1-D numerical models - Computational River Dynamics - page 232

After A , Q , and Q tk are solved, the sediment transport capacity, Manning n , bed
change, and bed-material gradation can be calculated using Eqs. (5.141), (5.182),
(5.184), and (5.185), respectively.
As compared to the iteratively coupled procedure, the fully coupled procedure is
much more complicated. In particular, if channel network routing, bank erosion, and
bed material consolidation need to be considered, the fully coupled procedure becomes
cumbersome and tedious.
5.4.4 Justification of decoupled and coupled models
Decoupled flowand sediment transport models have beenwidely used in the solution of
many real-life engineering problems. They are relatively easy to implement, and their
results may be justified due to different time scales in flow and sediment transport
and the use of empirical formulas for bed roughness and sediment transport capacity.
Most of the criticisms against the decoupled models are related to the equilibrium
sediment transport model. The application of the non-equilibrium transport model
and Wu's (1991) coupling procedure for non-uniform sediment transport simulation
has significantly enhanced the numerical stability of the decoupled flow and sediment
transport models. However, it is true that the applicability of the decoupled models is
restricted due to the assumption of low sediment concentration and small bed change
at each time step.
The coupled models take into account the physical coupling of water and sediment
phases, so that they should be more reasonable and could be applied in a wider range of
flow and sediment conditions. The coupled models are usually more stable and can use
larger time steps than the decoupled models (Saiedi, 1997; Cao et al ., 2002). However,
the implementation of the coupled models, especially the fully coupled models for
non-uniform sediment transport in looped channel networks, is very complicated.
Their efficiency may be offset by the required effort of iteration and matrix inversion.
Furthermore, because the time step for flow calculation is usually smaller than that
for sediment calculation, solving the nonlinear flow systemmight become a bottleneck
and restrain the efficiency of the coupled models.
It is diffficult to give a quantitative criterion as to when the decoupled models are
acceptable. Generally, in the lower flow regime with low sediment concentration,
the decoupled models are applicable; otherwise, the coupled models should be used.
Because the sediment concentration is usually low in most natural rivers, the decoupled
models can still play an important role in river engineering analysis.
5.5 DATA REQUIREMENTS OF 1-D MODEL
The following data are commonly required by 1-D models. They are also required by
2-D and 3-D models, with higher spatial resolutions.
Study domain
The study domain usually covers the channel reach of interest and additional tran-
sition reaches in upstream and downstream. Its inlets and outlets should be located