Drag of emergent and submerged rectangular obstacles in turbulent flow above bedrock surface

Flume experiments quantify the force balance equation for initial motion of isolated blocks. Experiments: (1) Initial motion on smooth concrete surface; (2) Initial motion on simulated rough bedrock. Three cases of initial motion: (1) the block is emergent and pivots; (2) the block is emergent and slides; (3) the block is submerged and slides. The drag coefficient (C d ) on rough bedrock was related to relative water depth and particle orientation. For submerged cases C d = 0.95 for sliding, but for near emergent/emergent conditions C d increases rapidly as relative depth reduces. For shallow water and emergent conditions large values of C d pertain as the relative depth decreases and blocks pivot. For emergence, the lift coefficient ( C l ) is zero. For submerged conditions negative C d decreased primarily as relative depth increased (with block orientation being a minor additional factor) and assumed a constant positive value of about 0.07 on rough bedrock and 0.17 on smooth bedrock for relative depths ≅ 4. Thus C l ≅ 0.12 C d . Due to the turning moment, the friction coefficient (μ f ) for the smooth bed was not constant, but on the bedrock μ f was constant, only varying between 0.5 and 0.8 depending on the lithology/degree of contact between bed/block.