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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.sub.*] as a function of a dimensionless fluid-sediment parameter [S.sub.*], 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.sub.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. CE Database keywords: Grain size; Settling velocity; Sediment.

Erosion tests were run on soft cohesive sediment beds freshly deposited from a concentrated slurry, in a large rotating annular flume. A total of seven tests were carried out with the bed age varying between 1.8 and 18.9 days. A surface type erosion was observed for the applied shear stress range (0.1 to 1 Pa). Following the increase in the water flow, the erosion rate increases quickly to a peak value and then drops off to about zero. The eroded depth was less than 8 mm for all beds, but the resistance to erosion at a given depth and for a given bed structure increased with increasing bed age. This phenomenon was attributed to thixotropy. The erosional strength of a given sediment layer is found to be one order of magnitude lower than its undrained and drained mechanical shear strengths measured with innovative testing methods. Hypotheses are given to explain that difference. However, at constant density and soil-water chemistry, the erosional and mechanical strengths seem to be affected by the same factors, i.e., bed age, structure, and temperature.

Comments on the article on the formula to estimate settling velocity of natural sediments. Use of the appropriate settling velocity for evaluation of sediment transport mode and suspended sediment transport; Prediction of suspension criterion and suspended sediment transport.

Discusses correlations of calculated time-integrated excess skin friction stress and sediment sand content in Quincy Bay in outer Boston Harbor, Massachusetts. Correlation of the excess shear stress with sediment lead; Site description; Existence of low contaminant concentrations in some low shear stress sites.