Study Guide week 5 Geog 427 1999

Geog 427/527: Fluvial Geomorphology
Winter 1999, University of Oregon

Study Guide for Week 5, Feb. 2-4

Test 1 is coming, on Tuesday, Feb. 9! See the Week 6 study guide for information on the test.

I've been putting some web links for flvuial geomorphology on the Geog 4/527 Links page, and the Study Guide for Week 10. Check them out.

Tuesday, Feb. 2: Bed Morphology (con.), Channel pattern, Meandering channels

Last Thursday we finished Bed Morphology through sand bed channels and part of riffle-pool sequences. Today I will start again with riffle-pool sequences. The study questions for all of the Bed Morphology reading are in the study guide for week 4. In today's lecture I plan to cover bed morphology of riffle-pool sequences and step pool sequences, as well as the channel continuum and meandering channels. In lecture, I will integrate discussion of riffle-pool bed morphology with discussion of meanders (p. 213-230). On Thursday, Feb. 4 we will cover braided and anastomsing channels. Large woody debris in channels will be covered after Test 1, on Thursday Feb. 11.

p. 205-207: Introduction to channel pattern	Channel pattern is "a mode of channel form adjustment in the horizontal plane." What topics (that we have already covered) are adjustment in the transverse mode or plane? What topics are adjustment in the lengthwise mode? You should be able to explain, or give examples of, how a channel can increase or decrease resistance to flow in each of these planes. What are the major characteristics used for classifying channel pattern? What is the difference between braided and anastomosing channels? Fig. 5.16 is very useful in showing different channel patterns, and the controls of channel pattern (left side of figure). You should know how sinuosity is calculated.
p. 207-213: The Continuum Concept	What is the continuum concept? Slope and discharge were originally considered the two most important variables for explaining channel pattern. Why slope and discharge? What other important variable(s) are slope and discharge related to? Note that on p. 208 Knighton introduces the term (and the equation for) specific stream power. Specific stream power is equivalent to unit stream power, which we discussed on Jan. 14. Fig. 5.17A shows a threshold line (or boundary line or envelope line), which is also given as an equation (equation 5.40). How is a threshold line different from a regression line (i.e., Fig. 5.14 and its equation; Fig. 5.10A and C; Fig. 5.8A, B and C)? You should know how to tell is a line is a regression line or a threshold line from its data plot. Note that Fig. 5.17B shows several threshold lines. For the same reference discharge, which type of channel has a steeper slope, braided or meandering? For the same slope, which type of channel is associated with greater discharge, braided or meandering? What is the order of anastomosing, braided, meandering and straight channels along the "flow strength continuum" or stream power continuum? What type(s) of channel is associated with high silt and clay content in the channel banks, and why? What other characteristic(s) might influence channel pattern in the same way as silt and clay content? In this discussion, Knighton lists a number of variables that influence channel pattern: slope, discharge, Froude number, channel width, channel width:depth ratio, %silt+clay, bed material size, bed material load (Qsb), and maybe some others. Which one to three of these do you think is most important? Why? What is significant about channels that lie close to the threshold line on one of the figures that differentiate among channel patterns?
p. 213-217: Meander geometry	What is the difference between regular and irregular meanders? Between confined and unconfined meanders? Between tortuous and non-tortuous meanders? What is meander wavelength? What is meander radius of curvature? Meanders have consistent form, scaled to channel width, over a wide variety of channels. The consistent form is described by the two relationships discussed on p. 215, l = 10w to 14w, and r_c = 2w to 3w. From Fig. 5.18B, you can see that one wavelength includes two meanders (one to the right and one to the left). How many pools would you expect to find in one wavelength? How many riffles? What pool spacing is implied by the relationship l = 10w to 14w, and is this consistent with the pool spacing discussed in the Bed Morphology section of Knighton? Table 5.9 lists a number of different reference discharges that have been related to meander geometry. Compare these reference discharges to the discussion of dominant discharge in Knighton, p. 162-167.
p. 217-225: Theories of meander development	The goal of meander geometry analysis is to quantitatively describe meander patterns in ways that allow testing of hypotheses about meander development. The goal is to answer questions such as: What is the underlying process that gives meanders their regular form? Why do meanders have the specific form that they do? When meander form is less regular (i.e., more than one dominant wavelength), why? In this section, you don’t need to concentrate on the methods of analyzing meander geometry. Instead, focus on explanations of meander geometry. What is a secondary current? Understand the typical 3-D pattern of flow through meander bends, as described on p. 217 and in Fig. 5.19. Compare Fig. 5.19 with Fig. 5.15 to see the similarities. How does this pattern of flow relate to formation of meander bends? To the location of pools? Look at Fig. 5.19A: where does maximum shear stress occur? Knighton presents a series of different ideas about how and why meanders and their associated features (cut banks, pools, point bars, etc.) develop: 3-D flow structure (helicoidal flow); macroturbulent flow and bursts; bar theory; unified bar-bend theory; minimum stream power; minimum variance. Read through Knighton’s discussion of these theories to get a sense of how each theory explains meander development.
p. 225-230: Meander instability and lateral migration	How do meanders migrate? What is the difference between translation and extension? What is cutoff? Knighton discusses several valuable points in this section: development of asymmetrical or irregular bends; the role of curvature in influencing rates of lateral migration; the cutoff process. Read through this section to understand the major issues.

Thursday, Feb. 4: Braided and anabranching channels

Note that a new book is now on reserve for this course in the Science Library:
R. J. Naiman, R. E. Bilby, and S. Kantor, eds., 1998, River Ecology and Management : Lessons from the Pacific Coastal Ecoregion. Sci QH 104.5 .P32 R57 1998.
I will probably not assign any readings from this book, but it has chapters on stream classification, hydrology, geomorphological processes and dynamics, and other topics that you might find of interest.

p. 230-236: Braided channels

What is a braided channel (as opposed to other channel types)?
What is the braiding index? How does braiding index (or degree of braiding) vary with slope?
What are the four characteristics (controls) that Knighton discusses as associated with braiding? Which of the four controls seem to have the strongest association with braiding, and which have questionable association with braiding?
What is the difference between lateral bars and mid-channel bars? Which are strongly associated with braided channels?
How do mid-channel bars form? (Fig. 5.23 is useful.)
What is convergence? Divergence? How do these two processes relate to scour, bed morphology and bar growth?
How is stream power related to braiding?

p. 236-241: Anabanching channels

What is avulsion?
What is the persistence of an island in an anabranching channel, compared to a bar in a braided channel?
What is the typical sinuosity of individual channels in an anabranching river?
What criteria did Nanson and Knighton (1996) use to define types of anabranching channels?
Sub-types of anabranching rivers: What is an anastomosing river? What is a wandering river?
You should review the six types of anabranching rivers to understand their major differences in controls and in behavior.
What does Knighton mention as the major controls that create anabranching rivers in general (not the specific types)?

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last update: 03/25/00 11:06 AM
Department of Geography, University of Oregon, Eugene OR, 97403-1251