Coastal cliffs hazardNatural and human-induced hazards

Stefano FURLANI, Stefano DEVOTO

Department of Mathematics and Geosciences

Program of the lecture

Types of landslides

Geohazards

Accidents and perception of risks

References

Type of landslidesLandslide classification

Types of coastal landslides

Source: Devoto, 2013

FALL

TOPPLE

ROTATIONAL SLIDE

DEBRIS FLOW

TRANSLATIONAL SLIDE

SOIL SPREADING

ROCK SPREADING

EARTH FLOW

Landslide types

Rock fall

very common

it usually involvessmall volumes ofrock

it is associated tocracks generated bystructural settingand/or enlarged bythe action ofdifferent processessuch as marine,karst, etc

very dangerousbecause this type oflandslide causescasualties Photo: Rock falls

Rock topple

Photo: Rock topple

Rock slide

This type ischaracterized by thepresence of a slipsurface, which canbe circular or not

The theory ofequilibrium limit wasborn for investigatingcoastal landslidelocated in the port ofGotheborg

Rotational landslideusually involves hugevolumes

Photo: Rotational slide witnessed by rotated limestone block

Rock spreads

this type of landslide involves clayey terrains capped by resistant materials

they are extensive and are characterized by slow speeds

Photo: View of a extensive rock spread, which evolves into a block slide

Rock spreads

they generate persistent cracks, that can assist geomorphologists to detectthem

rock spreads favour the occurence of other types of landslides such as rockfalls and block slides

Photo: Persistent crack generated by rock spreading phenomena

Rock spreads

Rock spreads

Photo: Lowered blocks detached and moved by rock spreading phenomena

Flows

Earth slides/flows not common as onmountains

They usuallyinvolves smallquantities ofmaterial

They are associatedmainly to clayeyterrains

They are triggeredby intense rainfalls.For these reasons,they are moredangerous forhuman structuresrespect to tourists orvisitors

Natural and human-induced hazardsHazard and risks

Coastal instability and related risk issues In recent years the interest for coastal hazards has increasedsignificantly due to extreme events - sometimes related toclimate change – which often induce high risk situations

Recent tragedies occurred in Italy at the Island of Ventotene(20 April 2010) and near Brindisi (21 October 2010): three persons were killed due to coastal landslides

Coastal instability and related risk issues

Conversely respectto inland slope-failure processes,coastal small eventsare more dangerousrespect to extensivelandslide processes.

The volume of fallenmaterial that causedthe examplecausuality wasextremely small (2-3m3), witnessing thateven small-sizedlandslide events canbe extremelydangerous andshould not beoverlooked in theprocess of hazardassessment

Coastal instability and related risk issuesPredisponing factors

The profiles of marine cliffs are the result of the interplay of different factors, such as:

•Geology

•Climate

•wave and tidal regime

•vegetation

•nearshore water depth, type and amount of beach material

•topography of the cliff-top area

•Rslc

The profile of cliff is crucial for the occurence of landslide

Vertical cliff favour rock falls, whereas low-lying coast are not affected by landslide

Coastal instability and related risk issuesPredisponing factors

Joint system is apredisponing factorfor some types oflandslides (rock falls,topples and slides)

Geomechanicalsurveys can assistgeomorphologists forinvestigating sea cliffretreat

Source: Furlani et al., 2011

Sea action favoured erosion of weakened layers situated at the bottom of rocky cliff. Thelatter is made up of resistant limestone at the top and soft sandstone at the bottom

In 1970 a 12 years oldboy was killed by arock fall on the NWcoast of Malta, atGnejina Bay and otherinjuries were reportedmore recently

Coastal instability and related risk issuesTriggering factors

Coastal landslides can be caused or accelarated by earthquakes

Coastal instability and related risk issuesTriggering factors

Different hazard (and vulnerability)for different landslide types!!

