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Laboratory Experiment on Flow Characteristics at a Beach Step
Magnus Larson (1), Tsuguo Sunamura (2)
Flow structure and sediment movement over the beach step, commonly present at the foot of the beach face, were studied in a laboratory wave tank. Two sets of experiments were made: a preliminary run using 2-s waves and a colored mixture of fine, medium, and coarse sand to observe details of sediment movement over the step and the beach face, and a set of runs with periods of 1.5 s, 2.0 s, and 2.5 s and coarse sand, in which the flow field was visualized and measured using small, neutrally buoyant suspended particles.
By the flow characteristics over the step, six distinct phases of the swash cycle were identified: (1) uprush of incident wave, (2) flow reversal, (3) backwash, (4) occurrence of critical flow, (5) development of a hydraulic jump, and (6) development of a backwash vortex. The time history of local flow velocity at the top of the step during a swash cycle has a saw-tooth shape, with maximum onshore and offshore speeds of the same magnitude. The velocity on the beach face was quite uniform through the water column during most of the swash cycle. The maximum Froude number over the step during the backwash was about 1.4. A backwash vortex formed after the onset of flow separation at the beach step during the final phase of the swash cycle.
The backwash vortex traps suspended fine sediment at the step, and the flow up the step slope induced by the vortex impedes avalanching of coarse sediment down the slope, facilitating high-angle sedimentation. Once caught by the vortex, most of the coarse sediment is eventually deposited either on the step or on the lower part of the beach face, and only a small amount may be deposited farther up on the beach face. The dip of the seaward-dipping stratification in the step (25-32°) is somewhat higher than in the field (about 20°), probably because of the presence of irregular waves in the field together with tidal fluctuation in water level, two factors not included in the experiments.
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