Hazard mapping
The analysis using above hydrologic and stability models could be readily incorporated in the Arc View either using map calculator or avenue scripts. The results give the factors of safety in the form of maps or contours. The task of engineer becomes easier to identify, visually which areas are critical that deserve further attention. Hazard rating could be assigned based on suitable criterion. Natural hazard is defined as the probability of occurrence of potentially damaging phenomena within a specified period of time and within a given area. Zonation also refers to the division of the land in homogenous areas or domains according to the degree of actual or potential hazard (Varnes, 1984). Following gives such a rating criterion, based on a quantitative estimation of landslide occurrence over a given region, without mentioning the time period.
Table 1. Quantitative Hazard Rating of Landslides (Miles et al.,1999)
| Stability Criterion |
Hazard rating |
| F > 1.5 |
Stable slope |
| 1.25 > F < 1.5 |
Low Hazard |
| 1.00 > F < 1.25 |
Medium Hazard |
| 0 > F < 1.00 |
High Hazard |
F is the factor of safety
An Illustrative example
The preliminary analysis of the slope stability is done in Arc View GIS, with hypothetical slope and simulated earthquake conditions, to validate and explore the capabilities of GIS. The area of analysis is 80X80m, with the crest of 85m and mean slope as 53°. The slope, soil properties (cohesion, friction angle, and unitweight) are given as input in attribute table and developed as themes through event theme menu. Then interpolation for the elevation and soil properties for whole slope area is done through spline interpolation. The following table gives the statistics of the given inputs.
Table 2: Input Parameteres for the hazard predition model
| Statistics |
c (kPa) |
f degree) |
g(KN/m) |
Slope (°) |
| Mean |
38.4 |
46.2 |
18.12 |
53.4 |
| Standard Deviation |
1.9 |
1.467 |
0.86 |
16.57 |
| Coefficient ofVariation (%) |
5 |
3 |
4.7 |
31 |
| Minimum value |
28 |
41 |
16 |
- |
| Maximum value |
42 |
49 |
21 |
- |
In stability calculations, it is assumed the failure is planar surface and hence equations 2 and 3 are used for computing safety factors. As this stability model is capable of including the earthquake effects, a pseudo-dynamic analysis is also performed. The results of this analysis are given in table 3. The factors of safety maps/contours with hazard rating are shown in figures. Assuming that the probability distribution of factors of safety follows normal distribution, the reliability index and the corresponding probabilities of failure are computed, for both static and pseudo-static results.
Table 3. Results of static and pseudo-static analyses
| |
Static FS |
Dynamic Factors of Safety (Aah) |
| 0.1 |
0.2 |
0.3 |
0.4 |
0.5 |
| Mean of FS |
2.6 |
1.95 |
1.52 |
1.21 |
0.98 |
0.8 |
| s of FS |
0.23 |
0.19 |
0.19 |
0.19 |
0.19 |
0.21 |
| C.O.V (%) |
8.8 |
37 |
12.5 |
15.8 |
20.3 |
26 |
| b |
6.89 |
5 |
2.73 |
1.11 |
-0.01 |
-0.97 |
| P[f] |
0 |
0 |
0.02 |
0.11 |
1.5 |
1.54 |