Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 93

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U nstea dy

Steady

(a)

t

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Figure 5.2. Hydrographs in an
irrigation canal: (a) typical,
(b) schematised.

(b)

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5.2 FRICTION FACTOR PREDICTORS

5.2.1 Introduction

Roughness relates the mean velocity with the water depth and bottom
slope and consequently influences the sediment and fluid characteristics of
a canal. The correct prediction of the roughness is critical as it determines
the construction costs and the type of maintenance to be expected in the
future. The choice of a low roughness results in smaller cross sections
and lower construction costs, but will require more maintenance to meet
the design criteria. On the other hand a design roughness higher than the
actual one will result in a lower water depth for the design discharge and
a large number of water level regulators will be needed to raise the water
level to the design level. This will increase the operational cost and change
the flow pattern and the sediment transport in the canal.
Roughness of a canal can be measured under uniform flow conditions,
which forms the basis for the design. Measured values are the overall
roughness of the whole cross section and include the influence of various
factors. Major factors that influence the roughness are (Chow, 1983):
•

size and shape of the material that forms the wetted perimeter;

• vegetation on the bed and sides;

• canal surface irregularities due to localized erosion/deposition, bed

forms, poor canal construction or maintenance, etc.;

• shape of the cross section and the B / y ratio;