Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 156

the sediment transport concepts for the conditions prevailing in irrigation
canals the following conclusions can be drawn:
General
•

there are several theories to estimate friction factors and (equilibrium
and non-equilibrium) sediment transport rates. These theories rely on
field and laboratory data, but they are not able to predict very accurately
the friction factors and sediment transport rates in irrigation canals;
Bed forms
• for the lower flow regime in irrigation canals all types of bed forms

(ripples, mega ripples and dunes) can be expected;

• the van Rijn method describes the bed forms more accurately than

other methods; more than 75% of the observed bed form types were
well predicted by the van Rijn method;
Friction factor predictors
•

the van Rijn method to predict the friction factor due to the grains and
bed forms gives good results when compared with measured values of
the friction factor. This method was able to predict more than 90% of
the measured values within an error band of 30%;

• several existing friction factor predictors consider only the bottom fric-

tion. The sidewalls of non-wide canals have an important impact on
the overall friction and therefore a weighed composite friction factor
is required. The proposed method for predicting the equivalent rough-
ness in a trapezoidal canal with different roughness along the bottom
and sidewalls performs better than the existing methods. The method
predicted more than 90% of the measured values as well predicted and
resulted in the minimum value of the standard error and the narrowest
range of variation of the predicted values;

• the existing methods for predicting the equivalent roughness in a rect-

angular canal with different roughness along the bottom and vertical
sidewalls cannot be explicitly applied. Mendez proposed a new method
to estimate the effective roughness in a rectangular canal with compos-
ite roughness. The method predicted more than 95% of the measured
values within an error band of 15%;

• the roughness of the side slopes is due to the type of material and the

vegetation, which may be present at the beginning or likely to grow
during the irrigation season, depending upon the type of maintenance.
The roughness has been divided into two categories, namely sides with-
out any vegetation and sides that are covered with vegetation. The
second category has been further divided into various weed factors
from densely grown to 1.0 ideally clean. For ideally maintained condi-
tions a weed factor may vary from 0.9 to 1.0. For fully-grown vegetation
the weed factor ranges between 0.1-0.3;

• maintenance conditions are divided into three possible maintenance sce-

narios: no maintenance, well maintained and ideally maintained. For
poorly maintained canals, at each time step starting from the beginning