Geog 427/527: Fluvial Geomorphology
Winter 1999, University of Oregon
Study Guide for Week 2, Jan. 12-14
Notes on Exercise 1: The station number for Grande Ronde at Troy was incorrect on the sign-up sheet. Go to the sign-up list on the Exercise page to get the correct number. The N. Fork Alsea R. at Alsea does not have data for WY 1990; use WY 1989 instead. If you can't locate your data on the USGS web site, e-mail Suzanne or me.
Tuesday, Jan. 12: Mechanics of flow (con.)
On Thursday, Jan. 7, we started mechanics of flow and completed through steady vs. unsteady and uniform vs. non-uniform flow. On Tuesday, Jan. 12 we will start with laminar vs. turbulent flow.
Lecture outline for Tuesday Jan. 12:
I. Laminar vs. turbulent flow, significance of turbulence, Reynolds number (p. 96-99)
II. Tranquil vs. rapid flow, energy in streamflow, Froude number (p. 96-99)
III. Equations of flow and resistance (p. 99-105)
IV. Stream power (p. 105-107)
The study questions for this material are included in the Study Guide for Week 1.
Thursday, Jan. 14: Thresholds of erosion; bank erosion
Your annual hydrograph from Part B of Exercise 1 is due at the beginning of class today!
On Tuesday, Jan. 12, I finished through tranquil vs. rapid flow and the Froude number. Today I'll start with equations of flow and resistance (see study guide above), then finish through entrainment (study guide below).
Exercise 2 is coming soon! Maybe it'll be posted on the web by Friday....
| p. 107-109: Basic concepts of entrainment and bed erosion |
What is the definition of boundary shear stress? What is the equation for
boundary shear stress? What is the definition of critical shear stress, and what is the difference between boundary shear stress and critical shear stress? What is the equation for critical shear stress (equation 4.9a)? According to equation 4.9a, what term is most important in determining the amount of shear stress needed to entrain a grain? What forces are acting on a grain at rest to entrain it? (Fig. 4.4B) What important aspect of shear stress is not included in equation 4.9a? |
| p. 109-113: Predicting entrainment |
The basic questions of sediment entrainment are 1) to determine whether
entrainment will or will not occur for given flow conditions and given sediment
characteristics; or 2) to determine the highest flow conditions (threshold or critical
flow) that a given channel with given sediment characteristics can withstand without
entrainment of the bed material. We’ll briefly discuss several approaches commonly
used to solve sediment entrainment problems: threshold velocity approach (Hjulstrom curve,
Fig. 4.5B); Shields criterion (equation 4.9b, Figure 4.5A); Williams approach (Fig. 4.5C);
and Komar’s selective entrainment function. Read through equation 4.9b to understand
the terms, and examine Fig. 4.5. Additional material will be presented in lecture. What is dimensionless shear stress, and how is it used to predict whether or not entrainment will occur? What was Shield’s original value for q (or k), and what are some later proposals for the value of q ? Why isn’t there a single value for q ? What are the factors that influence whether or not a grain will be entrained? What is the hypothesis of equal mobility? Is this idea included within equation 4.9a? How is entrainment of cohesive material different from entrainment of non-cohesive material? What are the three processes of entrainment of cohesive material? |
| p. 113-118: Bank erosion: |
We won’t get to this on Jan. 14; we’ll do this on Jan. 19. |
Here is the table I showed in lecture on Jan. 14. This table is based on Bathurst, J.C., 1997, Environmental flow hydraulics. In Thorne, C.R. et al., eds., Applied Fluvial Geomorphology for River Engineering and Management, p. 69-93.
| Channel types | Relative submergence, d/D50 |
Primary source of flow resistance |
| sand bed | 1000 or greater | varying bedforms |
| gravel bed | 5 - 100 | grain resistance; backwater effect of pools |
| boulder bed | < 1 | grain resistance, including surface distortion by protruding boulders |
| step pool/fall | < 1 | backwater effects; hydraulic jumps below falls; reduced during high flows |
last update: 03/25/00 11:06 AM
Department of Geography, University of Oregon, Eugene OR, 97403-1251