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Landslide types and causes1

Introduction to landslidesPart 4: Use of GIS for landslide mapping

Cees van Westen

International Institute for Aerospace Survey and Earth

Sciences (ITC), Enschede, The Netherlands.

E-mail: [email protected]


� First define the objective of the study. � Danger exists that the data that will be collected will not be in

accordance with the scale of analysis, or the method of analysis.� This might lead to a waste of time and money if too detailed

data is collected, or an oversimplification if too general data is collected.

� The following things should be considered:� The objective of the study� The scale of the study � The type of analysis that will be followed� The types of input data that will be collected.


Defining objective

� Landslide hazard studies can be made for any different purposes. Some of these might be:

• For an environmental impact study for engineering works; • For the disaster management of a town or city;• For the modelling of sediment yield in a catchment ;• For a watershed management project;• For a community participation project in disaster management;• For a the generation of awareness among decision makers; • For scientific purposes.

� Each of these objectives will lead to specific requirements withrespect to the scale of work, the method of analysis and the type and detail of input data to be collected.


Scales of analysis

� National scaleSmaller than 1:1.000.000, covering an entire country, mainly intended to generate awareness among decision makers and the general public. Maps on this scale are often intended to be included in national atlases.


Scales of analysis

• Regional scaleBetween 1:100.000 and 1:1.000.000, covering a large catchment area, or a political entity of the country. The maps at this scale are mostly intended for reconnaissance phases for planning projects for the construction ofinfrastructural works, or agricultural development projects.


Scales of analysis

• Medium scaleBetween 1:25.000 and 1:100.000, covering a municipality or smallercatchment area. Intended for the detailed planning phases of projects for the construction ofinfrastructural works, environmental impact assessment and municipal

planning.


Scales of analysis

• Large scaleBetween 1:2.000 and 1:25.000, covering a town or (part of) a city. They are used for disaster prevention and generation of risk maps, as well as for the design phase of engineering works.


Scales of analysis

• Site investigation scaleBetween 1:200 to 1:2.000, covering the area where engineering works will be carried out, or covering a single landslide. They are used for the detailed design of engineering works, such as roads, bridges, tunnels, dams, and for the construction of slope stabilisation works.


Data layers for slopeinstability hazard zonation

Scale of analysis

Regional Medium Large

GEOMOR 1. Terrain Mapping Units 3 3 3

PHOLOGY 2. Geomorphological (sub)units 2 3 3

3. Landslides (recent) 1 3 3

4. Landslides (older period) 1 3 3

TOPOGRAPHY 5. Digital Terrain Model 2 3 3

6. Slope map 2 3 3

7. Slope direction map 2 3 3

8. Slope length 2 3 3

9. Concavities/convexities 1 1 3

ENGINEERING 10. Lithologies 2 3 3

GEOLOGY 11. Material sequences 1 2 3

12. Structural geological map 3 3 3

13. Seismic accelerations 3 3 3

LANDUSE 14. Infrastructure (recent) 3 3 3

15. Infrastructure (older) 3 3 3

16. Landuse map (recent) 2 3 3

17. Landuse map (older) 2 3 3

HYDROLOGY 18. Drainage 3 3 3

19. Catchment areas 2 3 3

20. Rainfall 2 3 3

21. Temperature 2 3 3

22. Evapotranspiration 2 3 3

23. Watertable maps 1 1 2


General trends in landslide hazard assessment

Type ofanalysis

Techniques Required datafrom table 1

Regional1:100,000

Medium25.000

Large10.000

Inventory Landslidedistribution analysis

3 Yes,but... (*)

Yes Yes

Landslide activityanalysis

4,5,14,15,16,17 No Yes Yes

Landslide densityanalysis

1,2,3 Yes,but... (*)

No No

Heuristicanalysis

Geomorphologicalanalysis

2,3,4 Yes Yes,but.. .(**)

Yes,but.. .(**)

Qualitative mapcombination

2,3,5,6,7,8,9,1012,14,16,18

Yes,but...(***)

Yes,but.. .(**)

No

Statisticalanalysis

Bivariate statisticalanalysis

2,3,5,6,7,8,9,1012,14,16,18

No Yes No

Multivariatestatistical analysis

2,3,5,6,7,8,9,1012,14,16,18

No Yes No

Deterministicanalysis

Safety factoranalysis

6,11,12,13,16,20,21,22,23

NO No Yes,but.. .(****)


Landslide distribution analysis


Activity analysis


Density analysis


Geomorphological hazard analysis


Qualitative


Hazard zonation

� Slope instability hazard zonation is defined as:

The mapping of areas with an equal probability of occurrence of landslides within a specified period of time (Varnes, 1984).

� A landslide hazard zonation consists of two different aspects:

• the assessment of the susceptibility of the terrain for a slope failure, in which the susceptibility of the terrain for a hazardous process expresses the likelihood that such a phenomenon occurs under the given terrain conditions or parameters.

• the determination of the probability that a triggering event occurs.


Hazard zonation

� Often slope instability hazard assessment uses the assumption:

conditions which led in the past to slope failures, will also result in potential unstable conditions in the presentorThe past is the key to the present


Direct / Indirect hazard mapping

� Division of hazard mapping methods:� (1) Direct hazard mapping

� experience driven applied geomorphological approach, where the earth scientist evaluates the direct relationship between the landslides and the geomorphological and geological setting during the survey at the site of the failure.

� (2) Indirect hazard mapping� the mapping of a large amount of parameters and the

(statistical or deterministic) analysis of all these possible contributing factors in relation to the occurrence of slope instability phenomena, determining in this way the relation between the terrain conditions and the occurrence of landslides. Based on the results of this analysis statements are made regarding the conditions under which slope failures occur.


Relative / absolute hazard

� Another useful division in techniques for assessment of slope instability hazard is given by Hartlen and Viberg(1988) :

� (1) relative hazard � The relative hazard assessment techniques differentiate the

likelihood of occurrence of mass movements for different areas on the map, without giving exact values.

� (2) Absolute hazard� Absolute hazard maps display an absolute value for the

hazard, such as a factor of safety or a probability of occurrence.


Landslide hazard analysis methods

Another division (Carrara, 1983; Hartlen and Viberg, 1988):

(1) White box models,based on physical models (slope stability and hydrological models), also referred to as deterministic models;

(2) Black box models, not based on physical models but on statistical analysis;

(3) Grey box models, based partly on physical models and partly on statistics.