Abstract
In this paper, the findings of an experimental analysis aimed at investigating the flow generated by waves propagating over a fixed rippled bed within a wave flume are reported. The bottom of the wave flume was constituted by horizontal part followed by a 1:10 sloping beach. Bedforms were generated in a previous campaign performed with loose sand, and then hardened by means of thin layers of concrete. The flow was acquired through a Vectrino Profiler along two different ripples, one located in the horizontal part of the bed and the second over the sloping beach. It was observed that, on the horizontal bed, near the bottom, ripple lee side triggered the appearance of an onshore directed steady streaming, whereas ripple stoss side gave rise to an offshore directed steady streaming. On the sloping bed, a strong return current appears at all positions, interacting with the rippled bottom. The turbulence is non-negligible within the investigated water depth, particularly when velocities were onshore directed, due to flow asymmetry. Turbulence caused a considerable flow stirring which, above a non-cohesive bed, could lift the sediment up in the water column and give rise to a strong sediment transport.
Highlights
In coastal regions, waves propagating over a slope may become skewed, with sharp crests and flat troughs, and asymmetric, with steep fronts and gentle rear faces
On the horizontal bed, near the bottom, ripple lee side triggered the appearance of an onshore directed steady streaming, whereas ripple stoss side gave rise to an offshore directed steady streaming
Asymmetry consists in a difference between the time derivative of the velocity during the two half cycle, it is referred as acceleration skewness [1]
Summary
Waves propagating over a slope may become skewed, with sharp crests and flat troughs, and asymmetric, with steep fronts and gentle rear faces. In the presence of sandy beds, the interaction of asymmetrical and/or skewed waves with the bottom can lead to the appearance of bedforms that may exhibit a degree of asymmetry and this in turn may affect the flow [8] Due to their relevance for coastal hydro-morphodynamics, a large number of studies investigated ripple formation and evolution (e.g., [9,10]). Asymmetric bedforms were studied theoretically by Blondeaux et al [15] through a weakly nonlinear stability analysis They found that, depending on wave and sediment characteristics, a flat sandy bottom subjected to an oscillatory flow may lead to asymmetric ripples with the onshore flank of the ripples steeper than the offshore flank.
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