In the study of mechanics of sediment transport, the soil particles are always considered as incoherent.
Most of the river beds are made up of sand and gravel and as such assumption in regard to soil being incoherent is correct.
The basic mechanism that controls the sediment transport, is the drag force exerted by water in the direction of flow on the channel bed.
This drag force is also known as tractive force or shear force.
This force is nothing but a pull of water on the wetted area of the channel.
Consider a channel of length L and cross-sectional area A.
The volume of water in this length of channel would be A × L.
If w is the unit weight of water, then weight of water stored in this length (L) of the channel will be wAL. Weight of water acting in vertical direction = wAL.
The horizontal component of this weight = wAL sin θ.
But θ is slope of the channel which is represented by S. Horizontal component of weight = wALS.
Flow of water in channel and forces acting on silt at bed.
This horizontal force exerted by water is nothing but tractive force.
If average tractive force per unit of wetted area is τ, then
Hence average unit tractive force also know shear stress is given by τ = wSR.
where
w = Unit of weight of water
S = Longitudinal slope of the channel
R = Hydraulic mean depth H.M.D.
In the case of wide open channels, the value of hydraulic mean depth (H.M.D.) is almost equal to depth of the channel.
Hence tractive force can be written as follows also
τ = wSD
If tractive force becomes greater than the frictional resistance between particles, the particles are set in motion.
The resistance of sediment to motion in proportional to the diameter d of the particle and the submerged weight of sediment in water.
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