Introduction - Computational River Dynamics - page 9

Depending on the calculation procedure , flow and sediment transport models can be
classified as fully decoupled, semi-coupled, or fully coupled. Fully decoupled models
ignore the influence of sediment transport and bed change on the flow field by assuming
a low sediment concentration and a small bed change, and calculate the flow and
sediment transport separately at each time step. Fully coupled models compute all
the flow and sediment quantities simultaneously. Semi-coupled models calculate some
quantities in coupled form and the others separately. For example, flow and sediment
modules may be decoupled, whereas sediment transport, bed change, and bed material
sorting in the sediment module may be coupled. Because flow, sediment, and bed
material always interact with each other in an alluvial river system, fully coupled
models are more general and physically reasonable, whereas the applicability of fully
decoupled and semi-coupled models is limited. However, coupled models are more
sophisticated and may require more computational effort than decoupled models. In
addition, the results from decoupled models may be justified due to the difference in
time scales of flow and sediment transport and the use of empirical formulas for bed
roughness and sediment transport capacity. Fully decoupled and semi-coupled models
are still used by many investigators.
Depending on how to conceptualize sediment, sediment transport models can be
discerned as particulate and continuous-medium models. Particulate models treat sed-
iment as a group of particulate entities and describe the movement of single particles,
whereas continuous-medium models assume sediment as a kind of pseudo-continuous
medium. The assumption of continuous-medium models is only valid when the char-
acteristic size of the sediment particles is much shorter than the characteristic length
of the processes of study and the volume under consideration has enough sediment
particles. Apparently, particulate models are not limited in this way. From a strictly
theoretical point of view, particulate models should be preferred. However, because of
the limitations of computer capacity, considerable difficulties are encountered in the
simulation of the behavior of millions or even billions of irregularly shaped particles
that may collide randomly. In reality, particulate models are only feasible when the
sediment concentration is extremely low. Therefore, continuous-medium models are
more widely applied in the study of sediment transport in rivers. A typical continuous-
medium model is the diffusion model that is most often used for suspended-load
transport.
1.5 COVERAGE AND FEATURES OF THIS TOPIC
The subjects of this topic include physical principles, numerical methods, model clo-
sures, and application examples in computational river dynamics. It is organized into
twelve chapters.
Chapter 1 provides a general overview of computational river dynamics and the
arrangement of this topic. Chapter 2 introduces the mathematical descriptions of
flow, sediment transport, and morphological change processes in rivers. Chapter 3
presents the fundamentals of sediment transport. Chapter 4 introduces the numerical
techniques widely used to solve open-channel flows with sediment transport, such as
the finite difference method and the finite volume method. These methods are applied
and extended in the remaining chapters of this topic. Chapter 5 describes the 1-D