| Characterizing incipient motion within the wave bottom boundary layer is necessary to understand large-scale sediment transport phenomena and fluid-sediment-structure interactions. However, the shear stress criterion for incipient motion, generally established with steady flow experiments, has been limited in some oscillatory flows where pressure gradients may induce sediment motion. During this research, the relative strength of the shear stresses and the pressure gradients at incipient motion were examined to determine whether sediment motion was triggered by one of the two mechanisms, or a combination of both.;Near-bed in-situ measurements of incipient motion were made in the Small-Oscillating Flow Tunnel at the Naval Research Laboratory and the field-scale Large Wave Flume at Oregon State University over a range of oscillatory flows and sediment types. Particle Image Velocimetry and Accoustic Doppler Velocimeters measured fluid velocities, from which the shear stress and pressure gradient effects were computed. Additionally, the first Lagrangian measurements of sediment response to oscillatory flows were made with newly-developed electronic Smart Sediment Grains. These sensors have the potential to resolve fundamental hypotheses regarding the incipient motion of coastal sediments. Angle of repose experiments verified that the sensor enclosure has mobility characteristics similar to coarse gravel.;Results suggest that incipient motion was induced by the pressure gradients in flows with large accelerations and small orbital excursion amplitudes; by the shear stresses in flows with smaller accelerations and bigger orbital excursion amplitudes; and by the combined effects in intermediate flows. As more sediment grains were added in the mobile bed layer, the turbulent stresses increased and became more widespread across the domain. For the more densely packed bed configurations, turbulent vortices shed from neighboring sediment grains may also trigger sediment transport.;The data were synthesized to evaluate a more comprehensive incipient motion criterion, |Upsilon|, to account for the effects of shear stresses, pressure gradients and sediment characteristics including size, density and bed configuration. For each bed configuration, |Upsilon| at motion remained fairly constant, capturing the combined effects of the shear stresses and pressure gradients for each flow. However, as more sediment grains were added in the mobile bed layer, |Upsilon| at motion increased. The threshold at motion was determined to be a function of the static coefficient of friction and packed bed concentration. |Upsilon| may be more generally applicable in the marine environment than previous incipient motion criteria that were typically solely dependent on the bed shear stresses. |
