- A flowing water in a channel, always tries to scour its surface.
- Silt, gravel or even larger boulders are first detached from its bed or banks, and then swept D/S by moving water.
- The phenomenon of detaching and then sweeping
- The silt or gravel from bed or banks is know is sediment transport.
- The phenomenon is of great economic importance.
Following are some of the important works where sediment transport has its effects.
- The design and execution of flood control schemes is chiefly governed by the peak flood level.
- Which in turn depends upon the scour and deposition of sediment.
- Silting of channels and reservoirs also depends upon the sediment transport.
- Sediment deposited in rivers and harbours requires costly dredging.
- The quantity of solids (silt) entering the channel, is know sediment load.
- It is a single important factor which controls the shape and cross-section of the true regime channel.
- The sediment moving in a fluid can be broadly divided into two parts:
1. Bed load and
2. Suspended load.
1. Bed load.
- It is the load of bed material, in the bottom most layer of the flow.
- Suspension of sediment is not possible in this layer, because, of fluid dynamic reasons.
- The grains of bed load are not supported vertically by the flow but rest on the bed while rolling, sliding and jumping.
- The weight of the bed load particles is borne by the stationary grain particles of non-moving bed.
- The particles of bed load move regularly and exchange places with the similar particles of non-moving bed.
2. Suspended load.
- With increase in the velocity, smaller particles are thrown in suspension by the upward component of the turbulent velocity of flow.
- The particles always settle due to gravity effect.
- The flowing water provides an upward motion due to turbulent exchange.
- Due to turbulent exchange the water is regularly exchanged between various horizontal layers of water over a definite distance.
- The ascending water originates from lower layers of higher concentration and descending water originates from higher layer of lower sediment concentration.
- In this process of exchange of turbulences, there is a net upward surplus force.
- Which provides an upward motion to sediment particles and counterbalances.
- The settlement effect of the particles due to gravity.
- Suspended load also causes additional hydrostatic pressure on the bed of the channel.