Understanding the cause and effect relationship of debris slides in Papum Pare district, Arunachal Himalaya, India

Slope failures are recurrent phenomena during the Indian Summer Monsoon (ISM) season in the mountainous regions of Arunachal Himalaya, NE India, with a consequent damaging impact on the landscape, life, and property of the people. Geodynamic conditions, fragile lithology, high magnitude seismicity together with high-intensity rainfall from ISM provide the potent mixture of conducive factors contributing towards a very high frequency of landslides in the region. The problem is further accentuated by anthropogenic activities that have increased in recent times. Here we present a case study of a vulnerable road segment between Doimukh and Haj along the National Highway (NH-229) in Papum Pare district of Arunachal Pradesh that witnesses recurring debris slides during the rainy season. We suggest that a minimum threshold value of 433 mm of rainfall can be a triggering factor for the landslides in this Himalayan terrane. Considering similar climatic and geologic factors, this threshold value may be extrapolated for other areas in the Arunachal Sub-Himalayan region. An attempt is also made to analyze the stability of slope (dry/saturated condition) through 2D numerical modeling. The results of the finite element method (FEM) are validated with the limit equilibrium method (LEM). Both critical strength reduction factor (SRF) and factor of safety (FoS) are significantly reduced due to saturation of the debris material. Pre-event simulation for dry and saturated conditions gives the critical SRF values of 1.12 and 0.72 in FEM and FoS of 1.009 and 0.786 in LEM, respectively. A conservative estimate shows at least similar to 15,352 m(3) of sediments were generated from a single debris slide event. Predictably, a combined sediment volume from all the events contributes significant sediment load into the adjacent south-flowing Dikrong river. Heavy sediment flux leads to a cascading downstream impact with devastating floods, erosion, and aggradation besides frequent channel changes. This study is expected to contribute towards a better understanding of the disaster potential, cause and effect relationship in a highly landslide-prone terrane of the eastern Himalayas besides enhancing our understanding of upper catchment sediment generation and their trajectory in the Himalayan rivers. This also calls for the further analysis of space-time variability of the ISM to enable developing a landslide early warning system in Arunachal Himalaya.

Automated summary

Slope failures are common during the Indian Summer Monsoon in the Arunachal Himalaya region, causing significant damage to the landscape, life, and property. Factors such as rainfall intensity, geodynamic conditions, and human activities contribute to the high frequency of landslides. The study aims to improve understanding of disaster potential, cause and effect relationships, and sediment generation in Himalayan rivers.