Influence of seagrass meadow length on beach morphodynamics: an experimental study

A novel flume experiment was conducted to compare the sheltering effect of surrogate seagrass meadows of two different lengths against a bare beach (benchmark). The analyses focused on assessing the impact of meadow cross-shore extent on wave height attenuation, behaviour of wave orbital velocity components, sediment transport, and shoreline erosion. Throughout the tests conducted in the large-scale CIEM wave flume at LIM/UPC Barcelona, meadow density and submergence ratio remained constant, while irregular waves were run over an initial 1:15 sand beach profile. In both meadow layouts, a persistent decrease in wave height from the offshore area in front of the meadow to the breaking zone was found. This reduction was directly correlated with the length of the seagrass meadow. As a result of the reduction in wave energy, less erosion occurred at the shoreline in accordance with the decrease in wave height. The mean velocities exhibited changes in the velocity profile from the meadow area to the immediate zone behind the meadow, a phenomenon not observed in more onshoreward positions. Orbital velocities displayed a reduction exclusively for the long meadow case. This decrease was persistent up to the breaking zone. As a consequence of these changes, the long meadow layout led to a decrease in the volume of sediment transport and a breaker bar closer to the shoreline. The short meadow layout resulted in a higher volume of sediment transport compared to the long meadow layout, although still less than the benchmark layout. Furthermore, in the short meadow layout, the final bar was situated in a location similar to that observed in the benchmark layout.
This work acknowledges the REST-COAST project, which provided relevant information on the seagrass meadows located off the Catalan coast. The REST-COAST project receives funding from the European Union's Horizon 2020 Research and Innovation action under grant agreement No. 101037097. Also, we acknowledge the DANUBIUS-IP (grant number 101079778). The first author acknowledges funding from the National Agency for Research and Development (ANID) /Scholarship Program/DOCTORADO BECAS CHILE/2018–72190552. We wish to thank the technical staff of the CIEM wave flume (UPC) for their support and contributions to the experiments: Joaquim Sospedra, Oscar Galego and Ricardo Torres.
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