Rainfall thresholds for landslides in the Nepal Himalaya
Please refer "Dahal, R.K., Hasegawa, S., (2008), Representative rainfall thresholds for landslides in the Nepal Himalaya, published in Geomorphology, 100(3-4), 429-443, DOI:10.1016/j.geomorph.2008.01.014" for detail of this information. The reference cited in this information are given in the full paper.
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Until 2007, for the Himalaya, no generalized studies exist for landslide and debris-flow initiating rainfall thresholds although these mountains have tremendous landslide problems compared to other parts of the world. A few studies report the relationship between rainfall amount or intensity and size of landslides and debris flows. In early 2008, considering rainfall as a main factor triggering landslides in Nepal, Dahal and Hasegawa (2008) established relationships between landslide occurrence and rainfall characteristics in the form of empirical equations. These empirical relationships of rainfall with landslide occurrence described a threshold rainfall necessary for triggering landslides in Nepal. For the purpose of establishing an empirical relation for rainfall intensity–duration threshold in the Himalaya, Dahal and Hasegawa (2008) considered 193 landslide events through 55 years (1951–2006) for establishing rainfall threshold relationships. The method employed by Dahal and Hasegawa (2008) in establishing the rainfall thresholds is similar to that by other researchers (e.g., Caine, 1980; Cancelli and Nova, 1985; Larsen and Simon, 1993; Aleotti, 2004; Guzzetti et al. 2007) for such estimations in different parts of the world.
Dahal and Hasegawa (2008) described rainfall threshold for Nepal in two different approach: intensity-duration threshold and normalized rainfall intensity threshold. They defined intensity-duration threshold as:
I = 73.90D-0.79…………………………………….. (1)
Where I is hourly rainfall intensity in millimeters (mm/hr) and D is duration in hours. According to this threshold relation, for rainfall events of shorter duration, such as below 10 hours, a rainfall intensity of 12.0 mm/hr is necessary to trigger landslides, while an average precipitation of less than 2 mm/hr appears sufficient to cause landsliding if continued for more than 100 hours. Similarly, if 24-hour rainfall exceeds 144 mm, there is always risk of landslides in Nepal.
Dahal and Hasegawa (2008) also analyzed the landslides and the corresponding rainfalls respect to the mean annual precipitation (MAP), which is another approach to rainfall-threshold analysis. The ratio between the critical rainfall of the event and the mean annual precipitation of the site is defined as normalized critical rainfall (NCR), and it is expressed in percentage (Guidicini and Iwasa, 1977; Aleotti, 2004). By normalizing rainfall intensity (Jibson, 1989; Polloni et al., 1992; Aleotti, 2004; Guzzetti et al., 2007) with NCR, Dahal and Hasegawa (2008) expressed following threshold for landslides in Nepal:
NI = 1.10D-0.59…………………………………….. (2)
Where NI is normalized rainfall intensity (hr-1) and D is duration in hours. The threshold relation indicates that for rainfall events of 1-day, a normalized rainfall intensity of 0.17 hr-1 (i.e. 17% of MAP) is required to trigger landslides, while a normalized rainfall intensity of less than 0.07 hr-1 (7% of MAP) appears sufficient to cause landslides if continued for more than 100 hours.
