Magnitude and phase of major forces that act on a loose non-cohesive particle (sediment) on single layer fixed rough bed (longitudinal slope 2%) were determined from experiments in a laboratory flume under waves. The loose particles were glass spheres of diameter 1.58 ± 0.1 mm and specific gravity 2.5. The range of wave-height-to-water-depth (H/h) ratio in the experiments was 0.366 < H/h < 0.521. The measurement plane was parallel to the bed and located at a height of ½ loose particle diameter (ds) above the rough bed. Grayscale morphological image processing methods were used to separate the fluid and loose sediment phases from the same oblique particle image velocimetry (OPIV) image based on their signature sizes. The OPIV calibration method is presented and validated with conventional particle image velocimetry (PIV) method. Loose particle velocity and accelerations along with the associated fluid velocity and fluid total accelerations in the wave direction were determined simultaneously by processing OPIV used to compute magnitude and phase of major forces that act on the loose sediment particle. It was observed that for same wave period (T), an increase in H/h ratio has a dominant effect on sediment displacements onshore. The phase along with magnitude of the major driving force (drag and fluid accelerations) plays an important role at initiation of loose sediment from its rest position. It is suspected that the loose particle overcomes a critical bed friction force with higher H/h ratio as magnitude of drag force is higher. The resultant force then displaces the sediment onshore which experiences sliding and or rolling motions very close to bed, in a thin fluid layer over maximum protrusion of bed sediments. At the instance, the gravitational force plus bed frictions overcomes the lift force the loose particle attains a new position onshore.

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