Bedload Transport Under Low Frequency Waves

The concept of a bedload spectrum, showing the distribution of energy with frequency in the bedload process, is proposed. The bedload process is thus analysed in the frequency rather than in the time domain. In order to perform such spectral analysis of the bedload process it is necessary to measure bedload transport at higher rates (>1Hz) than has usually been possible. The development of a self generated noise device for the measurement of gravel transport as bedload is reported, and deployments on an English Channel beach are described. Concurrent measurements of the surface elevation and of the oscillatory, nearbed flow beneath the beach waves were taken with an ultrasonic current meter. The results from typical sets of 2048 measurements at 2Hz over 17.07 minute intervals of the flow and bedload characteristics were analysed spectrally and provide the first estimates of the spectral transfer functions which relate the flow and the mass transport rate. The results demonstrate the dominance of long period energy in the bedload transport process and confirm that coarse sediment transport is attenuated with frequency in the manner suggested by a momentum analysis of particle forces.