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Experiments by Invited Researchers

 

Wave propagation over Posidonia Oceanica

Project acronym: HyIII-UPC-08
Name of Group Leader: Panayotis Prinos (prinosp@civil.auth.gr)
User-Project Title: Wave propagation over Posidonia Oceanica
Facility: CIEM Flume
Proceedings TA Project: Wave propagation over posidonia oceanica: large scale experiments
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Posidonia oceanica, the most abundant seagrass species in the Mediterranean  is a plant which has a fundamental role in the coastal sand budget and in protecting the shoreline from erosion.

No large scale experiments had been conducted. The objectives and scientific aims of the project are described below:

  1. Measurement of wave attenuation, transmission and energy dissipation over Posidonia oceanica during low and high energy events in intermediate and shallow waters
  2. Investigation in the effects of  submergence ratio hs/D (hs=height of seagrass, D=water depth) and seagrass density (number of shoots per squared meter) on the above characteristics
  3. Measurement of mean velocities above and within the seagrass and estimation of wave induced flow within the seagrass which influences processes such as nutrient uptake, waste removal and larval dispersion.
  4. Measurement of bed friction and the associated damping due to the seagrass which is associated with sediment deposition and re-suspension.
  5. Investigation in the effect of Posidonia oceanica on sand deposition and suspension by measuring suspended sand concentration and video recording the threshold of suspension within the canopy.
  6. To provide experimental data for validation of numerical models on wave propagation over vegetation fields and to contribute to the formulation of parametric models, accounting for Posidonia oceanica effects, which are used in wave propagation, nearshore circulation and morphodynamic models.

The experiments, in prototype conditions (1:1 scale), have been conducted in the CIEM wave flume in Barcelona, using a 10.7m long meadow of artificial polypropylene plants, simulating Posidonia oceanica.

Short description of the work carried out

The achievements of the project in meeting the above objectives are:

  1. Measurements with water level varying between 1.80m and 2.40 m at the paddle, and values of Ts between 2.0 sec and 4.0 sec for regular and irregular waves, simulating low and high energy events.
  2. The submergence ratio (hs/D) was varied between 0.50 and 0.36 (a mean height, hs, of 0.55 m for the P.O. meadow was set and the water depth, D,  at the meadow was between 1.10 m and 1.50 m). The  effect of the sea grass density was investigated by using two different meadow configurations: (a) High Density (360 stems per m2  arranged in a non-staggered pattern) and (b) Low Density (180 stems per m2 arranged in a staggered pattern)
  3. Measurements of mean velocities were taken at the three locations within the meadow (front, middle and end of Posidonia oceanica meadow)
  4. The motion of the artificial plants was video recorded together with the motion of the leaves for determining their bending angle
  5. An experimental database has been developed for numerical models

The difficulties encountered are:

  1. The setup of the instrumentation and the meadow took longer than predicted
  2. Problems in the calibration of the AWGs
  3. The wave maker could not cope with irregular waves with period less than 2 sec
  4. Problems with stabilization of the meadow at the bottom of the flume
  5. Measurements with OBSs were not possible in some cases, as the instruments were covered by sand due to sand motion

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