Flume instrumentation for measurement of drag on flexible elements under waves

Abstract

Understanding energy dissipation by vegetation is critical for the effective management of shoreline erosion. Current methods for estimating energy dissipation require plant-specific parameters that are difficult to estimate for the large variety of plant morphologies used in shoreline protection, requiring testing on each species of interest. A simple and fast method to characterize drag in terms of wave interaction and obstruction natural frequency is needed to fully explore drag forces on vegetation. Our method directly measures hydrodynamic forces on individual plant shoots using a torque sensor mounted beneath the bed of a flume. This sensor allows data to be collected simply and inexpensively with high temporal accuracy that provides insight into drag forces and torque frequency from a variety of flexible elements when coupled with wave monitoring. The technique can evaluate several types of obstructions quickly without the need to set up an entire obstruction field. The data collected also suggest that more flexible objects result in less drag force on each element and suggest that frequency response is related to the frequencies existing in the driving wave and the natural frequency of the obstruction element, although harmonic synchronization appears to occur in some cases doubling the expected drag force magnitude.

The differential response of kelp to swell and infragravity wave motion

“…However, the extent of this dissipation depends strongly on the flexibility of the plant (Mullarney and Henderson ), which controls its ability to “go with the flow.” Indeed, Riffe et al () found wind wave dissipation in salt marshes was substantially reduced (to 30% of that for equivalent rigid stems) by the tendency of the sedge Schoenopletcus americanus to bend with the waves. This ability to move with the surrounding water reduces the drag forces experienced by the plant and ultimately, rates of breakage (e.g., Koehl ; Denny and Gaylord ; Chapman et al ).…”