Characterizing debris flow run-out and path selection in landslide-susceptible zones using RAMMS simulation

Debris flows, resulting from the downslope movement of earth materials under gravity, can significantly impact human populations and infrastructure, leading to loss of life and property damage. These flow-type landslides also contribute to sediment deposition in stream courses, agricultural areas, and infrastructural developments, necessitating the need for a better understanding of their susceptibility and runout characteristics. However, the existing landslide susceptibility maps have traditionally prioritized only the landslide initiation zones, often neglecting to account for the full extent of run-out and deposition areas. Consequently, communities have settled, engaged in agricultural practices, and undertaken construction in low and non-susceptible zones without adequate awareness of the potential risks of landslide material movement. The present research aims to identify debris flow run-out patterns, specifically the path and volume of materials displaced across different landslide-susceptible zones. The study is carried out in the Idukki district and utilizes the state-of-the-art numerical simulation model RAMMS through back-calibration of debris flow events from the existing landslide inventory. The study reveals that landslides predominantly follow two distinct paths: either along existing stream channels (SC) or down the steepest hill slope (SH). The research identifies the key drivers behind these two types of landslides. Notably, the initial depleting volume of the landslide emerges as a crucial factor influencing path selection, with a specific threshold volume of 13250 m³ identified to discriminate between SC and SH-type landslides. Landslides with a depleting volume surpassing this threshold tend to follow the steepest hill slopes, while those below this value prefer existing stream channels.