ice floe is a large piece of drift ice that might range from tens
of metres (yards) to several kilometres in diameter.
Drift and Pack Ice
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• are large areas of pack ice formed from seawater
• They significantly change their size during the seasons.
• Over the past decades a significant shrinkage of the arctic ice sheets was observed.
Polar Ice Packs
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• Impediment to shipping, icebreakers are needed to keep shipping lanes and harbours open.
• Ships can be become trapped in pack ice, be squashed and sunk.
• Drifting ice can damage jetties and embankments.
• It is generally not possible to build structures on the seabed that reach to the sealevel in such areas.
•
Pack Ice Hazards
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Breaking the ice ...
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• Breaking-up ice sheets and debris may block flow and cause flooding • Piling-up ice can destroy bridge pillars and foundations • Flowing ice will scour embankments and other civil engineering structures
Riverine ice hazards
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Periglacial Hazards
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An Ice-Age legacy: Permafrost • Permafrost soil is soil at or below the freezing
point for two or more years. • Permafrost exists in 24 % of exposed land in
the Northern Hemisphere - a considerable area of the Arctic is covered by permafrost.
• The extent of permafrost can vary as the climate changes.
• Overlying the permafrost is a thin active layer that thaws during the summer.
• The active layer thickness varies by year and location, but is typically 0.6–4 m.
• In areas of continuous permafrost and harsh winters permafrost may reach down to 1,493 m (e.g. Siberia).
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The extent of permafrost
International Permafrost Association http://ipa.arcticportal.org/
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Permafrost phenomena • Thawing-freezing cycles and the buoyancy of ice in surrounding water,
mud and soil leads to a variety of forms of ground movement that result in characteristic surface processes and patterns
• Talik • Ice-wedges • Stone rings • Pingos • Palsas • Thermokarst • Solifluction • ‚Drunken forests‘
• Multi-lingual permafrost glossary: http://nsidc.org/fgdc/glossary/ • Some of these features and processes can pose significant civil
engineering problems in arctic regions
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Permafrost: solifluction • is a type of mass wasting, where waterlogged sediment moves slowly
downslope over impermeable material • it occurs over permafrost, when the active layer becomes water
saturated, causing a form of downslope flow or creep • the creep is due to frost heave that occurs normal to the slope, as well as
to small-scale slippage • it can occur on slopes as shallow as 0.5 degrees and at a rate of between
0.5 and 15 cm per year
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Permafrost hazards • the active layer can make travel cross-country treacherous or areas impassable • thermokarst can impede cross-country travel due to the irregular surface • tracked vehicles with low specific load per axle may be needed
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Permafrost building hazards • frost heaving, thawing of pingo or palsas and similar processes moves
trees and man-made structures located over permafrost ouf of their vertical alingment
• Thawing permafrost can make buildings sink in or to slide down hills
• Thermokarst can lead to the collaps of roads and other infrastructure
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Flooding
• Riverine floods
• Estuarine floods
• Coastal floods
• Dam failure
• Animal activity (beavers)
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• triggered by prolonged or high-intensity rainfall events, snowmelt (particularly when combined with rainfall)
• run-off exceed the capacity of the river channels
• obstruction of drainage by debris, landslides, rock-falls, avalanches can lead to flooding upstream
• flooding proceeds usually slowly
• but can be very fast, when dams fail, or obstructions are broken through - flash floods
Riverine floods
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Predicting riverine floods
• based on run-off models for the catchment area • run-off models describe the water levels in a surface water body
resulting from precipitation events over parts or all of the catchment area
• the numerical models are calibrated against real rain events - dependent on historical data
• run-off depends on characteristics of the catchment area, such as topography, shape, vegetation cover, sealed areas, soil permeability and previous saturation, and the shape and form of the river bed
• substantial changes in the catchment area invalidate the calibration - failure to accuractely predict flooding as to time and height of flood waves
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Surface run-off
Elements of run-off Catchment area
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Hydrograph
Measured hydrographs are used to calibrate the storm run-off models
storm run-off
baseflow
rain event
flood event
time
run-off
dry weather run-off
flood peak
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Flooding prevention and mitigation • flooding peaks can be lowered by
– ‚re-naturing‘ river courses – storm water retention basins – increasing infiltration in the catchment area / reducing sealed areas
• floods can be retained by dykes of levees (American English) • in Europe and N-America many rivers prone to flooding are managed • flood waves are controlled by weirs that can be opened to divert waters
into storm retention basins, polders or flood plains without settlements • emergency measures include strengthening of dykes with sandbags or
breaching dykes to let flood areas of less value • European Flood Action programme:
http://ec.europa.eu/environment/water/flood_risk/com.htm • Web-site for real-time flood alerts in France:
http://www.vigicrues.gouv.fr/
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Flooding risk assessment • river floodplains can be mapped
based on topographical data and hydrographs
• on this basis flooding risk maps can be developed and the zoning regulations developed accordingly
• in the past people used ‚traditional‘ knowledge and avoided settling on low-lying floodplains, preferring the high-terraces
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Impacts • flood control measures
– require significant resources – have a significant footprint
• floods can – cause massive damage to infrastructure, such as dams, bridges, roads,
sewers, canals, gas and electricity networks – destroy or damage private property such as houses, factories, cars,
gardens, life-stock – contaminate drinking water wells and supply systems, agricultural land – cause the spread of diseases due to cadavers – permanently damage vegetation – severly disrupt economic activities
• However: in history several cultures, e.g. Egypt and Mesopotamia, depended on the annual flood for nutrient supply to agricultural lands
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Elbe flood August 2002
• Heavy rainfall over the Ore Mountains (CZ/D) lead to widespread flooding along the whole length of the Elbe River
• In some places the normal water level was exceeded by 12 m !
