1. Deterministic landslide hazard assessment
Deterministic landslide hazard assessment. Case study Manizales, Colombia
Cees van Westen
International Institute
for Aerospace Survey and Earth Sciences
(ITC), Enschede, The Netherlands.
Landslide case study Manizales
Introduction to GIS

2. Objective
In this exercise, a simple slope stability model (the infinite slope model) is used to calculate safety factor maps for different conditions.
The effect of groundwater depth and seismic acceleration is evaluated using input maps of these factors for different return periods of rainfall (related to the groundwater level) and earthquakes (related to the seismic acceleration).
In ILWIS, the model is represented by a user-defined function. Different scenarios are calculated by changing the variables of this function. The model is applied on a data set of the city of Manizales, in central Colombia.
Landslide case study Manizales

3. Shear strength / stress
Shear stress = W sin  / A
Shear strength (Mohr-Coulomb criterion)
s = c +  tan 
 = normal stress = W cos  / A
c = cohesion (KPa)
 = angle of internal friction (degrees)
 and c are geotechnical properties, which are measured in the laboratory using triaxial tests or shearbox tests.
Landslide case study Manizales

4. Safety Factor
The degree of slope hazard can be expressed by the Safety Factor (F) which is the ratio of the forces that make a slope fail and those that prevent a slope from failing.
F < 1 unstable slope conditions,
F = 1 slope is at the point of failure,
F > 1 stable slope conditions.
Landslide case study Manizales

5. Infinite slope g = unit weight of soil (N/m3). Infinite slope:
Conditions at crest and toe of the slope may be ignored.
Resulting forces from left and right are equal
Weight of the block:
g = unit weight of soil (N/m3).
Shear component of weight:
Normal component of weight:
Landslide case study Manizales

6. Infinite slope Stress = Force / area Shear component of weight:
Shear stress:
Normal stress:
Normal component of weight:
Safety factor:
Landslide case study Manizales

7. Infinite slope & water pressure
Height watertable above failure surface
Weight of the water:
Normal component of water weight:
Pore pressure on JK:
Factor of safety including pore pressure:
Landslide case study Manizales

8. Landslide case study Manizales

9. Case study Manizales
Landslide case study Manizales

10. Manizales 3D view
Landslide case study Manizales

11. City growth, Manizales
Landslide case study Manizales

12. Mass movement types
Landslide case study Manizales

13. Landslide activity
Landslide case study Manizales

14. Surficial materials
Landslide case study Manizales

15. Input data
Landslide case study Manizales

16. Slope information
ILWIS Mapcalc functions work with radials and not with degrees
e.g. 360 degrees = 2 *  radials = 2 * 3.14 =
With some Mapcalc calculations in which you will use the inbuilt ILWIS functions DEGRAD( ), SIN( ), COS( ), and SQ( ), first some maps are prepared that will be frequently used in this application:
map SI, sine of slope
map CO, cosine of slope
map CO2, squared cosine of slope
Landslide case study Manizales

17. Maps showing relation Z/Zw (m) for different return periods
During two months
a year (rather dry):
Gamma=14000,
m=M016
once a year:
Gamma=14000, m=M1
once in every 20 years:
Gamma=14000, m=M20
once in every 50 years Gamma=14000, m=M50
Landslide case study Manizales

18. User-defined function
The Safety Factor formula as presented above will be transformed into a user-defined function FS. This function already contains the known parameters (maps ASHT, SI, CO, CO2 and the known constants) but it also contains the variables Gamma and m.
The function can then be easily applied for various heights of the watertable (zw), by filling out the variables Gamma and m. This will result in a number of Safety Factor maps for specific watertable heights.
Function FS reads: (10000+((Gamma-m*10000)*ASHT*CO2*0.58)) / (Gamma*ASHT*SI*CO)
Landslide case study Manizales

19. Scenarios
(10000+((Gamma-m*10000)*ASHT*CO2*0.58)) / (Gamma*ASHT*SI*CO)
For the dry scenario, Gamma = 11000, and without water zw=0 and thus m=0 as well. To obtain a map with Safety Factors for the dry scenario (FSDRY), function FS is applied using parameters and 0. FDRY = FS(11000,0)
For the saturated scenario, Gamma = 16000, and when all soil above the failure surface is saturated with water, zw=z thus m=1. A map with Safety Factors for the saturated scenario (FSAT) is then calculated by using function FS with parameters and 1. FSAT = FS(16000,1)
Subsequently, function FS is applied repeatedly on the command line and Safety Factor maps are calculated for a number of watertable height scenarios. For each scenario, a value for Gamma is provided and the values for m are found in a number of existing groundwater maps (M016, M1, M20, M50).
Landslide case study Manizales

20. Results Safety factors are calculated for each scenario
Also earthquake acceleration can be incorporated
Landslide case study Manizales