landslides and airphoto interpretation

8/2/2019 Landslides and Airphoto Interpretation

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Landslide interpretation from airphotos1

Introduction to landslidesPart 2: Mapping landslides from

airphotos

Cees van Westen

International Institute

for Aerospace Survey and EarthSciences

(ITC), Enschede, The Netherlands.

E-mail: [email protected]


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Landslide mapping The information on landslides extracted from remote

sensing images, is based on:

morphological,

vegetational and

drainage conditions of the slope.

A particular type of slope failure is seldomly directlyrecognized as such, but interpreted by the analysis of acertain number of elements pertaining to the slopeinstability feature.

As a consequence, the categorization of slope movements,as obtained from the interpretation of aerial photographs, isnot that detailed as the classifications of Cruden and

Varnes.


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Landslide mapping

Practice has shown that a photo-interpretation has to bedone with the use of a simple classification.

Type Subtype Activit Depth Ve etation Body

1

2

3

4

5

6

Fall

Topple

Slide

Lateral spread

Flow

Debris avalanche

Rotational

Translational

Complex

Unknown

Stable

Active

Surficial

Deep

Bare

Low

High

Landslide scar

Run out body


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PortdeVi

elha,

ValldAran


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Vegetation characteristics


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FiumedA

cquaFred

da,

Basilicata,

Italy


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Drainage characteristics


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Active / Non-active


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Manizal

es,Caldas,

Colombia


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Mapping landslide types


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Mapping slide types


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Rockfall and toppling

Morphology:Distinct rockwall or free face in association with scree slopes(20 -30) and dejection cones. Jointed rock wall (>50) with fallchutes.

Vegetation:

Linear scars in vegetation along frequent rock fall paths.Vegetation density low on active scree slopes.

Drainage:

No specific characteristics.


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Fall examples


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Sturzstrom Morphology:

Extreme large (concave) scars on mountain, with downslided blocks of

almost geological dimensions. Rough, hummocky depositional forms,sometimes with lobate front.

Vegetation:

Highly irregular/chaotic vegetational conditions on accumulative

part,absent on strurzstrom scar.

Drainage:

Irregular disordered surface drainage, frequent damming of valley and

lake formed behind the body.


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Sturzstrom

intheSierraLasNieves

(prov.

Malaga,

Spain)


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Rotational slides

Morphology:

Abrupt changes in slope morphology, characterized by concave(niche) - convex (run-out lobe) forms. Often step-like slopes.Semi-lunar crown and lobate frontal part. Backtilting slopefacets, scarps, hummocky morphology on depositional part. D/Lratio 0.3 - 0.1 , slope 20 - 40.

Vegetation:

Clear vegetational contrast with surroundings, the absence oflanduse indicative for activity. Differential vegetation accordingto drainage conditions.

Drainage:

Contrast with not failed slopes. Bad surface drainage or pondingin niches or backtilting areas. Seepage in frontal part of run-outlobe


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LaTriev

eslandslid

earea,

Fra

nce


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Compound slides

Morphology:

Concave - convex slope morphology. Concavity oftenassociated with linear graben-like depression. No clear run-outbut gentle convex / bulging frontal part. Backtilting facetsassociated to (small) antithetic faults. D/L ratio 0.3 - 0.1 ,relatively broad in size.

Vegetation:

As with rotational slides, although slide mass will be lessdisturbed.

Drainage:Imperfect or disturbed surface drainage, ponding in depressionsand in the rear part of the slide.


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Translational slide Morphology:

Joint controlled crown in rockslides, smooth planar slip surface.Relatively undeep (shallow), certainly in surface mat. overbedrock. D/L ratio


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Alianoarea,

Basilicata,

Italy


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Lateral spread

Morphology:

Irregular arrangement of large blocks, which are tilting in variousdirections. Blocksize decreases with distance and morphologybecomes more chaotic. Large cracks and linear depressions areseparating the blocks. Movement can originate on very gentle

slopes (


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Lateralspread,

RioS

uarez,

Col

ombia


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Mudslides

Morphology:

Shallow concave niche with flat lobate accumulative part, clearlywider than transportational path. Irregular morphology iscontrasting with surrounding areas. D/L ratio 0.05 - 0.01, slope15-25.

Vegetation:

Clear vegetational contrast when fresh, otherwise differentialvegetation is enhancing morphological features.

Drainage:

No mayor drainage anomalies associated to mudslides, besideslocal problems with surface drainage.


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Earthflows Morphology:

One large or several smaller concavities, with hummocky relief

in the source area. Main scars and several small scarsresembles slide type of failure. Path following streamchanneland body is infilling valley, contrasting with V shaped valleys.Lobate convex frontal part. Irregular micromorphology withpattern related to flow- structures. Slope > 25, D/L ratio verysmall

Vegetation:

Vegetational on scar and body strongly contrasting withsurroundings, landuse absent if active. Linear pattern in

direction of flow.

Drainage:

Ponding frequent in concave upper part of flow. Paralleldrainage channels on both sides of the body in the valley.

Deflected or blocked drainage by frontal lobe.


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Earthflow

,Torvalley,

Spanish

Pyrenees


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Flowslides

Morphology:

Large bowlshaped source area with step-like or hummockyinternal relief. Relative great width. Body displays clearflowstructures with lobate convex frontal part (as earthflow).Frequent associated with cliffs (weak rock) or terrace edges.

Vegetation:

Vegetational pattern are enhancing morphology of scarps andblocks in source area. Highly disturbed and differentialvegetation on body.

Drainage:

As on earthflows, ponding or deranged drainage at the rear part

and deflected or blocked drainage by frontal lobe.


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Debris avalanche

Morphology:

Relatively small, shallow niches on steep slopes (>35) withclear linear path. Body frequently absent (eroded away bystream).

Vegetation:

Niche and path are denudated, or covered by secondaryvegetation.

Drainage:

Shallow linear gully can originate on the path of the debrisavalanche.


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Debris flow


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Debris flow

Morphology

Large amount of small concavities (associated to drainagesystem) or one major scar is characterising source area. Almostcomplete destruction along path, sometimes marked bydepositional levees. Flattish desolated plain, exhibiting vague

flow structures is body. Vegetation:

Absence of vegetation everywhere, recovery will take manyyears.

Drainage:

Deranged on body, while original streams are blocked ordeflected by the body.


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Aberfan


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Santa Lucia


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Armero (Colombi


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Sagging

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