Application of laser anemometry for measuring critical bed shear stress of sediment core samples

This work details the application of the Laser Doppler Anemometry (LDA) technique for the direct measurement of the critical bed shear stress of natural sediment beds. This property was determined from variations in the velocity profiles obtained at the interface, for various Reynolds numbers. The onset of erosion was identified as a sudden transition in the local stress. Samples of sediment were collected from Cávado (Portugal) and Westerschelde (the Netherlands) estuaries. Experiments were carried out in laboratory using a circular mini flume, 126.8mm in diameter, enabling the reproduction of the flow characteristics close to the sediment. It was found that the deformation at the sediment surface is a very important factor affecting the bed shear stress. That deformation is affected by the mud characteristics and composition, namely the particle size. It was also verified that the erosion process is not continuous, the sediment being eroded in layers. From the various experiments, it was concluded that the LDA technique is appropriate for measuring the critical bed shear stress of sediments.