Geoscience Reference
In-Depth Information
Simulation of flow and sediment
transport in vegetated channels
Vegetation can markedly affect the fluvial processes in streams. To understand and
analyze this, fundamental research and numerical modeling of flow and sediment
transport in vegetated channels have been conducted in recent years. The concepts,
governing equations, and numerical methods of the vegetation effect models are
introduced in this chapter.
10.1 EFFECTS OF VEGETATION ON FLOW
AND SEDIMENT TRANSPORT
10.1.1 Geometric characteristics of vegetation
Vegetation height and diameter
Vegetation is classified to be either flexible or rigid, based on whether it is subject
to deformation by the flow. Herbaceous species, such as grasses, usually are flexible,
whereas woody species, such as trees, usually are rigid. However, it is recognized that
different portions of the plant and the same plant in different stages of its life cycle
can behave in significantly different ways. For example, the stems of trees and shrubs
are often rigid, while their branches, twigs, and leaves are flexible. Saplings usually
act as flexible stems until they mature sufficiently to be able to withstand deformation
by the flow.
Because the shape of vegetation is highly irregular, it is challenging to represent a
vegetation element with simple geometry. As an approximation, a vegetation stem
(such as tree trunk) is often conceptualized as a cylinder with a height, h v , and a
representative diameter, D , as shown in Fig. 10.1. The stem height h v is defined as
the one without any deformation by the flow. Thus, the height of a rigid stem can be
readily measured. However, a flexible stem bends under the shear of flow, as shown
in Fig. 10.2, and its actual height, h v , is related to stem properties and flow conditions
(Kouwen and Li, 1980).
The rep resentative diameter D can be related to the stem volume, V ,by D
=
4 V
h v . In practice, one may directly measure the stem diameter. Due to the fact
that the diameter of many species (such as shrubs) may change along the height, the
convention of foresters and ecologists suggests that the stem diameter of trees should
be measured at the breast height. As an approximation, the stem diameter may be
measured at h v
/
4 (Freeman et al ., 2000).
 
 
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