Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 157

the effect of vegetation is accounted for in the roughness calcula-
tion. The maximum value occurs if the simulation period equals or
exceeds the total growing period of the vegetation. For well-maintained
canals, the effect of vegetation on the roughness is considered from the
beginning. Weed factor 1 is taken for the initial stage and it increases
linearly up to the periodic maintenance time. Méndez (1998) defined
the periodic maintenance interval at approximately two months; how-
ever it depends upon the type of vegetation in terms of total growing
time and its effect on roughness;
Sediment transport predictors
• based on the typical flow conditions and the sediment sizes usually

encountered in irrigation canals, the sediment will be transported as
bed load and suspended load, separately or combined. Therefore the
sediment transport predictors for irrigation canals should be able to
compute both types of sediment transport;

• existing predictors of the sediment transport capacity of wide canals

do not take into account the geometry and its effect on the velocity
distribution over the cross section; they do not describe the sediment
transport in irrigation canals in a reliable or realistic way. The best pre-
diction methods give only 60% of the measured values as well predicted
within an error band of 100%;

• the Ackers-White and the Brownlie predictors provide more accurate

estimates of the sediment transport capacity under the prevailing flow
conditions and sediment characteristics in irrigation canals.