We are one class meeting behind the lecture schedule that is in the syllabus.
| p. 151-153: Introduction to Ch. 5, channel classification |
This section provides a brief introduction to channel classification, a
topic that we will cover in more depth later. The value of this section is to illustrate
the range of channel morphology and to discuss the important channel morphologic
characteristics and channel morphology controls that have been used in classification
systems. Schumm’s (1963) system is an important early channel classification
approach. The classification based on channel boundary material shown in Table 5.1 is a
basic approach that is widely used, informally, in practice. The Rosgen system (Fig. 5.1)
is a highly detailed classification system that is currently being widely adopted by U. S.
land management agencies. It also has been strongly criticized by many geomorphologists.
We’ll cover the Rosgen system and the Montgomery and Buffington system (not presented
in Knighton) in more detail in a couple of weeks. |
| p. 153-158: Introduction to Characteristics of Adjustment |
- What are the independent controls (external controls) of channel morphology, and what is
the difference between independent controls and internal variables of channel morphology?
- Over what time scales are discharge and sediment load relatively constant (therefore we
should expect channels to reach an equilibrium with these controls)? Are all independent
controls constant at these time scales?
Among internal variables, what are the flow variables and what are the channel geometry
variables?
- What is meant by the concept that stream channels are indeterminate?
- What are the basic hydraulic geometry equations (the first three equations)? What
relationship links together the channel geometry variables in the hydraulic geometry
equations?
- What are the four components of adjustment (i.e., "degrees of freedom") in
channel morphology?
- Fig. 5.3 is a very useful diagram in showing the relative time scales of persistence (or
adjustment) of the various components of adjustment. You should have a general sense of
which channel variables adjust frequently and which adjust over longer time scales.
|
| p. 158-162: Concept of Equilibrium |
- We won't cover this section in detail, but you should read it to get the important
points. It will be a useful reference section if you go on to work in fluvial
geomorphology. Here are some specific points to get from the reading:
- Why does Knighton say that "true equilibrium never exists" and that the
equilibrium form is "recognizable as statistical averages" in channel
morphology?
- Is the principle that controls and determines stream equilibrium known? What kinds of
hypotheses have been proposed as explanations for equilibrium?
- What is the difference between equilibrium, disequilibrium and non-equilibrium? What are
some specific terms that could be used as the vertical axis in Fig. 5.5?
|
| p. 162-167: Concept of Dominant Discharge |
- What is meant by "dominant discharge"?
- In the 1950s and 60s, it was thought that 1) bankfull discharge, 2) the most effective
flow (flow that cumulatively transports the most sediment load), and 3) the flow that
shapes the channel in cross-section and planform were about the same. What was the typical
recurrence interval for this flow? What are some of the problems with this simple concept
of dominant discharge? Has the concept of "dominant discharge = bankfull
discharge" been abandoned?
- Be sure to look at Figure 5.6A to understand what is meant by "flow that
cumulatively transports the most sediment load."
|
| p. 167-171: Equilibrium cross section |
- We won't this section in detail, but you should read it to get the important points.
It will be a useful reference section if you go on to work in fluvial
geomorphology. Here are some specific points to get from the reading:
- Table 5.3 is a good summary of the cross section morphologic variables.
- How is cross section morphology compared among sites of different drainage areas (i.e.,
going downstream)?
- What is the goal of equilibrium cross section analysis? What are the two main approaches
in equilibrium cross section analysis, and how are they different?
- What is regime theory in fluvial geomorphology?
- Which approach is more successful and more widely used?
|
| p. 171-177: Spatial Variation – downstream |
- For this and the following sections on hydraulic geometry, you might start by reading
the Leopold and Maddock reading in your reading packet. If you have never studied
hydraulic geometry relationships before, you may find that this first publication on hg
lays out the basics of this approach more clearly than the Knighton reading.
- What is a downstream hydraulic geometry relationship? How are these relationships
typically portrayed?
- At what spatial scale is downstream hydraulic geometry typically analyzed? (For example,
approximately how far apart would cross sections typically be placed?)
