Environmental Engineering Reference
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of the B / y ratio (procedure 3). The predictability of the Brownlie predictor
improves when the hydraulic mean depth R is used instead of the water
depth. The accuracy of the Ackers-White predictor improves when the
water depth is used instead of the hydraulic mean depth ( R ). The Engelund-
Hansen prediction shows no significant change in accuracy for the two
cases (procedures 1 and 2). The use of the bed width or the average width
( B
B s )/2 as the representative width will not influence the results as
all the collected data are from rectangular cross sections. However, in
a trapezoidal canal section the method of taking a representative width
should have a significant effect on the total sediment transport. The most
logical option could be the use of the average of the bed width and the top
width.
As discussed earlier, sediment movement is a function of the hydraulic
and sediment characteristics. A proper estimate of the design discharge
and the roughness will support an accurate prediction of the hydraulic
conditions. An understanding of the influence of the canal geometry on
the sediment transport capacity for a given discharge will assist in the
proper selection of the bed width, bed slope and side slope during the
design.
Although the accuracy of the prediction will improve when the correc-
tion factor α is used for the sediment transport predictors, the accuracy of
prediction will still be low. The accuracy is less than 50% for an error fac-
tor of 1.5. Maybe one predictor method predicts well for one data set, but
for another data set the predictability is still very poor. This clearly indi-
cates the limitation of the available sediment predictors. The predictors are
derived from a limited range of hydraulic and sediment characteristics and
their use outside this range should be made with utmost care. Therefore
the predictors should be tested for the field conditions for which they are
to be used and adjustments should be made once these modifications are
found necessary.
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5.3.9 Sediment transport in non-equilibrium conditions
Sediment transport in a steady and uniform flow has a unique equilibrium
transport rate, but the transport rate under variable conditions differs from
this steady uniform and equilibrium situation. Non-equilibrium conditions
are very significant for the suspended load transport because the travel
length of a suspended particle is in general much longer that the step length
of a bed load motion (Nagakawa et al., 1989). While it is possible to relate
the bed load transport to the local and instantaneous characteristics of
water flow and bottom composition, no distinct relation can be established
for the suspended load transport since this part of the transport mode is
substantially influenced by upstream conditions (Armanini and Di Silvio,
1988).
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