Debris flows occur without warning and carry destructive energy. It is imperative to understand its flow mechanisms and interaction with structural countermeasures to prevent the loss of human lives and damage to infrastructure. Trapezoidal cross-sectional channels can more realistically model the flow path and improve mobility predictions, but the influence of varying side wall inclination on flow mechanisms is not well understood. Furthermore, an array of baffles is commonly used as a structural countermeasure and is designed using empirical methods since its interaction mechanism is not well understood. The principle objectives of this research are to study the flow mechanisms in trapezoidal channels with varying sidewall inclination and flow interaction with baffles.

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Debris flows occur without warning and carry destructive energy. It is imperative to understand its flow mechanisms and interaction with structural countermeasures to prevent the loss of human lives and damage to infrastructure. Trapezoidal cross-sectional channels can more realistically model the flow path and improve mobility predictions, but the influence of varying side wall inclination on flow mechanisms is not well understood. Furthermore, an array of baffles is commonly used as a structural countermeasure and is designed using empirical methods since its interaction mechanism is not well understood. The principle objectives of this research are to study the flow mechanisms in trapezoidal channels with varying sidewall inclination and flow interaction with baffles.

Two research methodologies, namely physical and numerical modelling was adopted for this research. A newly developed 5-m long flume model adjustable in cross section (symmetrically variable sidewalls) was used to investigate both mobility in trapezoidal channels and flow interaction with landslide debris baffles. Side wall angles were varied from 30° to 90° for studying mobility and a rectangular channel was adopted for modelling debris-resisting baffles. The effects of baffle configuration, namely baffle height, number of rows, row spacing, and degree of transverse blockages were studied. Numerical back-analysis of flume experiments was then conducted using the discrete element method (DEM) to study flow mechanisms influencing mobility in trapezoidal channels and interaction mechanisms with an array of baffles.

The study of flow mechanisms in trapezoidal channel reveals that increasing sidewall inclination decreases mobility. Decreasing side wall inclination from 90° to 30° leads to 50% increase in runout distance. Steep sidewalls lead to thicker flow depths which promote longitudinal spreading. A new dimensionless group, π₁, is presented to characterise the flow mechanism of longitudinal spreading. Longitudinal spreading is responsible for attenuation of the flow mass during transportation and decreasing mobility. The consideration of longitudinal spreading and channel sidewall angle is demonstrated to be necessary for a comprehensive mobility assessment.

The investigation of debris flow resisting baffles reveals that baffles can be categorized relative to the approach flow depth (h), namely tall baffles (1.5h) and short baffles (0.75h). Tall baffles are characterized by effectively developing upstream subcritical flow conditions, whereas short baffles exhibit supercritical upstream conditions. Tall baffles facilitate the suppression of peak overflow by up to 73% compared to short baffles and prevent supercritical overflow conditions. A single row is ineffective in reducing frontal velocity and suppressing downstream flow depth, whereas a three row staggered array provides up to 57% reduction in frontal velocity and 97% downstream flow depth suppression. Staggered rows of both tall and short baffles should be positioned as close as possible to optimize energy dissipation (0.25 times the channel width in this study).