Fundamentals of Fluvial Geomorphology and Channel Processes
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 3.19)
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, planform, and vegetation. Lane's balance (Figure
3.19) 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 generally it is most significant during the first few years following closure of the dam. In some
situations, a channel may shift from a degradational to an aggradational phase in response to slope flattening
due to degradation, increased sediment inputs from tributaries and bed and bank erosion, and reduction
in the dominant discharge.
System instability can also be introduced by the diversion of water into or out of the stream.
Channel diversion structures are designed to divert a portion of the water and/or sediment from a
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