USING LAGRANGIAN PARTICLE SALTATION OBSERVATIONS FOR BEDLOAD SEDIMENTTRANSPORT MODELING

A Lagrangian model for the saltation of sand in water is proposed. Sim ulated saltation trajectories neglecting particle rotation and turbule nce effects compare fairly well with experimental observations. The mo del for particle motion is coupled with a stochastic model for particl e collision with the bed, such that a number of realizations of the sa ltation process can be simulated numerically. Model predictions of mea n values and standard deviations of saltation height, length and strea mwise particle velocity agree fairly well with experimental observatio ns. Model predictions of the dynamic friction coefficient are also in good agreement with experimental observations, but they underestimate the value of 0.63 proposed by Bagnold for this coefficient. The saltat ion model is applied to the estimation of bedload transport rates of s and using a Bagnoldean formulation. Modelled values of the bedload tra nsport rates overestimate those predicted by commonly used bedload for mulae, which appears to be a consequence of problems in the definition of the dynamic friction coefficient, These results seem to indicate a few problems with the Bagnoldean formulation, particularly regarding the continuum assumption for the bedload layer, which would be valid o nly for very high particle concentrations and small particle diameters , and also regarding the evaluation of the shear stress exerted on the bed by the saltating particles, (C) 1998 John Wiley Sons, Ltd.