Turbulent Kinetic Energy in Submerged Model Canopies Under Oscillatory Flow

Abstract

Laboratory experiments measured the velocity inside a model meadow of submerged, flexible vegetation under 1 and 2 s period waves. The model plant consisted of a rigid stem and strap-like blades, similar to the seagrass Zostera marina and the freshwater eelgrass Vallisneria Americana. The ratio of wave excursion (Aw) to stem spacing (S) determined whether, or not, plant-generated turbulence enhanced the turbulence level within the meadow, compared to bare bed. Specifically, near-bed turbulence was enhanced for conditions with Aw/S > 0.5, and for these conditions the turbulence (TKE) normalized by the RMS wave velocity squared, TKE/Uw,RMS2, increased monotonically with the plant solid volume fraction, ϕ. The plant-generated turbulence was greater in the stem region than in the blade region, and this was attributed to the greater relative motion between the waves and rigid stem, compared to the flexible blades. A model previously developed to predict TKE in unidirectional flow through a rigid emergent canopy was modified by replacing the time-mean current with the RMS wave velocity. With a fitted scale coefficient, the modified model predicts TKE as a function of RMS wave velocity in the meadow, stem and blade geometry, and solid volume fraction. Wave decay was also measured and shown to have a linear correlation with the measured TKE within the canopy, providing a second method to predict meadow TKE in the field.