Experimental observations on sediment resuspension within submerged model canopies under oscillatory flow

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=0.6-1.6. 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=0.6-1.2Hz) was observed together with stem Reynolds numbers Rep above 250. As a result, sediment resuspension for both sediment populations was higher than that of the unimpeded experiment. However, at higher frequencies (F=1.4 and 1.6Hz) and higher plant densities (SPF=5%, 7.5% and 10%), the δTKE inside the canopy decreased, coinciding with stem Reynolds number Rep 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 Rep<250 and no production of δTKE was observed. The real case of a canopy of R. maritima behaved similarly to the flexible model canopy