Contents preface (VII) introduction 1—37

7.5. TRANSPORT OF SEDIMENT
When the average shear stress τ_o on the bed of an alluvial channel exceeds the critical shear τ_c, the sediment particles start moving in different ways depending on the flow condition, sediment size, fluid and sediment densities, and the channel condition.
At relatively low shear stresses, the particles roll or slide along the bed. The particles remain in continuous contact with the bed and the movement is generally discontinuous. Sediment material transported in this manner is termed contact load.
On increasing the shear stress, some sediment particles lose contact with the bed for some time, and ‘hop’ or ‘bounce’. The sediment particles moving in this manner fall into the category of saltation load. This mode of transport is significant only in case of noncohesive

270 IRRIGATION AND WATER RESOURCES ENGINEERING
materials of relatively high fall velocities such as sand in air and, to a lesser extent, gravel in water.
Since saltation load is insignificant in case of flow of water and also because it is difficult to distinguish between saltation load and contact load, the two are grouped together and termed bed load, which is transported on or near the bed.
With further increase in the shear stress, the particles may go in suspension and remain so due to the turbulent fluctuations. The particles in suspension move downstream. Such sediment material is included in the suspended load. Sediment particles move in suspension when u_*/w_o > 0.5. Here, w_o is the fall velocity for sediment particles of given size.
The material for bed load as well as a part of the suspended load originates from the bed of the channel and, hence, both are grouped together and termed bed-material load.
Analysis of suspended load data from rivers and canals has shown that the suspended load comprises the sediment particles originating from the bed and the sediment particles which are not available in the bed. The former is the bed-material load in suspension and the latter is the product of erosion in the catchment and is appropriately called wash load. The wash load, having entered the stream, is unlikely to deposit unless the velocity (or the shear stress) is greatly reduced or the concentration of such fine sediments is very high. The transport rate of wash load is related to the availability of fine material in the catchment and its erodibility and is, normally, independent of the hydraulic characteristics of the stream. As such, it is not easy to make an estimate of wash load.
When the bed-material load in suspension is added to the bed-material load moving as bed load, one gets the total bed-material load which may be a major or minor fraction of the total load comprising bed-material load and wash load of the stream depending on the catchment characteristics.
Irrigation channels carrying silt-laden water and flowing through alluvial bed are designed to carry certain amounts of water and sediment discharges. This means that the total sediment load transport will affect the design of an alluvial channel. Similarly, problems related to reservoir sedimentation, aggradation, degradation, etc. can be solved only if the total sediment load being transported by river (or channel) is known. One obvious method of estimation of total load is to determine bed load, suspended load, and wash load individually and then add these together. The wash load is usually carried without being deposited and is also not easy to estimate. This load is, therefore, ignored while analysing channel stability.
It should, however, be noted that the available methods of computation of bed-material load are such that errors of the order of one magnitude are not uncommon. If the bed-material load is only a small fraction of the total load, the foregoing likely error would considerably reduce the validity of the computations. This aspect of sediment load computations must always be kept in mind while evaluating the result of the computations.