# Rainfall Induced Landslide Studies in Indian Himalayan Region: A Critical Review
## 3 Results
### Landslide Monitoring and Investigation  
Landslide monitoring is a very significant aspect of landslide assessment, especially in the Indian Himalayan region where the slopes are generally creepy in nature, which could immediately fail during an abrupt rainfall event or seismic occurrence. Landslide monitoring is generally categorized into three types [53]: \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Threshold Equation** & **Region** & **Methodologies** \\ \hline \(I\) =  **1.82 D\({}^{-0.23}\)** & Chamoli, Uttarakhand & Empirical thresholds \\ \(I\) =  **58.7 D\({}^{-1.12}\)** & NH 58, Uttarakhand & Empirical thresholds \\ \(I\) =  **3.52 D\({}^{-0.41}\)** & Kalimpong, West Bengal & Empirical thresholds \\ \(E\) =  **(4.2 \(\pm\) 1.3) D\({}^{(0.56\pm 0.05)}\)** & Kalimpong, West Bengal & Semi-automated algorithm approach \\ \(I\) =  **100 D\({}^{-0.92}\)** & Gangtok, Sikkim & Empirical thresholds \\ \(I\) =  **43.26 D\({}^{-0.78}\)** & Sikkim & Empirical thresholds \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Threshold equations generated for various Himalayan regions using several methods (I is Rainfall Intensity (mm/h), while D is Duration (h)). Figure 6: Comparison of various ID thresholds developed for various Himalayan regions with global thresholds defined by [PERSON] [52]. 1. Observing the changes of topography and cracks on the surface during the site investigation. This is a traditional monitoring technique used by geologists at regular time periods. The major limitation of using such a technique is that it does not provide the variations for a short time interval and it is difficult to accurately determine the time and location of future landslide incidences. 2. Remote sensing techniques such as satellite imagery analysis, GPS synthetic aperture radar (SAR) interferometry, and light detection and ranging (LiDAR). These methods can be helpful as they can measure slope displacement over a large area independent of the weather conditions. 3. In situ ground-based observation of slope movement using various instruments (extensometers, inclinometers, and tiltmeters) and installing rain gauges to accumulate local rainfall data. For example, [PERSON] et al. [54] used tilt sensors for Darjeeling Himalayas, while [PERSON] et al. [55] used Electrical resistivity tomography (ERT) for Garhwal Himalayas. Of the various monitoring techniques, the use of remote sensing (RS) data along with traditional monitoring instruments would prove to be very helpful especially in understanding long-term deformation and the advancement in RS technology would gradually overcome the use of traditional equipment. In the case of Indian Himalaya, monitoring studies include remote sensing techniques and ground based observation. [PERSON] et al. [56] used the Persistent Scatterer Interferometry (PSI) technique using ENVISAT satellite for Lesser Himalaya, Nainital. PSI is based on InSAR technique, which utilizes several SAR images and has proved to be successful in identifying the creeping zones in Nainital. [PERSON] et al. [57] monitored the landslide dammed lake in Zanskar Himalayas for five months (January-May 2015) using multi-temporal high-resolution satellite images after the landslide event (December 2014) and depicted the variation in the dimension of the impounded lake during the monitoring period. [PERSON] et al. [54] and [PERSON] and [PERSON], [22] used tilt sensors at shallow depths to analyze the variation in tilting angle of the instrument, which is related to the lateral displacement of the slope. The study also validated the empirical models thereby encouraging similar studies to be conducted in other Northeast Himalayan regions, which could help in setting up a preliminary early warning system. The investigation of unstable slopes in the Indian Himalaya has been conducted using Ground Penetrating Radar (GPR) or 2D Electrical Resistivity Tomography (ERT). [PERSON] et al. [58] conducted ERT investigation for Naitwar Bazar landslide in Uttarakhand for six sites, which was active after the 2004 event.