Concerning the prediction of the hydraulic gradient, a first empirical correlation is the one proposed by Durand and coworkers [15]. They used sand particles of one specific mass with diameter up to 25.4 mm in pipes ranging from 38 mm to 558 mm in diameter (the maximum relative diameter in the pipe of 104 mm was 4.5%). The dimensionless excess of pressure drop caused by the presence of the particles U is expressed using the two ‘‘Froude’’ numbers FD and Fdp . These two parameters are grouped to form W ¼ F2 DF1 dp . They found a general correlation that best represents all their data