Tidal influence on downstream fining in gravel-bed rivers Public Deposited

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  • This research examines the downstream fining phenomenon as it operates in coastal gravel-bed rivers of Oregon. Downstream fining is a change in bed composition toward smaller sediment sizes in the downstream direction. Changes in stream flow discharge and channel slope affect the rate of bed-load transport, thereby altering the downstream fining regime. This research focuses on ascertaining the rate of downstream fining and the characterization of tidal influence on bed-load transport in the lower-river reaches. For this purpose, a combination of physical and numerical analysis techniques were used. Variations of particle size distributions and specific gravity values were assessed along the main channel. Numerical analysis techniques included a MATLAB program for simulating bed-load transport as affected by tide. The numerical model developed for this investigation, TIMM (Tidally Influenced Movement Model) uses physically based excess shear stress as the underlying mechanism. Namely, an undulating water surface is applied to Shields criterion for incipient motion and bed-load transport. The Generalized Stream Tube model for Alluvial River Simulation version 2.1 (GSTARS 2.1), developed by the Bureau of Reclamation, was used to validate conclusions drawn from field data analyses. The five rivers of the Tillamook Basin were the sites of field data collection. The Tillamook Basin is located approximately 60 miles (96.6 kilometers) west of Portland, Oregon and 60 miles (96.6 kilometers) south of the Columbia River mouth at the Pacific Ocean. The basin has a total area of 570 square miles (1476 square kilometers) including Tillamook Bay, which is the second largest estuary in Oregon. All rivers empty into the Tillamook Bay. From north to south, the rivers are the Miami, Kilchis, Wilson, Trask and Tillamook. The Kilchis River was the primary field research site and the other four rivers allowed expansion of field research for added understanding of downstream fining. Bulk sampling of the armor and sub-armor layer of the Kilchis River was completed for five sidebars along the river, from river mile 0 to river mile 14 (0 - 22.5 km). Photo frame sampling was carried out for the armor layer of sidebars along the four additional rivers. In total, 21 sampling locations with 141 individual sampling points were used for the particle size analyses. Assessment of longitudinal variation in specific gravity of bed particles by size fraction was performed for all five rivers. Particle size analyses showed a distinct downstream fining trend. Kilchis River surface particle sizes decreased from 216 mm at river mile 14 (22.5 km) to 10 mm at river mile 0. Miami River surface particle sizes decreased from 43 mm at river mile 9 to 29 mm at river mile 1.5 (2.4 km). Wilson River surface particle sizes decreased from 51 mm at river mile 27 to 23 mm at river mile 0. Trask River surface particle sizes decreased from 55 mm at river mile 18 to 26 mm at river mile 4 (6.4 km). Diminution coefficients (rates of size reduction) were found to be 0.55 km⁻¹ for the armor layer and 0.48 km⁻¹ for the sub-armor layer of the Kilchis River. The R-squared values for the armor and sub-armor coefficients are 0.92 and 0.99, respectively. Results of regression analyses performed for the photo frame sampling data were 0.02, 0.03, and 0.04 km⁻¹ for the Miami, Wilson, and Trask Rivers, respectively. R-squared values of 0.19, 0.78, and 0.81, respectively. Diminution coefficients reported for all rivers were far outside the value reported for abrasion-dominated systems (0.089 km⁻¹), yet were within the range of diminution coefficients reported for selective sorting-dominated systems (0.001 to 0.05 km⁻¹). Average specific gravities for bed material were 2.78, 2.68, 2.73, 2.56, and 2.76 for the Miami, Kilchis, Wilson, Trask, and Tillamook Rivers, respectively. Simulations of sediment transport within the tidal portion of the Kilchis River (river mile 0 to 3 or 0 to 4.8 km) using TIMM at moderate river streamflow above the threshold for transport of material showed that tidal influence causes distinct deposition zones during periods of high, low, and moderate tide levels. Depositional zones were found to propagate downstream with increases in river discharge, such that at elevated river stage the location of depositional zones associated with tide levels were undistinguishable. It can be concluded that tide has a significant influence at flows below, and moderately above the threshold for transport. Simulations of a simplified version of the Kilchis River using GSTARS 2.1 produced comparable results to the TIMM simulations. GSTARS 2.1 was run using three scenarios, 1) a uniform bed and incoming sediment supply set at 7.9 mm, 2) a mixed bed with mean sediment diameter of 7.9 mm and coarser incoming sediment supply, and 3) a mixed bed with mean sediment diameter of 7.9 mm and finer incoming sediment supply. Each scenario had output data that show maximum deposition in the zone of tidal influence. The location of head of tide for the simplified Kilchis River was found to occur at river mile 5 instead of river mile 3 used for the head of tide in TIMM simulations. GSTARS 2.1 simulations showed that variations in particle size distribution of incoming sediment supply influence rates of downstream fining. An incoming sediment supply that had a coarser particle size distribution than the particle size distribution of the bed resulted in an observable increase in deposition of large particle sizes in the upstream reaches; however, there was no observable increase in deposition of large particle sizes in downstream reaches. An incoming sediment supply that had a finer particle size distribution than the particle size distribution of the bed resulted in an observable increase in deposition of smaller particles in the downstream reaches, with no observable increase in deposition of smaller sizes in the upstream reaches. Therefore, simulations show evidence that sediment supply of particles coarser than the bed causes increased rates of fining in reaches near the sediment source. Key contributions of this research are in the categories of methodology, numerical analysis, and basic understanding of the fate and transport of sediment in the zone of tidal influence. It has been shown that particle size data, collected in detail on sidebars, can be used in conjunction with specific gravity data to categorize in-stream particles based on probable origin and type. Characterization of sediment transport in the zone of tidal influence using numerical models showed the tide cycle influences the downstream fining trend in lower reaches by shifting the zone of deposition farther upstream than would the case without tidal influence, with a net effect of increasing the rate of downstream fining. Moreover, tidal influence was found to have an inverse relationship with water discharge. Finally, it was shown that numerical modeling of river reaches in the tidal zone should include consideration of tidal fluctuations in order to predict erosion and depositional areas more accurately.
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