Planform Geometry

Fundamentals of Fluvial Geomorphology and Channel Processes
separation zone caused by centrifugal forces in the bend, and secondary flow. Middle bar is
the term given to areas of deposition lying within, but not connected to the banks. Figure 2.7
shows a typical middle bar on the Mississippi River. Middle bars tend to form in reaches
where the crossing areas between bends are excessively long and occasionally in bends due
to the development of chutes. Alternate bars are depositional features that are positioned
successively down the river on opposite sides (Figure 2.8). Alternate bars generally occur in
straight reaches and may be the precursor to a fully developed meander pattern.
2.1.4.3 Planform Geometry
Sinuosity is a commonly used parameter to describe the degree of meander activity
in a stream. Sinuosity is defined as the ratio of the distance along the channel (channel length)
to the distance along the valley (valley length). Think of sinuosity as the ratio of the distance
the fish swims to the distance the crow flies. A perfectly straight channel would have a sinuo-
sity of 1.0, while a channel with a sinuosity of 3.0 or more would be characterized by tortuous
meanders.
The meander wave length (L) is twice the straight line distance between two
consecutive points of similar condition (i.e. pools or crossings) in the channel as depicted in
Figure 2.9. This is sometimes referred to as the axial meander wavelength to distinguish it
from the channel length between inflection points which is also sometimes referred to as the
meander wave length. The meander amplitude (A) is the width of the meander bends
measured perpendicular to the valley or straight line axis (Figure 2.9). The ratio of the
amplitude to meander wavelength is generally within the range 0.5 to 1.5. It should be
noted that the meander amplitude and the width of the meander belt will probably be unequal.
The meander belt of a stream is formed by and includes all the locations held by a stream
during its development history. In many cases, this may include all portions of the present
flood plain. Meander wave length and meander width are primarily dependent on the water
and sediment discharge, but may also be modified by confines of the material in which the
channel is formed. The effects of bank materials is shown by the irregularities found in the
alignment of natural channels. If the material forming the banks was homogeneous over long
distances, a sinusoidal alignment having a unique and uniform meander wavelength would be
expected although this rarely occurs in nature.
The radius of curvature (r) is the radius of the circle defining the curvature of an
individual bend measured between adjacent inflection points (Figure 2.9). The arc angle ( )
is the angle swept out by the radius of curvature between adjacent inflection points. The
radius of curvature to width ratio (r/w) is a very useful parameter that is often used in the
description and comparison of meander behavior, and in particular, bank erosion rates. The
radius of curvature is dependent on the same factors as the meander wavelength and width.
Meander bends generally develop a radius of curvature to width ratio (r/w) of 1.5 to 4.5, with
the majority of bends falling in the 2 to 3 range.
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