Fundamentals of Fluvial Geomorphology and Channel Processes
To avoid this confusion the field indicators of degradation (knickpoints and knickzones) will
not be referred to as headcuts. Rather, a headcut (or headcutting) is defined as a headward
migrating zone of degradation. This headcutting may occur with or without the formation of
knickpoints or knickzones which are purely a function of the materials encountered.
Once headcutting is initiated it may proceed rapidly through the system. The rate of
headward advance is a direct function of the materials encountered in the bed and also the
basin hydrology. If the channel bed is composed primarily of non-cohesive sands and silts,
then no knickpoints or knickzones will form and headcutting will work upstream by parallel
lowering of the bed. However, if consolidated materials such as clays, sandstones, or other
resistant materials occur in the channel bed, then knickpoints or knickzones will form as
degradation encounters these resistant layers. When this occurs the headward migration rate
may slow considerably. Therefore, the dominant factor affecting the headward migration rate
is the relative resistance to erosion of the bed materials, and to a lesser degree the discharge
in the stream.
As degradation migrates upstream it is not restricted to the main stem channel. When
headcutting passes tributary junctions it lowers the base level of these streams. This initiates
the degradation process for the tributaries. The localized increased slope at the confluence
produces an excess sediment transport capacity that results in degradation of the stream bed.
This process can continue upstream rejuvenating other tributaries until the entire basin has
been affected by the downstream base level lowering.
Upstream Factors. System instability is often initiated by upstream alterations in the
basin. This may result from a change in any of the controlling variables, but is most commonly
associated with modifications to the incoming discharges of water and sediment. Looking at
Lane's balance (Figure 2.14) it can be seen that either an increase in the water discharge or
a decrease in the sediment load can initiate channel degradation. These factors are often
altered by dams or channel diversions. A brief discussion of the effects of these features on
the channel stability follows.
Channel response to flow regulation may vary considerably depending upon the
purpose and manner of operation of the dam. Construction of a dam has a direct impact on
the downstream flow and sediment regime. Channel adjustments to the altered flow duration
and sediment loads include changes in the bed material (armoring), bed elevation, channel
width, plan form, and vegetation. Lane's balance (Figure 2.14) indicates that a reduction in
the discharge and sediment load, as might be expected downstream of a dam, tends to
produce counter-acting results. Consequently, the response of a channel system to dam
construction is extremely complex. The specific channel response will depend upon the
magnitude of changes in the flow duration and sediment loads and the existing channel regime
downstream of the dam. Therefore, channel response downstream of a dam is very complex
and may vary from stream to stream. Generally, the initial response downstream of a dam is
degradation of the channel bed close to the dam and sedimentation further downstream due
to increased supply from the degrading reach. This is the typical response most commonly
anticipated downstream of a dam. Degradation may migrate downstream with time, but
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