In summary, a meandering river has regular inflections that are sinuous in plan. It consists of a
series of bends connected by crossings. In the bends, deep pools are carved adjacent to the
concave bank by the relatively high velocities. Because velocities are lower on the inside of the
bend, sediments are deposited in this region, forming a point bar. The centrifugal force in the
bend causes a transverse water surface slope, and in many cases, helicoidal flow occurs in
the bend. Point bar building is enhanced when large transverse velocities occur. In so doing,
they sweep the heavier concentrations of bed load toward the convex bank where they are
percent of the average channel velocity. The bends are connected by crossings (short straight
reaches) which are quite shallow compared to the pools in the bendways. Much of the
sediment eroded from the outside bank is deposited in the crossing and on the point bar in the
next bend downstream. At low flow, large sandbars form in the crossings if the channel is not
well confined.
The scour in the bend causes the bend to migrate laterally and sometimes downstream.
Lateral movements as large as 760 m (2,500 ft) per year have been observed in alluvial rivers.
The meander belt formed by a meandering river is often fifteen to twenty times the channel
width. When compared to most braided rivers, meandering rivers have relatively flat slopes.
The geometry of meandering rivers is measured quantitatively in terms of: (1) meander
A, and (5) bend deflection angle φ. These variables are shown in Figure 5.15.
The actual meanders in natural rivers are obviously not as regular as indicated in Figure 5.15.
The precise measurement of meander dimensions is therefore difficult in natural channels and
tends to be subjective. An example on how to measure these characteristics is presented in
Section 5.9 (Problem 1). The analysis of the mean meander dimension in nature shows that
the meander length and meander width are both related to the width of the channels. The
empirical relationships for the meander length λ and the bank-full channel width as well as the
meander amplitude, A, and the bank-full channel width are shown in Figure 5.16 and Table 5.1.
Figure 5.15. Definition sketch for meanders.
5.19
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