|
Wave ripples are common in shallow-water environments and are found in sedimentary records. Ancient wave ripples tend to be preserved in shallow-marine settings near fair-weather wave base. In such setting, a wave group in nature is irregular. Thus, it should contain some components which are large enough to mobilize the bottom sediment and other less strong components. These weaker components probably provide hiatuses, during which the flow is not strong enough to mobilize the sediment, for the velocity near the bottom. Our hypothesis is that such velocity hiatuses play a role in sediment dynamics. To examine this hypothesis, the present study was designed.
The wave flume used in this study was 25 m long, 0.4 m wide and 1.2 m deep. The direction of wave propagation was designated as onshore, and the opposite direction as offshore. A sand bed (4.0 m long, 0.4 cm wide and 80 mm thick) was constructed using well-sorted quartz sand with grain diameter of 0.30 mm. A total of 26 runs under different wave conditions were performed. The velocity near the bottom was measured with an acoustic Doppler velocimeter. Three types of wave patterns were generated: two types for simulating waves with intervening velocity hiatuses; and regular waves for comparison purposes. In the former two types, wave patterns consisted of alternate long and short periods. These wave periods were employed in such a way that the wavelengths of the shorter periods were smaller than twice a given water depth and those of the longer periods were large enough to mobilize the bed. The former two types were designed to be different in sequence of convexity and concavity of wave patterns. The sequence with convex-concave longer wave and successive convex-concave shorter wave was described as zero-up-crossing wave pattern. The sequence with concave-convex longer wave and successive concave-convex shorter wave was called zero-down-crossing wave pattern. The convexity and concavity of wave surface provided respectively onshore and offshore flow for water movement near the bed. Under the zero-up-crossing waves, the direction of flow velocity near the bed turned from onshore and successively offshore to the hiatus, and back to onshore.
The ripples developed under oscillatory flow with intervening hiatuses manifested the following characteristics in geometry and migration. 1) The ripples formed under the zero-down-crossing waves had symmetrical or onshore-skewed crests, and migrated onshore faster than those under regular waves with similar energy. 2) The ripples formed under the zero-up-crossing waves had symmetrical or offshore-skewed crests, and migrated offshore. Our observation suggested that the direction of the flow immediately before onset of the hiatuses controlled the migrating direction and crest geometry of ripples. In short, when the flow direction before onset of velocity hiatuses is offshore, the direction of ripple migration trends offshore.
|
