Investigating the Impact of the Rainfall System of April 2018 on the Occurrence of Landslides in Boyer-Ahmad and Dena Counties in Kohgilouye and Boyer-Ahmad Province

Landslide can be defined as mass movement of materials on sloping slopes under the influence of mass gravity and triggering factors such as earthquakes, floods and heavy rains. This phenomenon is one of the natural hazards that causes a lot of loss of life and money every year in mountainous, rainy and seismic areas. By zoning the risk of landslides, it is possible to identify sensitive areas with high potential for landslides and by providing solutions, appropriate control and conduting effective management methods, the occurrence of landslides can be partially prevented or the damages caused by them can be reduced.

In this research, the effect of the rainfall system of April 2018 on the occurrence of landslides in Boyer Ahmad and Dena counties in Kohgiluyeh and Boyer Ahmad province has been investigated. The zoning of landslide risk has been performed using Haeri-Sami experimental model and the integration of thematic rock maps based on geology, slope angle, length of fault, length of road and river, amount of rain, intensity of rain and earthquake have been done in the scale of 1:1500000. After preparing the landslide zoning map, by carefully examining the Google Earth images, the areas of the studied areas where the potential of landslides were expected to be higher were identified. Then, during the extensive field visits, the geographical coordinates of the observed landslides were recorded and by preparing the point coordinates of the landslides in the Arc Map software environment and transferring them to the Google Earth images, the location of the landslides was recorded. The identification and range of each location was prepared in the form of a polygon file and transferred to the Arc Map software environment, and in this way a digital map of the distribution of landslides in the studied area was prepared. The density ratio index (Dr) was used to evaluate the landslide risk zoning map.

The results showed that only in terms of the geological factor, about 23% of the area under study has a high sensitivity to the occurrence of landslides, while other factors including the slope angle (73%), fault length (99%), and length of roads and rivers (98%) have little sensitivity to the occurrence of landslides. In terms of the amount of rainfall, in the average rainfall mode (78% of the region) it has moderate sensitivity and in the mode of April 2018 rainfall, 82% of the region is of medium sensitivity and 10% of high sensitivity with regard to the occurrence of landslides. In terms of the intensity of rainfall (79%) and earthquakes, about 58% of the area under study has a medium sensitivity to the occurrence of landslides. Landslide risk in 4 classes: no risk, very low risk, low risk and medium risk, respectively, have an area distribution of 15.1, 36.07, 43.33 and 5.5% in the case of average rainfall and 22.54, 7.9, 38.03 and 31.53% in the case of April 2018 rainfall. In the average rainfall, about 94% of the studied area is in the classes without risk to low risk of landslides, and in the rainfall of April 2018, about 69% is in the low and medium risk range.

Based on the results of landslide risk zoning, it can be said that the occurrence of landslide risk in the study area based on the studied model and the mutual conditions of the effective parameters in the average rainfall is almost low and in the rainfall of April 2018, the risk is low to moderate. Therefore, it can be concluded that the rainfall in April 2018 has a relatively significant effect on the risk of landslides. In connection with the evaluation of the efficiency of the model using the density ratio index, the model has been able to separate the landslide risk zones well, and in the very low risk class, the amount of rainfall is more accurate in both cases.