Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 96

account for the shape and size of bed forms in the roughness prediction.
Brownlie (1981b) relates the discharge, slope and sediment characteristics
with the flow depth that is used to determine the roughness of the section.
White et al. (1980) gives a relation to compute the mean velocity from
water depth, slope and sediment characteristics that are used to predict
the friction factor. Engelund (1966) relates the mean flow velocity with
the slope, water depth and a mobility parameter.
The flow in most irrigation canals is sub-critical, meaning that the
Froude number is smaller than 1. More especially the flow in irrigation
canals is found in the lower flow region with Froude numbers smaller than
0.7 and even smaller than 0.4 (Ranga Raju, 1981). Bed features for these
low flow regimes can be described as a flat bed, as ripples or as dunes.
The latter two forms are characterized by rough triangles in the longi-
tudinal profile with a gentle sloping upstream face and a more inclined
downstream face (see Figure 5.4). No sharp distinction exists between rip-
ples and dunes, but they show some subtle differences. Engelund (1966)
described these bed forms as:
•

flat bed : a flat bed is a surface without any bed form. There is no motion
of the sediment on flat beds;

• ripples : ripples are a bed form to be found in canals with bed material

smaller than 0.6 mm; the wave length is shorter than 30 cm and the wave
height is a few centimetres. When the velocity is slightly greater than
the threshold value several ripples will be formed in the bed. The ripple
geometry is almost independent of the flow conditions;

•

dunes : dunes are a bed form that occurs in flows with a larger velocity.
Dunes develop for all sizes of bed sediment and their length and height
are greater than those of ripples. Moreover, the sediment transport in
canals with dunes is larger than in canals with ripples. The geometry of
the dunes strongly depends on the flow depth.

Deposition

Flow

Erosion

Figure 5.4. Schematic
representation of bed forms
for the low flow regime.

Several authors have analysed the bed forms as observed under flume
and field conditions and tried to explain the type of bed forms for certain
flow conditions. They presented graphical solutions for the prediction of
bed forms by using dimensional and non-dimensional plots. Some of these
authors are Liu (1957), Simons and Richardsons (1966), Bogardi (1974)
and van Rijn (1993). Each theory is based on a particular classification
parameter as presented in Table 5.1.