Fundamentals of Fluvial Geomorphology and Channel Processes
3.4
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.
3.4.1
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 main-
tains 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 dimensions (slope, width, depth), vegetation (type
and density), and climate. Dependent variables include: channel slope, depth, width, and planform.
A channel that has adjusted its dependent variables to accommodate the basin inputs (independent
variables) is said to be stable. Mackin (1948) gave the following definition of a graded stream:
A graded stream is one in which, over a period of years, slope is delicately adjusted
to provide, with available discharge and with prevailing channel characteristics, just
the velocity required for the transportation of the load supplied from the drainage
basin. The graded stream is a system in equilibrium.
Mackin did not say that a stream in equilibrium is unchanging and static. A more commonly used term today
for this type of stability is dynamic equilibrium. A stream in dynamic equilibrium has adjusted its width,
depth and slope such that the channel is neither aggrading nor degrading. However, change may be
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