The sand transport that causes the movement and deformation of a sand hill in a periodically oscillating flow is investigated in the laboratory. An experimental technique is developed to assess the variation of the state of concentration of sand particles in the process of the interaction of the particles with the fluid in motion around the sand hill as well as with the particles themselves. The method of measuring the concentration of sand particles is based on the digital image processing. It is confirmed that under the well-controlled experimental conditions of homogeneous distribution of sand particles, the decrease in brightness of the digital images of an area is determined by the concentration of the particles in the area. Taking a space average on the images with an appropriate scale to the size of sand particles to fade out the unfavorable effect of each particle makes it possible to predict inhomogeneous and continuous distribution of the concentration of sand particles from the brightness of the digital images with the limited resolution. We make a visualization experiment of the two-dimensional flow field around a sand hill on sand bed in a flow sinusoidally oscillating in a straight channel with square section. The digital images of the unsteady flow field with suspended sand particles around the sand hill is analysed to estimate the concentration of sand particles. The results show that in the periodically oscillating flow the sand particles repeat a basic motion every half period such as sand particles running up the slope of the hill by accelerating flow, running off the hill by flow separation at the top of the hill, running down the hill as suspended in a turbidity current, and running into the roll by the rotational motion of shed vortices. It is found that these periodic motions of sand particles have particular distinguishing features of the change in the state of concentration in the process of fluidization, movement, transport, sedimentation, and accumulation.