Controls on Isolated Boulder Incipient Motion: Experimental Insights into Froude Number, Relative Submergence, and Shape

Predicting boulder incipient motion is critical for designing hydraulic structures, river restoration, and hazard mitigation, yet the process remains poorly understood. This study investigates incipient motion through controlled flume tests on smooth beds with three slopes, varying discharge, and Froude number (Fr). Three concrete cube sizes were tested as angular reference cases, along with a worn cubical rock with rounded edges, to assess whether entrainment trends also apply to broader, rounded forms. The results indicate that the boulders began to move at an initial discharge (Qinit), but global transport was only achieved at a higher discharge (Qfull), indicating a transitional process rather than a single threshold. Entrainment was strongly controlled by the Froude number. While higher Fr≈2.3 (shallow, supercritical flow) required greater velocities for motion, near-critical conditions (Fr≈1.1, greater submergence) showed the least resistance despite lower velocities, as undular instabilities enhanced drag and reduced stabilizing lift. A new dimensionless threshold parameter, θcrit, was derived from a static force balance incorporating drag, lift, effective weight, and bed friction, to predict boulder entrainment on bedrock or artificial beds.