A simple formula to estimate settling velocity of natural sediments

A simple formula to calculate the settling velocity of natural sediment particles for grain sizes between 0.063 and 1 mm is presented. The formula has been derived from the previous work of Dietrich, and it predicts the dimensionless settling velocity W-* as a function of a dimensionless fluid-sediment parameter S-*, provided the sediment shape factor and roundness are known. In case no information on shape and roundness factors is available, this paper recommends using the formula with a shape factor of 0.7 and a roundness value of 3.5 for naturally worn particles. The formula is tested against several independent data sets, and its performance is compared to other existing simple settling velocity formulas. For fine sediments with nominal diameters d(N) between 0.063 and 0.25 mm, for which sediment suspension in natural conditions is most likely to occur, the proposed formula is shown to perform the best in terms of standard error of the estimation.