In addition to changes in planform location and size with time, the bed configuration of a river
can change with temperature, discharge, and concentration of silts and clays. At low flow or
with warm water the bed of a sand bed stream can be dunes, but at large flows or cold
temperature the bed may become plane or have antidune flow. With dunes, resistance to flow
is large and bed material transport is low. Whereas, with plane bed or antidune flow the
resistance to flow is small and the bed material transport is large. This topic is discussed in
detail in Chapter 3.
In summary, archaeological, botanical, geological, and geomorphic evidence supports the
conclusion that most rivers are subject to constant change as a normal part of their
morphologic evolution. Therefore, stable or static channels are the exception in nature.
1.2.2 Introduction to River Hydraulics and River Response
In the previous section, it was established that rivers are dynamic and respond to changing
environmental conditions. The direction and extent of the change depends on the forces
acting on the system. The mechanics of flow in rivers is a complex subject that requires
special study, which is unfortunately not included in basic courses of fluid mechanics. The
major complicating factors in river mechanics are: (a) the large number of interrelated variables
that can simultaneously respond to natural or imposed changes in a river system and (b) the
continual evolution of river channel patterns, channel geometry, bars and forms of bed
roughness with changing water and sediment discharge. In order to understand the responses
of a river to the actions of humans and nature, a few simple hydraulic and geomorphic
concepts are presented here.
River forms are broadly classified as straight, meandering, braided or some combination of
these classifications; but any changes that are imposed on a river may change its form. The
dependence of river sinuosity on the slope, which may be imposed independent of the other
river characteristics, is illustrated schematically in Figure 1.3. By changing the slope, it is
possible to change the river from a meandering one that is relatively tranquil and easy to
control to a braided one that varies rapidly with time, has high velocities, is subdivided by
sandbars and carries relatively large quantities of sediment. Such a change could be caused
by a natural or artificial cutoff. Conversely, it is possible that a slight decrease in slope could
change an unstable braided river into a meandering one.
The significantly different channel dimensions, shapes, and patterns associated with different
quantities of discharge and amounts of sediment load indicate that as these independent
variables change, major adjustments of channel morphology can be anticipated. Further, if
changes in sinuosity and meander wavelength as well as in width and depth are required to
compensate for a hydrologic change, then a long period of channel instability can be
envisioned with considerable bank erosion and lateral shifting of the channel before stability is
restored. The reaction of a channel to changes in discharge and sediment load may result in
channel dimension changes contrary to those indicated by many regime equations. For
example, it is conceivable that a decrease in discharge together with an increase in sediment
load could actuate a decrease in depth and an increase in width.
1.7
Previous Page
