Laser instruments for particle size and settling velocity measurements in the coastal zone

Within the last 2 years, a powerful new set of instruments have been developed which promise to address the measurement needs of the sediment transport community. The sediment transport researcher needs 3 measurements having to do with sediments: concentration of sediments with sufficient size resolution, the settling velocities of these size classes independently of each other, and the 'reference concentration' of these various size classes. The last of these lists the concentration at a small distance above the seafloor; it is the bottom boundary condition specifying the sediment concentration at, or flux off the bed in relationship to applied hydrodynamic stress. The suite of instruments described in this paper covers these needs, with only the continuing deficiency of assumed sphericity and homogeneity of individual particles. In this paper, the authors stress the engineering aspects, illustrating performance with data as available.