Instruments for particle size and settling velocity observations in sediment transport

In this paper we describe two sensors for measurement of particle size-distribution and settling-velocity distribution. These measurements are critical to the correct estimation of the true sediment concentration in the field, as well as to evaluating models for transport rates of sediments. A multi-angle measurement: of laser scattering is made and inverted to obtain the particle size distribution. Since small-angle scattering is relatively insensitive to particle composition, the size distribution measurements are robust, and do not require particle refractive index. It is shown that with a knowledge of the size distribution, true particulate volume concentration can be obtained, unaffected in calibration by changes in particle size distribution. The data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer. The importance of these observations lies in the implication that historical data acquired with single-parameter optical or other sensors needs to be revisited, In the second instrument, analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates. In this case, the history of concentration of each size class is examined to determine the settling velocity, without invoking any assumptions of settling regime. Settling velocity data from a field experiment off the New Jersey coast fit the model: w(f,n) = 0.45 x 10(-3) a(n)(1,2), where a(n) is radius in microns and settling velocity is in cm/s. (C) 2000 Elsevier Science B.V. All rights reserved.