Test: Sediment Transport Theory - Civil Engineering (CE) MCQ

Natural rivers used for navigation frequently get silted up which reduces the clear depth required for the same. This often requires costly dredging due to the sediment deposited in the rivers and harbours.

The force exerted by water in the direction of flow is the drag force or tractive force. If this force on the bed particles exceeds the force opposing their movement then, the bed particles will start moving to lead to the sediment transport.

BEAST stands for Benthic Environmental Assessment Sediment Tool. BEAST is one of the sediment erosion devices designed in order to quantify rates of sediment erosion.

Shield was the first person to analyze the experimental data on incipient motion condition using the critical tractive force approach. The assumption of the entry of clean and clear water in the channel is used to develop non-scouring highest possible flow velocity at the peak flow.

The maximum tractive stress = Y_w. R. S
= 9.81 x 1000 x 2.0 x 10^-4
= 1.962 kg/m²

The average shear stress = Y_w. R. S
= 9.81 x 1000 x 0.6 x 1/2500 = 2.35 N/m².
This value is more than 0.53 N/m², the soil grains will not be stationary and sediment transport and scouring will occur.

The unit tractive force in channels is uniformly distributed along the wetted perimeter. For the bed of the canal, the average shear stress is equal to the tractive force per unit area. For side slopes of the canal, average shear stress is equal to 0.75 times the tractive force per unit area.

Coherent means that there is a cohesive force between particles such as in the case of clays. The soil is assumed to be incoherent so that each soil grain is studied individually. Most of our river beds are made up of gravels and sands in which there is no cohesion.

The scouring and silting may create sharp and irregular curves. This increases the resistance to flow of the channel and thereby, raising the flood levels for the particular discharge.

This causes large scale scouring and siltation of irrigation canals thereby increasing their maintenance. A poorly designed artificial channel gets silted so badly that they sometimes become inoperable and leads to huge economic loss.