The incipient motion of sediment particles in gravel-bed
rivers is a very important process. It represents the
difference between bed stability and bed mobility. A field
study was conducted in Oak Creek, Oregon to investigate
incipient motion of individual particles in gravel-bed
streams. Investigation was also made of the incipient motion
of individual gravel particles in the armor layer, using
painted gravel placed on the bed of the stream and recovered
after successive high flows. The effect of gravel particle
shape was examined for a wide range of flow conditions to
determine its significance on incipient motion.
The result of analysis indicates a wide variation in
particle shapes present. Incipient motion and general
transport were found to be generally independent of particle
shape regardless of particle sizes.
A sample of bed material may contain a mixture of shapes
such as well-rounded, oval, flat, disc-like, pencil-shaped,
angular, and block-like. These are not likely to move in
identical manners during transport nor to start motion at the
same flow condition. This leads to questions about the role
of shape in predicting incipient motion and equal mobility in
gravel-bed streams.
The study suggests that gravel particles initiate motion
in a manner that is independent of particle shape. One
explanation may be that for a natural bed surface many
particles rest in orientations that give them the best:
protection against disturbance, probably a result of their
coming to rest gradually during a period of decreasing flows,
rather than being randomly dumped. But even when tracer
particles were placed randomly in the bed surface there was no
evident selectively for initiation of motion on the basis of
particle shape.
It can be concluded from analysis based on the methods of
Parker et al. and Komar that there is room for both equal
mobility and flow-competence evaluations. However, the equal
mobility concept is best applied for conditions near incipient
motion and the flow-competence concept is best applied for
larger flows and general bedload transport. Furthermore, with
an armored bed, such as that at Oak Creek, there is a tendency
for a more-nearly equal mobility (or equivalent) for the
normalized transport rates for the various size fractions when
incipient motion and moderate bedload transport occurs. / Graduation date: 1995
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/35018 |
| Date | 12 December 1994 |
| Creators | Matin, Habib |
| Contributors | Klingeman, Peter C. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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