Rolling is the primary mode of transport in gravel-bed streams, while saltation in which grains hop over the bed in a series of low trajectories is largely restricted to sands and small gravels.
[4] Variables used to characterize the bed material load transport as described by Kumar (2012) are as follows:[5] – Channel geometry: b (width of the channel), y (flow depth) and BF (bed form of the channel) – Dynamic properties: Q\ (channel discharge), Sf (friction/energy slope), τb (bed shear stress) and τc (critical shear stress or Shields’ shear stress) – Sediment properties: d (mean size of sediment), σ (gradation coefficient of the sediment particles) and Gs (specific gravity) – Fluid properties: ν (viscosity) Bed material load transport (C) is a function of all the above parameters, i.e.: C = f (b, y, BF, Q, Sf, τb, τc, d, σ,Gs, ν) Knowledge of sediment transport is important to such endeavors as river restoration, ecosystem protection, navigation, and infrastructure management.
Direct measurement is done through the use of a physical trap, placing the device in contact with the bed, “allowing the sediment transported as bedload to accumulate (or be trapped) inside the sampler for a certain amount of time, after which the sampler is raised to the surface and the material is emptied and weighed to determine a weight transported per unit time.
"[6] There are three types of direct samplers, which include a box or basket, pan or tray, and pressure difference as described by Hubbell (1964).
No one method is entirely satisfactory, but indirect channel surveys, provided they are detailed enough at the reach scale, can produce reliable results, and have the advantages of minimum disturbance to the flow and time-integrated sampling which averages out short-term fluctuations in the transport rate.