A head loss equation in the hydraulic transport of solids in horizontal pipelines has been developed theoretically assuming a flow model with both suspended solids and a sliding bed. By analyzing experimental results, two factors in the theoretical equation, or an apparent friction factor and the spatial solid concentration, were determined as a function of the flow parameter, ψ.=[V2/gD(s-1)]√Cd
　Two head loss parameters, φ=(im-iw)/iwC and φ*=(im-iw)/iwq, were examined against ψ. While, the ratio of the solid concentrations, q/C, has been discussed theoretically and experimentally. Considerations into the critical velocity made it possible to introduce a dimensionless parameter, Nc, which is availabe for judging the flow condition.
　The experiments were conducted with a 155.2mm pipe using sand of 0.91mm in diameter and gravel of 8.75mm, 27.5mm and 45mm, and with a 100.3mm pipe using sand of 0.25mm, 0.65mm and 1.28mm and gravel of 7.0mm. Besides these, the experimental data collected in 1957 with a 80.7mm pipe using sand of 0.75mm and gravel of 8.0mm were used.