Debris transport and impact dynamics under dam-break flow: Effects of initial conditions and bathymetric transitions

This study investigates positively buoyant debris entrainment, transport, and structural impact under dam-break flow across varying bathymetric configurations. Using instrumented debris models, multi-axial dynamometer measurements, and computer vision tracking, the experimental program systematically varied impoundment depth (0.6–0.8 m), debris placement (0.5–2.1 m from structure), submergence ratio (0 and 0.5), release angle (0°, 45°, 90°), and slope-to-berm transition geometry (smooth versus abrupt 8 mm step). Two debris models differing in size, mass, and draft were tested. Each configuration was repeated five times, yielding 135 trials. Hydrodynamic tests without debris isolated flow effects from debris-induced loading. Two primary findings emerge from this investigation. First, debris transport and impact exhibit inherent stochastic behavior. Despite controlled initial conditions with five repetitions per configuration, impact forces showed coefficients of variation frequently exceeding 50% and in some cases reaching 100%. This variability reflects fundamental randomness in how debris rotates, interacts with bathymetric features, and contacts structures. The turbulent flow field, debris-bed interactions, and three-dimensional motion produce outcomes that cannot be predicted deterministically. Current design provisions based on deterministic force calculations are inadequate. Probabilistic frameworks are necessary to account for this inherent variability. Second, individual debris objects produce multiple successive impacts on structures through surface roller interaction. Surface roller formation at the structure face redirects debris upstream after initial contact, causing secondary and sometimes tertiary collisions. These subsequent impacts can occur to a structure that is already damaged from the primary impact, creating cumulative loading scenarios. Current design codes treat debris impact as a single event and do not address this multiple impact mechanism. Several parametric effects govern debris transport and impact dynamics. Initial pitching motion during entrainment controlled the time lag between debris and bore front, directly governing cushioning from fully developed surface rollers or impacts during bore arrival. Trailing debris arrivals with large time lags experienced substantially reduced forces due to surface roller cushioning effects. Abrupt slope transitions amplified debris-bed interactions, producing pronounced rotation (up to twelvefold increases over transport distance) and erratic impact geometry. Smooth transitions yielded more predictable trajectories, though both configurations converged to similar peak loads at sufficient impoundment depth (0.8 m). Release angle determined impact character. Flow-aligned debris (0°) delivered concentrated loads approximately double those of perpendicular orientations (90°), which achieved hydrodynamic stability with minimal rotation. Partial submergence elevated first-impact loads by 16–61%, shifting transport from sliding to floating modes. Smaller debris exhibited more complex three-dimensional motion due to stronger surface roller influence relative to mass. Peak structural forces scaled nonlinearly with impoundment depth, a 14% depth increase produced a 150% force increase. Impulse measurements confirmed that momentum transfer occurred predominantly in the flow direction, with lateral and vertical components representing approximately one-third the longitudinal impulse magnitude. The findings reveal critical gaps in current design provisions, including omission of secondary impacts, lack of probabilistic treatment for inherent transport variability, and assumed impulse profiles (half-sine, rectangular, trapezoidal) that do not reflect observed triangular distributions.