Fundamentals of Fluvial Geomorphology and Channel Processes
With some modifications to Figure 2.12, Figure 2.13 is a combination of some
concepts of Schumm and Rosgen. Schumm's classification system was heavily dependent on
his Midwestern experience, while Rosgen's experience began in steep mountain streams. In
addition, Schumm's (1977) classification did not specifically include incised channels, which
are included in Rosgen's (1994) F and G classes. Figure 2.13 includes C, D, DA, and E
classes, and could be expanded to include all of Rosgen (1994) classes. The value of
Figure 2.13 is to demonstrate that moving from class to class is a predictable response that
manages energy, materials, and channel planform to reestablish a balance of sediment and
water discharge with sediment and water supply.
2.2 CHANNEL STABILITY CONCEPTS
Streambank protection measures often fail, not as the result of inadequate structural
design, but rather because of the failure of the designer to incorporate the existing and future
channel morphology into the design. For this reason, it is important for the designer to have
some general understanding of stream processes to insure that the selected stabilization
measures will work in harmony with the existing and future river conditions. This section
describes the basic concepts of channel stability. This will allow the designer to assess
whether the erosion at a particular site is due to local instability processes or is the result of
some system-wide instability problems that may be affecting the entire watershed.
2.2.1 THE STABLE CHANNEL
The concept of a stable river is one that has generated controversy between engineers,
scientists, landowners, and politicians for many years. An individual's definition of stability
is often subjectively based on past experiences or project objectives. To the navigation
engineer, a stable river might be one that maintains adequate depths and alignment for safe
navigation. The flood control engineer on the other hand is more concerned with the channel
maintaining the ability to pass the design flood, while to the local landowner a stable river is
one that does not erode the bankline. Therefore, bank erosion would not be an acceptable
component of these groups' definition of a stable river. Geomorphologists and biologists, on
the other hand, might maintain that bank erosion is simply part of the natural meandering
process of stable rivers and would be perfectly acceptable in their definition of a stable river.
Consequently, there is no universally accepted definition of a stable river. However, some
manner of defining stability is needed before the concept of instability can be discussed.
Therefore, the following paragraphs will attempt to establish a definition of a stable river to
be used for this manual.
River behavior may be influenced by a number of factors. Schumm (1977) identified
these as independent and dependent variables. Independent variables may be thought of as
the basin inputs or constraints that cause a change in the channel morphology. Independent
variables include: basin geology, hydrology (discharge of water and sediment), valley
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