Effect of two distinct patches of Myriophyllum species on downstream turbulence in a natural river

Velocity profiles upstream and downstream of two aquatic plant species that are similar in morphology but differ in patch structures were measured in a natural river. Turbulence statistics were analyzed after thorough data filtering. In the wake of the M. alterniflorum, which was a slender, 0.3 m wide and 1.2 m long patch of aspect ratio 1:4, there were distinctive peaks in both, turbulence intensity and turbulent kinetic energy, which indicated increased lateral mixing. In contrast to the M. alterniflorum, turbulence statistics in the wake of the M. spicatum, which was the larger, 2 m wide and 2.4 m long patch of aspect ratio 1:1.5, indicated increased lateral shear of a greater magnitude. The turbulent kinetic energy was diminished in the closest layer to the bed downstream the both plants, although, in the case of M. alterniflorum, the observed values were similar to those upstream. The occurrence of the mixing layer below the height of M. spicatum was visible in the power spectral density plot. In both cases, ejections in the wake diminished in favor of other coherent structures. The shape and configuration of a patch are decisive factors governing the occurrence of flow instabilities downstream of the patch.