Estimating Rainfall Thresholds for Landslide Occurrence in the Bhutan Himalayas

Abstract

Consistently over the years, particularly during monsoon seasons, landslides and related geohazards in Bhutan are causing enormous damage to human lives, property, and road networks. The determination of thresholds for rainfall triggered landslides is one of the most effective methods to develop an early warning system. Such thresholds are determined using a variety of rainfall parameters and have been successfully calculated for various regions of the world at different scales. Such thresholds can be used to forecast landslide events which could help in issuing an alert to civic authorities. A comprehensive study on the determination of rainfall thresholds characterizing landslide events for Bhutan is lacking. This paper focuses on defining event rainfall-duration thresholds for Chukha Dzongkhag, situated in south-west Bhutan. The study area is chosen due to the increase in frequency of landslides during monsoon along Phuentsholing-Thimphu highway, which passes through it and this highway is a major trade route of the country with the rest of the world. The present threshold method revolves around the use of a power law equation to determine event rainfall-duration thresholds. The thresholds have been established using available rainfall and landslide data for 2004-2014. The calculated threshold relationship is fitted to the lower boundary of the rainfall conditions leading to landslides and plotted in logarithmic coordinates. The results show that a rainfall event of 24 h with a cumulated rainfall of 53 mm can cause landslides. Later on, the outcome of antecedent rainfall varying from 3-30 days was also analysed to understand its effect on landslide incidences based on cumulative event rainfall. It is also observed that a minimum 10-day antecedent rainfall of 88 mm and a 20-day antecedent rainfall of 142 mm is required for landslide occurrence in the area. The thresholds presented can be improved with the availability of hourly rainfall data and the addition of more landslide data. These can also be used as an early warning system especially along the Phuentsholing-Thimphu Highway to prevent any disruptions of trade. initiated due to heavy monsoon precipitation and aggravated due to the increase in human activities. The increasing number of landslide events in Bhutan can be attributed to complex geological conditions, steep slopes, climate change, type of soil, and tectonic activity. Landslides in Bhutan mostly occur during the monsoons during which the torrential rainfall leads to several flash floods and landslide triggering, cutting off many parts of south Bhutan from the rest.The relationship between the amount of rainfall associated with landslide occurrences is generally studied using either an empirical or physical based approach [4][5][6][7]. Physical process models are based on numerical models which study the relationship between rainfall, pore water pressure, soil type, and volumetric water content that can lead to slope instability. Such a study is usually site specific due to variation in soil prop...

Spatial Landslide Risk Assessment at Phuentsholing, Bhutan

“…The temporal probability was calculated using the rainfall threshold determined by Dikshit et al [26] using the Poisson probability model. Readers are referred to [3,39,40] for the detailed explanation on determining the temporal landslide probability.…”

“…Gansser [31] described the regional geological setting of the Bhutan Himalayas and characterized the majority of the country as part of the Himalayan Crystalline Complex. The areas that are most prone to landslides are heavily fractured and weathered rocks of phyllite, slate, and schist that contain high amounts of clay [26,27]. The study region has been identified as geologically weak with the presence of several shear zones, including weathered and folded phyllite rocks turning to clay [25].…”

“…Geosciences 2020, 10, x FOR PEER REVIEW 3 of 16 unavailability of a detailed landslide inventory map results in the exclusion of the magnitude component. In the present study, the spatial component was calculated using a weighted sum analysis based on landslide contributing factors which was multiplied with the temporal probabilities determined based on regional rainfall thresholds [26] to develop the landslide hazard map. The predisposing factors for spatial aspect were identified based on a comprehensive field study conducted by the authors and examining the historical landslide locations in the study area.…”

“…The majority of the landslide incidences are triggered due to rainfall, therefore we have focused solely on rainfall-induced landslides. The previous research in Bhutan has been towards forecasting, which led to the development of several thresholds using various statistical [3,25,26] and probabilistic [27] approaches. However, landslide mitigation and understanding the elements at risk are yet to be conducted for any part of Bhutan.…”

Rainfall Induced Landslide Studies in Indian Himalayan Region: A Critical Review

“…The analysis of precipitation for landslide occurrence can be performed by estimating minimum rainfall conditions, sub surface monitoring, or slope stability analysis. The minimum rainfall conditions, also known as thresholds, can be classified as empirical and physical approaches [40][41][42]. Physical models assess the relationship between rainfall conditions and hydrological conditions of the soil, which affects slope stability.…”