SEDIMENT TRANSPORT

  • 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.

  1. The design and execution of flood control schemes is chiefly governed by the peak flood level.
  2. Which in turn depends upon the scour and deposition of sediment.
  3. Silting of channels and reservoirs also depends upon the sediment transport.
  4. Sediment deposited in rivers and harbours requires costly dredging.

SEDIMENT LOAD

  • 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.
Development of ripples at bed.
Development of ripples at bed.