- Do all channels have the same downstream hydraulic geometry relationships? What factors
(external controls) result in different downstream hydraulic geometry relationships?
|
| p. 177-180: Spatial Variation – local |
- How is cross section form different between pools and riffles?
- How would at a station hydraulic geometry relationships be different for pools and
riffles?
|
| p. 180-186: At-a station Hydraulic Geometry |
- What is an at-a-station hydraulic geometry relationship? (They are portrayed in the same
way as downstream hydraulic geometry relationships.)
- Do all cross sections have the same at-a-station hydraulic geometry relationships?
- What factors result in different at-a-station hydraulic geometry relationships?
|
| p. 186-187: Adjustability |
- This section returns to a theme introduced on p. 153-158: how rapidly do channels adjust
their cross section form? Several informative examples are presented.
|
| p. 187-189: Introduction to bed configuration |
- Table 5.8 is an important and useful summary of bedforms.
- What is the basic classification for small-scale bedforms of gravel-bedded streams?
- What are the large-scale bedforms of gravel-bedded streams?
|
| p. 189-193: Ripples, dunes, and anti-dunes |
- What kind of bed material do ripples, dunes and anti-dunes form in? How do the bedforms
in this sequence relate to variation in shear stress and Froude number?
- What is wavelength of a bedform?
- What is Yalin’s explanation for dune wavelength? What is a burst? Understand Fig.
5.12.
- What is the difference between an upper flow regime plane bed and a lower flow regime
plane bed?
- After they form, are ripples, dunes and anti-dunes fixed in position on the channel bed?
If not, how do they move?
- How do bedforms in this sequence relate to resistance (or roughness)?
At a given cross-section, will you always observe the same bedforms?
|
| p. 193-201: The riffle-pool sequence |
- What are riffles? What are pools? What kinds of bed material do riffle-pool
sequences form in? How are the riffle and the pool related to channel planform?
- What is the typical pool spacing in riffle-pool sequences?
- How is the bed material size of riffles different from that of pools? New results
(Clifford 1993a, Sear 1996) show that the sediment structure (fabric) is different between
pools and riffles – how?
- At low flow, how is depth, velocity and water surface slope different in a riffle as
compared to in a pool? How are riffles and pools different at high flows (i.e.
bankfull flow) in these flow characteristics?
- It is well documented that under a broad range of conditions, channels with initial
plane beds will develop riffle-pool sequences. Riffle-pool sequences are remarkably
regular in spacing from one stream to another. The formation of riffle-pool sequences
requires scour and deposition to be occurring at specific, regularly-spaced locations on
the bed of the channel. A number of different theories to explain why riffle-pool
sequences form, and to explain the differences in sediment size between riffles and pools,
have been proposed. These theories or models include: the velocity-reversal hypothesis (or
competence reversal hypothesis or shear stress reversal hypothesis); explanations related
to 3-dimensional flow structure (as illustrated in Fig. 5.15); the kinematic wave
explanation; explanations related to macroturbulent flow structures (i.e., bursts or
roller eddies); and several others. Read through this section and get a feel for
what these different theories represent. What does Knighton say is the explanation
for riffle-pool sequences?
|
| p. 201-205: Step-pool bedforms |
- What kinds of streams develop step-pool sequences? How are these streams different from
streams discussed in the previous sections?
- What are the typical slopes of step-pool streams?
- What do steps and the pools associated with them look like, and how are they different
from riffle-pool sequences? What kind of sediment is found in pool of step-pool sequences?
- What is the typical spacing or wavelength between steps, and how does it compare to the
typical spacing or wavelength of riffle-pool sequences?
- Within the range of step-pool channels, how does step-pool morphology vary as slope
increases?
- How would you expect typical roughness or resistance values to compare in riffle-pool
versus step-pool streams?
- What is the typical flow regime of step-pool streams?
- What is the theory of Abrahams et al (1995) about controls of equilbrium in step-pool
streams? Note that this is a specific example of an equilibrium theory as discussed on p.
158-162.
- How stable are step-pool structures? How frequently is the channel bed mobilized and
reworked in step-pool streams? How does this frequency compare to that for riffle-pool
streams and ripple-dune streams?
|
| p. 205: Synthesis |
- What are the common themes that link these different bed morphologies?
|