The Impact of Submerged Vegetation (Potamogeton malaianu) on Flow Structures: a Flume Study

Submerged plants have important ecological values in aquatic system, such as rivers and lakes, which are strongly interacted with flows. Many efforts have been paid to investigate the impact of the submerged plants on flow structures, roughness and turbulent diffusion. However, rigid plants are usually assumed in most of studies on the response of flows to plants. The natural submerged plants are essentially flexible and seriously depart from the rigid assumption. Thus, the simulated plastic plants sometimes are applied to investigate the impacts of submerged vegetation on turbulent flow structures. Although this consideration is closer with the real aquatic plant, but the submerged plants have various morpho structures which can hardly be simulated. Large differences may exist for flows through those various kinds of vegetations. In this work, we adopted Potamogeton malaianu, which widely distribute in shallow waters, such as lakes, as the target plant. Three groups of experiments under the averaged velocities of 5, 10 and 15 cm/s were conducted using Potamogeton malaianu with different distribution patterns and densities. The flume is partly filled by vegetation for simulating natural environment. The three-dimensional velocities were measured with Nortek Vectrino and Sontek MicroADV in and out of the vegetation patch of Potamogeton malaianu. The experiment results show that velocity pattern are changed by Potamogeton malaianu over region inside and surrounding vegetation patch: 1) The horizontal distribution of velocities are regulated by the vegetation patch. 2) The mean flow velocities are reduced inside vegetation patch and increase outside of patch; 3) Due to the flexibility of Potamogeton malaianu, a reversal of flow velocity gradient resulting in a secondary maximum velocity value was observed. The larger density the plants have, the more significant reversal flow can be observed; 4) The shape of velocity is S-type and velocity increases with depth within canopy;