Graduate Thesis Or Dissertation
 

Swash zone sediment suspension and transport

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5t34sm681

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  • Swash zone processes are of significant importance to foreshore morphologic change. This thesis addresses the importance of the uprush and backwash fluid motions and flow duration effects to the resulting sediment transport. The analysis relates the field observations to well known sediment transport formulations as well as investigates the importance of bore generated turbulence. The field study was conducted at Gleneden Beach, OR during February 25-28, 1994. Data were collected by continuously monitoring suspended sediment concentrations (SSC), sea surface elevations, and velocity (z=4 and 8 cm) at 3 crossshore locations within the swash zone spanning high tides. SSC and bed level fluctuations were monitored using a vertical stack of Fiber optic Optical Backscatter Sensors (FOBS) which penetrated the bed. Foreshore surveys and video run-up data were also collected. Results showed that the uprush suspension was high, concentrated in the leading edge and nearly vertically uniform over the water column. Shortly after the sensors were inundated by runup, the sediment rapidly settled out of the water column as observed at the sensor location. The observed settling was in part due to advection of the sediment laden uprush migrating passed the sensor. Through flow reversal, the SSC was small. SSC increased again in the backwash although vertical profiles were markedly different from uprush profiles with much of the suspension being confined to very near the bed where strong vertical gradients in SSC existed. The amount of backwash suspension appeared to be affected by flow duration. Suspended load transport comparisons with Bagnold-type energetics formulations did not yield strong relationships for instantaneous transport or total uprush or backwash transport. Suspended sediment transport in the leading edge of swash zone bores showed some dependence on the estimated energy dissipation across the bore front. The field observations and analysis supported the following uprush scenario: A surf zone bore migrates into shallow water and begins to influence local sediment transport processes, until the water depth in front of the bore approaches zero causing the bore to collapse. Bore turbulence and bore collapse dissipate energy, which may maintain sediment in suspension. The suspended sediment is advected with the uprush front as it travels up the beach face, but the ability to maintain the sediment in suspension decreases causing the sediment to settle out of the water column. near flow reversal.
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