Turbulence-induced forces in a freshwater macrophyte canopy

Drag forces on macrophytes in a steady turbulent flow consist of both a mean and fluctuating component. Macrophytes that form a canopy experience lower mean forces due to a reduction of velocities within the canopy. However, submerged canopies also generate additional turbulence. In this study, an experiment is conducted in a laboratory flume to investigate how canopy density influences both mean and unsteady forces on a macrophyte. Canopies with densities of 39, 156, and 625 stems m(-2) are constructed from 25 cm lengths of Lagarosiphon major. Drag is measured on a single stem within the canopy at four flow rates. The results show that although mean forces are reduced with increasing canopy density, the unsteady force (characterized by RMS of fluctuations from the mean) increases with the square-root of canopy density. The force response of the stem to the turbulence was frequency dependent with the strongest response at low frequencies.