Impact of submerged herbaceous vegetation on wind-induced current in shallow water

Abstract

A combined wind tunnel and water flume experiment was performed to study the effect of an aquatic herbaceous plant patch on the turbulent structure and flow field of wind-induced currents. Eel grass was chosen as the test plant. The plants were settled in patch form. Three-dimensional instantaneous velocities were measured using an acoustic doppler velocimeter (ADV). The results indicate that in the non-eel grass patch flow, turbulent shear stress −u′w′¯has a local minimum near the flow reverse point, turbulent intensity has the tendency to decrease continually from surface to bottom, and the dominant turbulent event is sweeping. In the eel grass patch with wind-induced flow, reverse flow occurs more quickly than in the non-eel grass patch case, and the reverse point shifts downward. Turbulent shear stress −u′w′¯ has two maxima below the canopy. Turbulent intensity u′2¯ decreases generally from water surface to flume bed, but there are maxima below the canopy. The dominant turbulent event is the fourth quadrant event below the canopy, and the two relative contributions of the second and the fourth quadrant event have similar values above the canopy. The mean bias and RMS values at centre of the eel grass patch are almost as two times larger as that at other measuring lines in non-eel grass region. This indicates that not only the shear stress and turbulent intensity become larger, and the vertical variations of the shear stress and turbulent intensity are also larger due to the effect of vegetation. The differences in the effects of submerged herbaceous vegetation on open channel flow and wind-induced flow were discussed in relation to time averaged velocity, turbulent shear stress, turbulent intensity and coherent structure based on the present test data and previous data.