Logjam Deformation: Experimental analogs with variable flow

Observed deformation of a massive, channel-spanning logjam in Big Creek, Idaho inspired a suite of physical experiments exploring logjam kinematics in a simplified but controlled setting. Using chopsticks as surrogates for logs, we conducted experiments in a 6 m long and 1.22 m wide channel with a semi-circular, textured bed. Nails driven into the bed restrain the chopsticks and initialize logjam formation. We conducted 24 hour experiments hours under two discharge conditions: (A) constant base discharge and (B) alternating discharge between the base flow and a doubled flow. After initial stabilization, we use high-resolution down-looking photographs at one-minute intervals to construct time-lapse videos and for Particle Image Velocimetry. Despite identical experimental protocols during stabilization, the starting configuration of chopsticks is markedly different for each run. In Experiment A, the orientations and packing of chopsticks is visibly less ordered than Experiment B. However, deformation in both experiments is accomplished by the same three mechanisms: rigid blocks that propagate downstream as v-shaped fronts bounded by shear planes, logjam-wide adjustments in response to the change in position of a key member, and independent logs whose trajectories either travel underneath the logjam or adjust unbounded in the backwater. Total compression is 46% and 80% for experiment A and B, respectively. Time-series of incremental displacements for both experiments decrease noisily over time, but zero displacement is never reached. Despite very different hydrologic forcings, cumulative rates of deformation for both experiments are similar, suggesting that the progressive deformation of disordered, elongate particles (chopsticks and logs) within a larger ensemble leads to denser packing, and that this mechanism best describes logjam deformation.