Quantcast Downstream Factors

Fundamentals of Fluvial Geomorphology and Channel Processes
consequences and receive considerable media attention, the most commonly encountered
system instability problems can generally be attributed, at least in part, to man's activities.
Any time one or more of the controlling variables (runoff, sediment loads, sediment
size, channel slope, etc.) in a watershed are altered there is a potential for inducing system
instability. The particular system response will reflect the magnitude of change and the
existing morphological sensitivity of the system. Therefore, each system is unique and there
is no standard response that applies to all situations. With this in mind it is not practical to
attempt to discuss all the possible scenarios of channel response. Rather, the aim of this
discussion is to present some of the more common factors causing system instability, and to
illustrate how a particular channel response might be anticipated using the stability concepts
discussed earlier.
A list and brief discussion of some of the more common causes of system instability are
presented in the following sections. For this discussion the causes have been grouped into
three categories: (1) downstream factors, (2) upstream factors, and (3) basin-wide factors.
Following this, a brief discussion is presented concerning complex response and the
complications involved when a system is subjected to multiple factors.
Downstream Factors. The stability of a channel system can be significantly affected
by a downstream base level lowering. Base level refers to the downstream controlling water
surface or bed elevation for a stream. One of the most common causes of base level lowering
is the implementation of cutoffs or channelization as part of channel improvement projects
(Figure 2.16). As indicated by Lane's relation (Figure 2.14) the increased slope must be offset
by one of the other variables. Consequently, there is an imbalance between the sediment
transport capacity and supply. If the discharge and bed material are assumed to remain
constant (which may not always be the case), then the channel must adjust to the increased
slope (i.e., sediment transport capacity) by increasing its bed material load. This increased
sediment load will be derived from the bed and banks of the channel in the form of channel
degradation and bank erosion. As the bed continues to degrade the zone of increased slope
will migrate upstream and the increased bed material load is transmitted downstream to drive
aggradational instability there.
The manner in which degradation migrates through a channel system is a very complex
process. Before this process is discussed some of the relevant terminology must first be
addressed. The following definition of terms is based on the terminology used by Schumm
et al. (1984). Channel degradation simply refers to the lowering of the channel bed. Field
indicators of degradation occur in the form of knickpoints or knickzones. A knickpoint
is a location on the thalweg of an abrupt change of elevation and slope (Figure 2.17). This
may also be visualized as a waterfall or vertical discontinuity in the stream bed. A steep reach
of channel representing the headward migrating zone is referred to as a knickzone (Figure
2.18). A knickzone is often composed of a series of small knickpoints. Knickpoints and
knickzones are often referred to as headcuts. While headcut is a commonly used term, it does
generate some confusion because it is also used as a description of the headward migration
process of degradation.


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