ORTHOGONAL WAVE CURRENT INTERACTION OVER A FIXED RIPPLED BED: PRELIMINARY RESULTS OF AN EXPERIMENTAL CAMPAIGN

This paper reports some results obtained in the framework of the TA WINGS, funded by the EU through the Hydralab+ program. The project was aimed at investigating the hydrodynamic effects of an orthogonal wave onto a current in order to understand the nature of the velocity distribution along the water column over fixed ripples. Velocity profiles were acquired within the DHI shallow water tank by means of several Vectrinos in order to investigate the effects of wave-current interaction. A comparison between wave only case and wave plus current case showed that, when the current overlaps to waves, the recirculating cells formed near the bed are flatter than those formed in wave only. Moreover, the vorticity increases outside the boundary layer and decreases inside it when the current superposes to the wave.

Wave-Current Flow and Vorticity Close to a Fixed Rippled Bed

An experimental study of wave and current interaction over ripples is presented in this paper. The campaign was carried out at the shallow water tank at the Danish Hydraulic Institute (DHI, Denmark), in the framework of the TA WINGS (Waves plus currents INteracting at a right anGle over rough bedS), funded by the European Union (EU) through the Hydralab+ program. Mean velocity profiles, measured with acoustic Doppler velocimeters for different flow conditions including current only, wave only and wave plus current were recorded and allowed to recover flow and vorticity fields. Recirculating cells in both wave only and wave plus current conditions form but they flatten when the current superposes over the wave. It was found that the superposition of current reduces the undertow present in the case of only waves and leads to an increase of vorticity outside the boundary layer. Instead, inside the boundary layer, the vorticity is dumped by the effect of current.

Steady Streaming Induced by Asymmetric Oscillatory Flows over a Rippled Bed

The flow induced by progressive water waves propagating over a rippled bed is reproduced by means of the numerical solution of momentum and continuity equations to gain insights on the steady streaming induced in the bottom boundary layer. When the pressure gradient that drives the flow is given by the sum of two harmonic components an offshore steady streaming is generated within the boundary layer which persists in the irrotational region. This steady streaming depends on the Reynolds number and on the geometrical characteristics of the ripples. Nothwithstanding the presence of a steady velocity component, the time-average of the force on the ripples vanishes.

Wave-Induced Oscillatory Flow Over a Sloping Rippled Bed

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.

NUMERICAL INVESTIGATIONS OF THE MECHANISMS ASSOCIATED WITH THE ONSHORE RIPPLE MIGRATION

Quantification of cross-shore sediment transport is one of most intriguing challenges in shoreline and coastal geomorphology. During the past decades, several key mechanisms associated with onshore/offshore sediment transport have been identified, such as wave skewness/asymmetry, progressive wave streaming and undertow current. However, applying these mechanisms to the migration of wave formed bedforms (ripples) is not straightforward. For example, recent field observations off Fire Island, NY showed that ripples migrated onshore even during periods of offshore directed wave skewness, which is contradictory to the prediction of empirical sediment transport formulations. The physical processes driving ripple vortex formation, ejection and boundary layer streaming associated with rippled bed can further complicate the bedload/suspended load sediment transport over ripples. To fully understand these mechanisms, a comprehensive model that can resolve the ripple dynamics and interactions between free surface wave and rippled bed is examined.