Improved parameterization of seagrass blade dynamics and wave attenuation based on numerical and laboratory experiments

Abstract

Seagrass blade dynamics were explored through numerical and laboratory experiments in order to improve parameterization of wave attenuation by submerged aquatic vegetation in the presence of a background current. In the numerical model, a single blade was modeled as a series of rigid plates attached by torsion springs. For the laboratory model, strips of low‐density polyethylene were placed in a recirculating wave flume. A new form of the Keulegan—Carpenter number based on the horizontal excursion of the blade tip was found to be an excellent predictor of drag coefficient. An algebraic model for predicting wave attenuation was developed based on the following observations. During the portion of the wave period when the fluid velocities are highest, the blade motion is almost completely arrested and the vast majority of the turbulence production occurs during this time. Turbulence production when the blade is pronated is accurately predicted by the maximum fluid velocity over the wave period. The relative contribution to the total turbulence production over the wave period is determined by the relative strength of the waves and the current. Therefore, using a simple algebraic fit, the total depth‐integrated, time‐averaged turbulence production can be accurately predicted by two flow parameters: the maximum fluid velocity over the wave period, and a non‐dimensional number that compares the wave and current velocities. By fitting the algebraic model to data from a particular site, it can be used to efficiently estimate wave attenuation and drag coefficient in seagrass exposed to waves with a background current.

A simple and practical model for combined wave–current canopy flows

“…Experiments were conducted in a recirculating wave flume in Stanford University's Bob and Norma Street Environmental Fluid Mechanics Laboratory (Weitzman 2013;Zeller et al 2014). The flume's test section is 4.9 m long and 1.2 m wide.…”

Efficient three-dimensional reconstruction of aquatic vegetation geometry: Estimating morphological parameters influencing hydrodynamic drag

“…For flexible vegetation, wave dissipation can be reduced as stems move back and forth with surrounding water (Méndez et al, 1999;Riffe et al, 2011;Utter and Denny, 1996). Models simulating stem motion require knowledge of hydrodynamic parameters and the geometric parameters such as N and d considered here, as well as plant material properties, such as the Young's modulus, which are beyond the scope of this paper (Mullarney and Henderson, 2010;Zeller et al, 2014). Surveys of vegetation geometry have focused on subsamples of the full canopy, with measurements confined to a few small (often 0.1 -1 m 2 ) quadrats (Feagin et al, 2011;Lightbody and Nepf, 2006;Paul and Amos, 2011;Riffe et al, 2011;Temmerman et al, 2005b).…”

Mechanisms for the Asymmetric Motion of Submerged Aquatic Vegetation in Waves: A Consistent‐Mass Cable Model

“…Luhar and Nepf () noted that the exact value of had little effect on their model results because the ratio of the calculated root‐mean‐square forces for and was distributed with mean and standard deviation . For simplicity, Zeller et al () selected an approximated value of 0.02. In contrast, Luhar and Nepf () used a larger value of because their model was found unstable for the cases with high Cauchy number if .…”

“…The cable model results were first compared with the laboratory experiments by Zeller et al () for the blades in combined waves and currents. During the experiments, six wave‐current conditions were produced, where the wave period –5.19 s, the amplitude of the horizontal wave velocity –25.8 cm/s, and the currents –13.7 cm/s (Table ).…”