Bedload transport in an Oregon Coast Range stream Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s7526g30q

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  • Utilizing both vortex and Helley-Smith samplers, bedload measurements were obtained for three storms during the 1977-78 winter at Flynn Creek. Flynn Creek drains a 2 km^2 undisturbed watershed in the Coast Range of western Oregon. The highest peak flow observed for the two-year study, including both 1977 and 1978 water years, was 0.79m^3 s^-1 km^-2, and represents approximately an average annual peak flow based on fifteen previous years of streamflow data. Results of measurements obtained with the vortex sampler indicated that as streamflow increased bedload transport rate increased. Temporal variations in bedload transport rate also indicated that as the storm season progressed, bedload transport rates increased even though streamflow did not necessarily increase above the streamflow of the previous storm. This may be a result of some critical discharge being necessary to initiate bedload transport. The calculated critical discharge for Flynn Creek was 0.72 m^3 s^-1 km^-2 and was exceeded several times during the storm season. The peak bedload transport rate of 325 kg/hr occurred at a discharge of 0.77 m^3 s^-1 km^-2. Significant (p = 0.01) differences in bedload transport versus water discharge relationships occurred between storms. Helley-Smith bedload transport rates were consistently larger than vortex rates observed during all storms except on December 13, 1977, when the rates were approximately equal. The higher transport rates measured with the Helley-Smith sampler are attributed to sampler design and indicate that the sampler is more efficient for sand-sized material. Sand-sized particles represent the predominant particle size transported as bedload in Flynn Creek. Bedload transport did not occur uniformly with time, but rather occurred in "pulses" that did not appear to follow any pattern. Variations in stream cross sections and scour and fill measurements indicated that the release of bedload material occurred nonuniformly at many locations along the length of the stream channel. Information collected with cross-sectional measurements during 1977 indicated that the bed slope upstream of the fish trap may have been adjusting to the change in height of the streambed in the fish trap. Consequently, sediment data from different years should be compared with caution until it can be determined that the stream has come to equilibrium. On a seasonal basis, as the amount of organic matter in transport decreased, bedload transport rates generally increased.
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