Particle size and settling velocity of bed and suspended sediments for mud-sand beds

The particle size and the settling velocity of sediments are key parameters in sediment transport studies. However, it remains surprisingly difficult to determine particle size and settling velocity distribution of fine-grained sediments (mud-sand). A large range of methodologies exist to measure either the particle size distribution or to measure the settling velocity. An important influential parameter is the shape of fine-grained sediments, with clay minerals being shaped as plates rather than as spheres. Furthermore, the settling velocity of fine particles is influenced by turbulent shear and flocculation processes. Sometimes, the sediment samples are pre-treated (destroying inter-particle bonds) to measure the primary particle sizes involved while in other cases samples are not pre-treated in order to represent the effect of flocs. As a result, a large uncertainty exists in the way particle size and settling velocity should be measured. A range of methodologies (sedimentation, video camera, and laser-diffraction) to measure the settling velocity and particle size distribution in the field and in the laboratory is used and compared. The labour-intensive sedimentation methods measure a particle size distribution which can be used for sedimentation studies. The particle size distribution measured by the most commonly applied laser diffraction method is representative of the plate diameter of the clay particles, but the corresponding settling velocity is not correct. This difference can be explained by the shape of the clay particles through a derivation of the settling velocity of non-spherical particles resulting in a simple relationship to convert the particle size measured by laser diffraction to a representative particle size to be used in sedimentation studies. A comparison of the settling velocity measured by an in situ settling method and by a video camera method shows good agreement for high concentrations (> 2000 mg/L) but deviating results for low concentrations (< 500 mg/L).