Do surrogates tell the true story? A comparison of wave attenuation for natural and artificial seagrass.

Maike Paul, Lucy Gwen Gillis

Monday 29 june 2015

17:51 - 17:54h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology, Poster pitches

Parallel session: Poster pitch: 3G. Environment - Ecohydraulic

The interaction of flexible vegetation with hydrodynamics has been recognised to alter hydrodynamic forcing. To unravel the influence of specific vegetation parameters on hydrodynamic forcing, artificial structures have been used that represent their natural equivalents. Studies with surrogates provide insight in underlying processes, but remain a simplification of natural conditions. It is therefore difficult to assess how well knowledge obtained with surrogates can be transferred to living equivalents. This study provides a comparison of the temperate seagrass Zostera noltii with three surrogates that have been used to mimic seagrass in flume experiments. It focuses on the wave attenuation capacity of this species which is important from an eco-engineering perspective, but also incorporated the effect of an underlying flow. A range of shoot densities and underlying flow velocities were applied to cover a wide range of natural conditions. Results suggest that that the interaction of waves and flow dominates wave evolution even in the presence of relatively high densities of Zostera noltii. But in the absence of flow, wave attenuation is strongly dependent on above ground biomass for natural as well as artificial seagrass. For the given shoot densities, none of the used surrogates match the data obtained for natural Zostera noltii. This could be attributed to the difference in leaf length as the natural leaves were almost half the length of the shortest surrogates and hence the surrogates may have caused a higher blockage, resulting in a higher increase of wave height. It may also be possible, that none of the surrogates is a suitable representation of Zostera noltii under wave forcing, even though the flexible one appeared to behave comparably under flow. In that case, similarity may not be transferrable between unidirectional and oscillatory hydrodynamic forcing and other parameters have to be used to produce surrogates which yield the same wave attenuation as natural seagrass.