Size-sorting during suspension transportation—lognormality and other characteristics

Grain size distributions of the suspended loads above a bed of bimodal size distribution (size range 2-00-0.04 mm) were studied in a laboratory flume at water velocities varying from 42 to 160 cm/s. With increase of velocity the phi (logarithmic) size distribution of the suspended particles (at 5-20 cm above the bed) changed from a strongly skewed to a nearly symmetrical, unimodal form (nearly lognormal) through an intermediate bimodal stage. At low velocity the skewness of the distribution changed from positive to negative with increase of height. The experiments indicate that lognormality of‘weight frequency’ distribution of grain sizes is a transitional feature, attained through size sorting within a critical range of velocity and height above a sand bed of a given composition. The observed changes in the size distribution patterns were effected by a differential rate of increase in weight in the different size classes in suspension with increase of flow velocity. The phenomenon could be explained by the equation of relative suspension concentration which relates the relative concentration of a suspended particle of a particular diameter to the flow velocity of the turbulent fluid and the height of suspension above the bed.