Sand and small stones moving on the riverbed cause topographical changes such as local lowering and scouring. The measurement of particle size distribution of bed load is important to prevent river disasters, and a robust and simple particle sizer without using an electric power supply is required. In this study, a passive piezoelectric sensor for the measurement of the particle size distribution was developed and the continuous measurement for sequential impacts was analyzed. The sensor consists of an aluminum circular disc and an annular piezoelectric transducer. Naturally shaped gravel was employed as the bed load. When the gravel hits the surface of the sensor plate, resonance flexural vibration modes were excited on the sensor and electric power was generated through the piezoelectric effect. The size-discrimination parameters were defined by the spectral amplitudes of the resonance modes. The output signals of the sensor overlapped owing to the sequential impacts and the particle size distributions were estimated by time–frequency analysis.
