Geoscience Reference
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On the other hand, considering numerical accuracy and sometimes stability in the
solution of bed-load transport equation, e.g., Eq. (2.153), the grid spacing should
be (several times) smaller than the adaptation length. However, because of limited
computer capacity, the grid spacing has to be given large values (sometimes much
larger than the length of the dominant bed form) in field cases, and to obtain feasible
solutions, L b is hence set to one or two times the grid spacing (Rahuel et al ., 1989;
Fang, 2003). This treatment perhaps is the choice under certain circumstances, but it
may give grid-dependent solutions.
Because bed-material load is a combination of bed load and suspended load, its
adaptation length can be given the larger of L b and L s (Wu et al ., 2004a):
{
L b , L s }
L t =
max
(2.155)
or a weighted average of L b and L s :
L t
= (
1
r s
)
L b +
r s L s
(2.156)
where r s is the ratio of suspended load to bed-material (total) load.
In cases where bed load and suspended load coexist, L s is usually larger than L b ,
and thus Eq. (2.155) gives
t , which is required in the derivation of Eq. (2.151).
Therefore, Eq. (2.155) was used by Wu (2004) in many cases. However, because L b
and
α = α
are usually treated as calibrated parameters, the difference between Eqs. (2.155)
and (2.156) is not important.
Because wash load does not have significant exchange with the bed, its adaptation
coefficient
α
and length L can be set to be zero and infinitely large, respectively.
It should be pointed out that because the values of L b and
α
vary by case, the methods
discussed above and in Section 2.5 are only empirical guidance for evaluating these
two parameters. Their calibration using available measurement data is recommended
to obtain more reliable results for real-life problems. Sensitivities of sediment transport
models to these parameters are demonstrated in Sections 5.6 and 9.2.
α
2.7 TRANSPORT AND SORTING OF NON-UNIFORM SEDIMENT
MIXTURES
2.7.1 Non-uniform sediment transport
In the case of non-uniform sediment transport, moving sediment particles collide and
interact; bed sediment particles experience the hiding and exposure effects, because
fine particles are more likely to be hidden and coarse particles have more chance to be
exposed to flow. However, if the sediment concentration is low, interactions among the
moving sediment particles are usually negligible, so that each size class of the moving
sediment mixture can be assumed to have the same transport behavior as uniform
sediment. This assumption is adopted in this topic, except where stated otherwise. As
an example, a depth-averaged 2-D non-uniform sediment transport model based on it
is presented below.
 
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