Effects of Posidonia oceanica seagrass on nearshore waves and wave-induced flows.

Abstract

This work focuses on the effects of the large Mediterranean seagrass Posidonia oceanica on coastal waves and wave-induced flows, which has significant implications for coastal protection. Investigations were made on both a natural shallow Posidonia oceanica bed and, in controlled conditions of full-scale Posidonia mimics under regular and irregular waves. In the field, waves and currents were monitored during low energy conditions and a Mistral wind
event. Data were collected on the distribution of Posidonia patches, density and canopy height, as well as bed sediment type and bathymetry. In the flume, measurements were made of water surface elevation along the flume and oscillatory flows at 3 locations and 4 elevations, under several wave conditions, water depths and for 2 canopy densities. The mimics were designed carefully to recreate the hydraulic behaviour of Posidonia plants under waves.

Field results indicate that shallow Posidonia meadows are effective at reducing wave energy under low wave energy conditions and small wave amplitudes. The flume experiments confirm this trend. Under both regular and irregular waves, drag coefficients decrease with increasing Reynolds vegetation numbers; wave dissipation factors decrease with wave orbital amplitude. Under spectral waves, most wave energy dissipation occurs at the peak spectral frequency and it is largest for the least energetic wave spectra. At high wave Reynolds numbers, the canopyinduced hydraulic roughness (r) appears to be a function of the canopy element density only, and the empirical formula of Nielsen (1992) is successfully applied. However, more work is required in low energy conditions to examine the range of validity of the formula. In natural conditions under small amplitude waves, attenuation of wave-induced flows is negligible in the upper canopy; flume experiments confirm this trend. The typical flow intensification at the canopy top, measured for other seagrasses, occurs only for tests with the largest wave amplitudes, whilst, under smaller waves, flow intensification is located within the upper part of the canopy. In the lower canopy, flows are always reduced and flows decelerate exponentially with increasing orbital amplitude. This is a novel observation in flexible canopies. The artificial canopy, like the natural Posidonia bed, enhances flow asymmetries at the canopy top, especially
under waves with large wave orbital amplitudes. This is thought to be a mechanism to enhance shoreward drift. Turbulence in the artificial canopy, under regular waves, peaks at the canopy top, as occurs under unidirectional flows and for other seagrass beds exposed to waves. Vertical
turbulent exchanges are enhanced at the edge of the seagrass patch and are larger for lower submergence ratios (the ratio of canopy height to water depth). A reduction in submergence ratio in the flume, also causes increased shear stresses at the top of the canopy, lower wave height decay and reduced oscillatory flow attenuation in the lower part of the canopy. The denser canopy, in the conditions tested, increases relative roughness (r/A), wave attenuation, in canopy oscillatory flow reduction and turbulent kinetic energy at the top of the canopy. Oscillatory flows characterised by small orbital amplitudes can penetrate further into the canopy than larger orbital velocities, inducing a larger drag, thus increasing wave dissipation, as
proposed for rigid canopies (corals). This is manifested as a thinner canopy boundary layer under small orbital amplitude waves than the large amplitude waves. A conceptual model is proposed to summarise these findings. Under storm conditions Posidonia is believed to be less
efficient at reducing wave energy, however it remains effective at reducing sediment transport locally and, by inducing a preferential shoreward drift, at preventing sand dispersal offshore

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Identifiers

ORCID for Ian Townend: orcid.org/0000-0003-2101-3858

ORCID for Charlotte Thompson: orcid.org/0000-0003-1105-6838

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