Moisture-Induced Shear Strength Degradation of Debris Materials and Its Implications for Debris Flow Behaviour in a Himalayan Catchment

Debris flows are predominantly rainfall-induced phenomena in which progressive
degradation of soil shear strength plays a critical role in flow initiation and mobility. This
study presents an experimental investigation into the moisture-dependent reduction of shear
strength parameters of debris material collected from an active debris-flow site in the Sidhra
region of Jammu, India. The site lies in the Outer Himalayan belt and is underlain by
colluvial debris, weakly lithified Siwalik sandstones, siltstones, and mudstones, which readily
disintegrate into fine-grained, clay-rich soils during intense rainfall.
Systematic laboratory testing was performed on samples obtained from both the source and
deposition zones, with direct shear tests conducted at moisture contents of 0%, 20%, 30%,
and 50% to simulate dry to highly saturated field conditions. The results reveal a pronounced
reduction in both cohesion and internal friction angle with increasing moisture content,
indicating a clear transition from frictional-cohesive behaviour under dry conditions to
predominantly frictional and flow-like behaviour at high degrees of saturation. Beyond a
critical moisture threshold, cohesion becomes negligible, leading to a drastic reduction in
shear resistance and a strong increase in susceptibility to rainfall-triggered debris-flow
initiation.
The experimental results are further integrated into numerical simulations using the Rapid
Mass Movement Software (RAMMS). The influence of the Voellmy-Salm friction
parameters: the dry Coulomb friction coefficient (μ) and the turbulent friction coefficient (ξ),
is examined. These parameters, calibrated using laboratory-derived shear strength values,
significantly control simulated flow height, velocity, runout distance, and flow path. The
study highlights the importance of incorporating moisture-dependent shear-strength
degradation into debris-flow hazard assessments and demonstrates that realistic calibration of
RAMMS friction parameters is essential for reliable prediction of debris-flow dynamics.