Investigations of wave-induced turbulent structures in vegetated flows

Abstract

Listen

Translate article

The role of coastal vegetation in mitigating shoreline erosion by damping of incoming waves and the resultant effect on sediment transport is a critical area of coastal management research. However our understanding of the underlying hydrodynamic processes is limited. Laboratory flume experiments were conducted where a naturally grown emergent vegetation channel and a vegetation-less sand channel were exposed to regular waves. Wave height data was collected using wave gauges located at various points along the channel in the direction of oncoming waves. Acoustic Doppler Velocimeters (ADVs) placed at various depths in the water column measured wave orbital velocity signatures. This thesis investigates the wave-damping phenomenon of the vegetation and attempts to quantify it using the wave attenuation factor. Variation of plant drag coefficient with distance in the vegetation field as well as dependence with Reynolds number, Keulegan Carpenter number and the Viscous Frequency Parameter â are investigated. Horizontal and vertical orbital velocity signals from ADV measurements were analyzed and variation of turbulent kinetic energy with depth has been presented. Comparisons with Linear and Stokes Wave Theory predicted values, calculated using wave height data from wave gauges, were made to understand the turbulence generated by the vegetation bed under wave action. Frequency analysis of the power spectra of wave orbital velocities was used to separate the wave and turbulent portions from the total component of the velocity and has been separately studied to understand the depth variation of the turbulent structures. Wave attenuation factor decreased with increasing distance in the vegetation field while the drag coefficient remained almost constant after a couple of meters. The drag coefficient decreased with both Reynolds number and Keulegan Carpenter number. The observed orbital velocities were less than the wave theory predicted values with the horizontal component showing a zone of decreased attenuation in the mid-depth region, while the vertical velocities showed greater attenuation near the free surface. This work advances the existing knowledge base of vegetative wave attenuation and turbulence studies involving emergent vegetation canopies under regular wave action by employing natural vegetation effects in the laboratory environment as an unique feature of the experiment.