Channel Pattern

Fundamentals of Fluvial Geomorphology and Channel Processes
2.1.4.1 Channel Pattern
Channel pattern describes the planform of a channel. The primary types of planform
are meandering, braided, and straight. In many cases, a stream will change pattern within its
length. The type pattern is dependent on slope, discharge, and sediment load.
The most common channel pattern is the meandering stream (Figure 2.3). A
meandering channel is one that is formed by a series of alternating changes in direction, or
bends. Relatively straight reaches of alluvial rivers rarely occur in nature. However, there are
instances where a reach of river will maintain a nearly straight alignment for a long period
of time. Even in these relatively straight reaches, the thalweg may still meander and alternate
bars may be formed. Straight streams generally occur in relatively low energy environments.
The braided pattern is characterized by a division of the river bed into multiple channels
(Figure 2.4). Most braided streams are relatively high gradient and relatively coarse streams.
2.1.4.2 Channel Geometry and Cross Section
The following paragraphs describe the channel geometry and cross sectional
characteristics of streams. Since meandering streams are the most common form of alluvial
channels this discussion will focus primarily on this stream type.
Pools and A schematic showing features associated with meanders and
straight channels is given in Figure 2.5. As the thalweg, or trace formed by the deepest
portion of the channel, changes from side to side within the channel, the momentum of the
flow affects the cross-sectional geometry of the stream. In bends, there is a concentration of
flow due to centrifugal forces. This causes the depth to increase at the outside of the bend,
and this area is known as a pool. As the thalweg again changes sides below a bend, it crosses
the centerline of the channel. This area is known as the riffle or crossing. At the point of
tangency between adjacent bends, the velocity distribution is fairly consistent across the cross
section, which is approximately rectangular in shape. The concentration of flow in the bend
is lost and the velocity decreases accordingly, thus causing deposition in the crossing.
Cross Section Shape. The shape of a cross section in a stream depends on the point
along the channel with reference to the plan the type channel, and the
characteristics of the sediment forming and transported within the channel. The cross section
in a bend is deeper at the concave (outer bank) side with a nearly vertical bank, and has a
shelving bank as formed by the point bar on the convex side. The cross section will be more
trapezoidal or rectangular in a crossing. These are shown in Figure 2.6. Cross section shape
can be described by a number of variables. Some of these such as the area, width, and
maximum depth are self explanatory. However, other commonly used parameters
warrant some explanation. The wetted perimeter (P) refers to the length of the wetted cross
section measured normal to the direction of flow. The width-depth (w/d) ratio is the channel
width divided by the average depth (d) of the channel. The average depth is
11