Spatial and seasonal variation in wave attenuation overZostera noltii

Abstract

[1] Wave attenuation is a recognized function of sea grass ecosystems which is believed to depend on plant characteristics. This paper presents field data on wave attenuance collected over a 13 month period in a Zostera noltii meadow. The meadow showed a strong seasonality with high shoot densities in summer (approximately 4,600 shoots/m 2 ) and low densities in winter (approximately 600 shoots/m 2 ). Wave heights and flow velocities were measured along a transect at regular intervals during which the site was exposed to wind waves and boat wakes that differ in wave period and steepness. This difference was used to investigate whether wave attenuation by sea grass changes with hydrodynamic conditions. A seasonal change in wave attenuation was observed from the data. Results suggest that a minimum shoot density is necessary to initiate wave attenuation by sea grass. Additionally, a dependence of wave attenuation on hydrodynamics was found. Results suggest that the threshold shoot density varies with wave period and a change in energy dissipation toward the shore was observed once this threshold was exceeded. An attempt was made to quantify the bed roughness of the meadow; the applicability of this roughness value in swaying vegetation is discussed. Finally, the drag coefficient for the meadow was computed: A relationship between wave attenuance and vegetation Reynolds number was found which allows comparing the wave attenuating effect of Zostera noltii to other plant species.Citation: Paul, M., and C. L. Amos (2011), Spatial and seasonal variation in wave attenuation over Zostera noltii, J. Geophys.

Wave attenuation by submerged vegetation: combining the effect of organism traits and tidal current

“…4), but the trends for shoot densities ≤2000 m −2 did not differ significantly. This finding agrees with observations made in the field, where a minimum shoot density was required to observe wave attenuation in Zostera noltii (Paul & Amos 2011) and Ruppia maritima (Newell & Koch 2004). However, it may be an artefact of the dataset, as data for 2000 shoots m −2 did not show good correlation (R 2 = 0.15) and the density of 4000 shoots m −2 lacks data for a submergence ratio of 1:1.…”

“…As a result of changing water depth, wave height and period are likely to have changed. Paul & Amos (2011) showed that waves of different periods are attenuated differently by a Zostera noltii bed, and it is possible that this response to differing wave periods is responsible for the difference in wave-attenuation capacity with varying water depth. Fonseca & Cahalan (1992) varied water depth during their laboratory study on 4 different seagrass species, and al though they applied waves with the same period, incident wave height varied between runs.…”

Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

“…Most studies investigating the impact of vegetation on coastal hydrodynamics have been performed over short durations and in the summer months, when above-ground biomass is high and incident wave conditions are slight to moderate (Paul andAmos 2011, Jadhav andChen 2012). Field studies on unidirectional and channelled flows have also tended to focus on low or moderate discharges, due to difficulties associated with planning field campaigns to measure highly unpredictable events (Bakry et al 1992 and references therein).…”

Flume instrumentation for measurement of drag on flexible elements under waves

“…This method can be cumbersome and the results cannot be translated over to other species easily (Bouma et al 2010), and needs to be performed on each species or plant morphology (USACE 2006). Even within a single species, the seasonal changes in plant foliage influence the bulk canopy drag (Schoneboom and Aberle 2009), so testing at several different stages of growth is needed to fully characterize some species (Paul and Amos 2011).…”

“…Use of a non-dimensional drag coefficient is advantageous as it is independent of plant area, which may change seasonally, but is dependent on the morphology and hydrodynamics. For Thalassia testudinum (Bradley and Houser 2009), Zostera noltti (Paul and Amos 2011), and artificial kelp (Kobayashi et al 1993;Mendez et al 1999), exponential functions of Reynolds number were determined for the estimation of drag coefficients with reasonable accuracy.…”