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