Research papers Experimental observations on sediment resuspension within submerged model canopies under oscillatory flow Àlex Ros n , Jordi Colomer, Teresa Serra, Dolors Pujol, Marianna Soler, Xavier Casamitjana Department of Physics, Campus Montilivi, Escola Politècnica Superior II, University of Girona, 17071 Girona, Spain article info Article history: Received 27 November 2013 Received in revised form 2 October 2014 Accepted 14 October 2014 Available online 23 October 2014 Keywords: Progressive waves Submerged aquatic vegetation Sediment dynamics Resuspension Sediment distribution abstract A set of laboratory experiments were conducted to study the effect of submerged aquatic vegetation in sediment resuspension under progressive waves. Three vegetation models (rigid, flexible and real plants of Ruppia maritima), six wave frequencies (in the range F ¼0.6–1.6 Hz) and four plant densities (Solid Plant Fractions, SPF in the range of 1–10%) were used. The sediment bed properties corresponded to a salt marsh wetland with a bimodal particle size distribution with two particle populations (population 1: particle diameters in the range of 2.5 to 6.0 μm, and population 2: particle diameters in the range of 6.0 to 100 μm). Within the canopy, wave velocities were attenuated for all the canopies studied and for all the frequencies analyzed. The change in the TKE (ΔTKE) compared with the case without plants was studied. For the rigid canopy model, in comparison to the unimpeded experiment, an increase in ΔTKE inside the canopy for smaller frequencies (F ¼0.6–1.2 Hz) was observed together with stem Reynolds numbers Re p above 250. As a result, sediment resuspension for both sediment populations was higher than that of the unimpeded experiment. However, at higher frequencies (F ¼1.4 and 1.6 Hz) and higher plant densities (SPF ¼5%, 7.5% and 10%), the ΔTKE inside the canopy decreased, coinciding with stem Reynolds number Re p below 250. As a result, sediment resuspension for larger canopy densities and larger frequencies was reduced. For the flexible vegetation model, in comparison with the unimpeded experiment, a reduction in the ΔTKE inside the canopy was nearly always found. Resuspended sediment concentrations were found to decrease as flexible canopy densities increased. For the flexible vegetation the stem Reynolds number was Re p o250 and no production of ΔTKE was observed. The real case of a canopy of R. maritima behaved similarly to the flexible model canopy. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Seagrasses and salt marshes are ecosystem engineers with the well-known function of reducing the action of waves and storm surges (Granata et al., 2001; Türker et al., 2006). Sheltering is characteristic of dense canopies, where turbulence cannot pene- trate deep into the canopy, and flushing is controlled by the stem- scale turbulence (Nepf et al., 2007). Under wave-dominated flows, near-bed turbulence levels within seagrass canopies are lower than those on bare soils (Granata et al., 2001; Hendriks et al., 2008; Pujol et al., 2013). As a consequence, wave energy and sediment resuspension are reduced by seagrasses (Terrados and Duarte, 2000; Bouma et al., 2005; Infantes et al., 2012; Paul et al., 2012). In addition, the reduction in sediment resuspension improves water clarity, which in turn, provides greater light penetration and consequently an increase in productivity, thus creating a positive feedback for the sea and wetland grasses growth (Ward et al., 1984; Koch, 2001). The reduction of sediment resuspension within the canopy is directly linked to the modification of currents, wave velocity and turbulence (Neumeier, 2007; Pujol et al., 2013) as well as to the intrinsic properties of the canopy itself, canopy density and flexibility of plants. As pointed out by van Katwijk et al. (2010) at the relatively wave-exposed, sandy sites, dense vegetation cause muddification (increase in fine sediments and organic content) of the sediments. In contrast, in sheltered sites with muddy sedi- ments, dense vegetation has no effect on the sediment composi- tion. In sparse sheltered vegetation, contrary to non-sheltered canopies, with muddy sediments, sandification (decrease in fine sediments and organic content) prevails. In events where the flow level is above the vegetation, sedimentation decreases with dis- tance, from the seaward marsh edge and from the creeks, in parallel with a grain-size fining or muddification (Neumeier and Amos, 2006). Coastal zones present a large variety of clay, silt and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research http://dx.doi.org/10.1016/j.csr.2014.10.004 0278-4343/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ34 972 418372; fax: þ34 972 418098. E-mail address: alex.ros@udg.edu (À. Ros). Continental Shelf Research 91 (2014) 220–231