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Coastal flooding
• can result from a combination of two or more events – high astronomical tide – storm surge – seiches – pile-up of waves due to the bathymetry
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When the tide comes in ... • Tides are caused by the gravitational orces of the moon and other
celestical bodies in combination with centrifugal forces
• Most costal areas experience two tides per 24 h
• The height of the tide depends on the topography of the seabed, the constellation of the celestical bodies and the latitude
• Open ocean tides are shallow, i.e. < 1 m
• Tides in peripheral seas, such as the Baltic, Mediterranean and Black Seas are also shallow
• Tides in estuaries can be enormous, e.g. Fundy Bay (16 m), Bristol Channel, due to the water being forced into them
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Spring tides and neap tides
• Very high tides are called spring tides and occur, when sun, moon and earth are in one line (in conjunction).
• Very low tides are called neap tides and occur, when sun, moon and earth are 90° apart.
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Predicting astronomical tides • Predicting tides pre-occupied
mathematicians and astronomers since antiquity
• Astronomical tides are composed of numerous harmonic variations of celestical bodies exerting gravitational forces onto the Earth‘s oceans
• These harmonic variations can be deconvoluted using Fourier analysis (Lord Kelvin)
• For predictive purposes (analogue) computers (tide predictors) were built from mid-19th century onward
• Today complex codes on digital computers are used that also include hydrodynamics
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Estuarine floods
• can result from a combination of two or more of these events
– high river discharge e.g. due to precipitation/ snowmelt upstream
– high (astronomical) tide or wind surge blocks river drainage
– built up of a tidal bore
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• Are standing waves in confined water bodies, such as lakes or peripheral seas (Baltic, Adriatic)
• Variable meteorologic pressure distributions over the water body causes long waves that are reflected by the margins
• Positive interference of the reflected waves give rise to high water levels
• Seiches are frequently responsible for flooding e.g. in Venice or St. Petersburg
Seiches
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Impacts
• destruction of coastal defences
• contamination of agricultural land by salt
• soil erosion
• coastal freshwater resources become brackish
• destruction of houses, infrastructure
• distress among humans
• spreading of diseases in crowded shelters
• economic losses
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The Netherlands: the February 1953 flood • Strong winds plus a spring-
tide brought water levels to 4 m above normal
• Dykes began to overtop and erode from the back, to break eventually
• Large areas of southern Netherlands became flooded
• More than 500 deaths • Afterwards the dykes were
considerably strengthened and the Schelde/Maas tide control was constructed
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The Netherlands: floods and flood protection
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Storm Xynthia, 27 February 2010
http://www.lefigaro.fr/actualite-france/tempete-xynthia.php
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Flash floods • are flood waves that occur
suddenly due to severe precipitation events
• the rainfall can occur far away in uplands of areas with low water retention capacity
• there are often no warning signs in the effected areas
• floods can carry large amounts of debris that causes additional impacts
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Flash floods: Australia
January 2011
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Dam failure: Barrage de Malpasset (F) • Constructed 1952-1954 upstream of Frejus for irrigation and water supply
• Following heavy rainfalls in late November/early December 1959 resulted in near-overtopping, the foot of dam became dislodged
• On 2 December the dam breached
• A 40 m high water wall raced down the valley towards Frejus at a speed of up 70 km/h
• The villages of Malpasset and Bozon were destroyed
• When the flood wave reached Frejus after 20 minutes, it was still 3 m high
• About 500 people died in the incident
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Dam failure: Kolontár tailings pond (H) • In October 2010 the retaining dam of
the talings pond of the alumnium smelter in Kolontár, Hungary, failed.
• A flood wave of ‚red mud‘ rushed down over several villages
• Several people died • The caustic red mud caused injuries
to others
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Mechanisms of dam failure
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Mechanisms of dike erosion
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Good dike design • allows the waves to run ‚dead‘ • prevents waves from breaking at the toe or crown • prevents erosion in the back after overtopping • prevents complete soaking and hydraulic failure of toe
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Mitigation • Land-use planning to prevent building in flood zones • ‚Natural‘ measures, such as overflow basins, polders • Weather reports • Prediction tools for flood events
• Early warning systems for flood events
• Good maintenance of flood defence works
• Strengthening of flood defence works
• Evacuation plans • Disaster relief plans
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Beaver dams • Next to humans, beavers are the species that most extensively shapes
ist own environment • beaver dams and the ponds created by them can cover large areas -
several metres high and wide and hundreds of metres long • beavers build/repair these dams quite quickly, so that flooding can occur
within days
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Next sequence
• Processes involving the solid earth