Biomechanical Properties and Resistance to Uprooting of Laboratory‐Scale Wood Logs

Wood dynamics affects riparian ecosystem functioning and river morphology. The spatial and temporal dynamics of wood pieces in river corridors, in particular of deposited rejuvenated wood logs, depend on their biomechanical properties and resistance to uprooting. The ability of stranded wood logs to withstand drag forces depends on how efficiently their roots have sprouted and on the interarrival time, magnitude, and duration of the moderate floods to which they are subjected. We performed static pullout tests on small-scale wood logs (Salix species) of four different sizes, growth stages, and sediment moisture content. Statistics of root biomass growth rate and related spatial distribution along the trunk reveal important insights for upscaling dynamics. Similarly, force-displacement curves indicate the maximum resistance and related energy for uprooting. Autocorrelation analysis of the sequence of force drops in the force-displacement signal reveals the statistical nature of the mechanism of load redistribution among roots. These results are then used to advance a physically based mathematical model of the resistance of wood log roots to flow-induced drag forces. Given that the magnitude, duration, and return period of hydrologic events are typically correlated, our model implies the existence of windows of opportunity for wood logs to either survive or remobilize.