Download - 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 interpretation from airphotos2
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 interpretation from airphotos3
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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Landslide interpretation from airphotos12
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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Landslide interpretation from airphotos15
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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Landslide interpretation from airphotos17
Sturzstrom
intheSierraLasNieves
(prov.
Malaga,
Spain)
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Landslide interpretation from airphotos18
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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Landslide interpretation from airphotos19
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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Landslide interpretation from airphotos21
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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Landslide interpretation from airphotos22
Alianoarea,
Basilicata,
Italy
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Landslide interpretation from airphotos23
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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Landslide interpretation from airphotos25
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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Landslide interpretation from airphotos26
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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Landslide interpretation from airphotos27
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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Landslide interpretation from airphotos29
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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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Landslide interpretation from airphotos31
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