| Summary: | 博士 === 國立中央大學 === 土木工程研究所 === 92 === Field experiments were conducted to investigate the wave, current and sediment suspension across the surf zone by deploying wave gauges, 3D-ACM current meters and OBS turbidity meters at Taichung and Ampin, in the west coast of Taiwan. A synchronized video camera was also installed underwater to observe sheet flows and sediment movement on ripple beds.
Generation of sand drift in surf zone is mainly due to energy released from shoaling waves. Wave groups in approaching waves and surf beats usually imply energy undulation of low frequencies, which may affect the movement of bed sediments. A new approach using Hilbert transform and Intrinsic Mode Functions analysis to explore the effect of energy grouping and undulating on sediment suspension is presented.
An interesting bursting phenomenon of near-bed sediment concentration was found dominating in surf zone, which was closely related to incident wave groups. Peak concentrations of suspended sediment associated with wave groups were 2~3 times larger than that without wave groups.
It is found the main mechanisms of sand drift in the surf zone are sheet flows and sediment suspension. The criterion for inception of sheet flow at depth of 1.7m, ripple height of 5.5cm and sand of 0.27mm in diameter, is about 40cm/s in current speed. The suspension load, bed load and sheet flows are found all responsible for the sand drifts in surf zone. This study is far from being complete but should provide readers a good insight on sediment transports in surf zone. It needs further observations to clarify the amount of sand actually moved by sheet flow alone.
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