Different SPEED

Type of material and type of movement

Coastal instability and related risk issuesStrategies

Hazard Map (very difficoult)

Landslide Inventory Map

Susceptibilty Landslide Map

Coastal instability and related risk issuesStrategies

It is important to define for extensive landslide

Coastal instability and related risk issuesSpeed classes

Source: Devoto, 2013

It is important to define for extensive landslide

Coastal instability and related risk issues

Landslide InventoryMap(LIM)

LIM are producedby means of outputsof traditionalactivities such asgeomorphologicalmapping and aerialphoto interpretation

Good quality isessential for furthersteps as susceptibiltymap

Source: Devoto et al., 2012

Coastal instability and related risk issuesLandslide InventoryMap (LIM)

Coastal instability and related risk issuesLandslide susceptibility

Identification andclassification of active massmovements within the area ofinterests

Definition of factorscontributing to slopeinstability and creation ofrelated dataset

Source: Piacentini et al., 2015

Coastal instability and related risk issuesLandslide susceptibility

We used predisponing factors such as

slope angle

curvature

distance from coastline

distance from scarp

distance from faults

distance from joints

Topographic Position Index

Source: Piacentini et al., 2015

Coastal instability and related risk issuesLandslide susceptibility

Prediction rate curve of the predisposing factors

Source: Piacentini et al., 2015

Coastal instability and related risk issuesLandslide susceptibility

A susceptibiltity map wasproduced at a regionalscale and classify the coastinto 5 classes:

- Very low;

-Low;

- Medium;

-High;

-Very High

Source: Piacentini et al., 2015

Technique Main Advantages Main Limits

Interferometry Identify pastdeformations

No information on planar components of displacements

GPS Reliable Post-processing

Tape extensometer

Easy to be carriedout and cost-effective

Measures only along some scanlines

Automaticfissurimeters

Possibility to correlate displacements and precipitation

Installed only in secure areas, toavoid vandalism

Coastal instability and related risk issuesLandslide Hazard

we have to insert time on susceptibility

Source: Furlani et al., 2013

Acquisition of necessary knowledge to define methods to analysecoastal geo-hazard

Definition of field protocols which can be used in different coastalenvironments for investigation of coastal geo-hazard

Production of thematic maps also taking into account issues related toclimate change (sea-level change, more frequent extrememeteorological events)

Accidents and perception of riskAccidents and suicides

Accidents and injuries from cliffsCliff instability may be the scene of accidentes or deaths of people

People have been killed or injured because of activities carried out on the cliffs, such as climbing, walking

There are also injuries and deaths regarding suicides or attempted suicides by jumping from high cliff tops, in particular in romantic and easy to reach contexts (e.g. the Rilke path at Duino, Trieste -Italy, Beachy Head, Sussex - England)

The perception of risks

As noted by Williams and Williams (1988), the perception of risks changes between social groups and individuals

People living along the coasts are more aware of cliff hazard than occasional tourists because of their own experience

As noted by Bird (2016), geologists, as well as botanists, ecologists, etc, face the problem of relating geological research and training with cliff hazard

ReferencesMain references to scientific literature

ReferencesBIRD E. (2016). Coastal Cliffs: Morphology and Management. Springer, pp.92.

CRUDEN D.M. & VARNES D.J. (1996). Landslide Types and Processes. In: Turner A.K. AndShuster R.L. (Eds), Landslides: Investigations and Mitigation. National Research Council,Special Report 247, 36-75.

DEVOTO S., BIOLCHI S., BRUSCHI V., FURLANI S., MANTOVANI M., PIACENTINI D., PASUTOA. & SOLDATI M. (2012). Geomorphological map of the NW coast of the Island of Malta(Mediterranean Sea). Journal of Maps, 8:1, 33-40.

DEVOTO S. (2013). Geomorphological map of the NW coast of the Island of Malta(Mediterranean Sea). Unpublished PhD thesis, University of Modena and Reggio Emilia.

FURLANI S., DEVOTO S., BIOLCHI S. & CUCCHI F. (2011). Factors triggering sea cliffinstability along the Slovenian coasts. In: Micallef A. (ed.), MCRR3-2010 ConferenceProceedings, Journal of Coastal Research, Special Issue, 61, 387-393.

FURLANI S., ANTONIOLI F., BIOLCHI S., GAMBIN T., GAUCI R., LO PRESTI V., ANZIDEIM., DEVOTO S., PALOMBO M. & SULLI A. (2013). Holocene sea level change in Malta.Quaternary International, 288, 146-157.

PIACENTINI D., DEVOTO S., MANTOVANI M., PASUTO A., PRAMPOLINI M. & SOLDATIM. (2015). Landslide susceptibility modeling assisted by Persistent ScatterersInterferometry (PSI): an example from the northwestern coast of Malta. NaturalHazards, 78, 681-697.

WILLIAMS M.J. & WILLIAMS A.T. (1988). The perception of, and adjustment to,rockfall hazards along the Glamorgan Heritage Coast, Wales. Ocean and ShorelineManagement, 11, 319-338.