Fundamentals of Fluvial Geomorphology and Channel Processes
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 geometry, 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 3.6.
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 calculated by dividing the cross section area by the
channel width. The hydraulic radius (r), which is important in hydraulic computations is defined as the
cross sectional area divided by the wetted perimeter. In wide channels with w/d greater than about 20 the
hydraulic radius and the mean depth are approximately equal. The conveyance, or capacity of a channel
is related to the area and hydraulic radius and is defined as AR2/3.
Channel Bars. Channel bars are depositional features that occur within the channel. The size and
location of bars are related to the sediment transport capacity and local geometry of the reach. The
enlargement of a bar generally results in caving of the opposite banks in order to maintain conveyance of
the discharge. The primary types of bars are point bars, middle bars, and alternate bars.
Point bars form on the inside (convex) bank of bends in a meandering stream. A typical point bar
is shown in Figure 3.3. The size and shape of the point bar are determined by the characteristics of the
flow. The development of a point bar is partially due to the flow 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 3.7 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 3.8). Alternate bars generally occur in straight
reaches and may be the precursor to a fully developed meander pattern.
3.1.3.3 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 sinuosity of 1.0, while a channel with a sinuosity of 3.0 or
more would be characterized by tortuous meanders. The
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