Abstract: To understand the changes in the characteristics of flow structure caused by an emergent vegetation patch over a curved channel, laboratory experiments were conducted on a 180-degree U-shaped flume. Wooden cylinders were used to simulate a sparse vegetation patch and solid cylinders mimicked a dense vegetation patch; in each case, the patch position in the U-bend was the same to clarify the bend effect. An acoustic Doppler velocimeter (ADV) was used to measure the three-dimensional velocity. Experimental results show that the adjustment length upstream from the patch and the length of the stable wake region are adjusted significantly by the curved channel. And both the vegetation density and curved channel work together on the wake structure behind the patch, including but not limited to the difference in vertical turbulent kinetic energy (TKE), while the upstream adjustment length is not affected by vegetation density. Behind the vegetation patch, turbulence structure changes significantly, and large vortices appear on a scale comparable to the patch size, resulting in an obvious peak of the longitudinal TKE distribution. The peak value is also affected by the curve. However, it becomes smaller once the vegetation patch is moved to a position closer to the bend outlet. This trend is also found in turbulence spectrum analysis, showing the channel bend increases the number of small-scale turbulent eddies behind the patch. Therefore, small peak values caused by small-scale turbulent eddies do not depend on the relative patch position in the curved channel.

Key words: curved channel, emergent vegetation, longitudinal velocity, turbulent kinetic energy, turbulence spectrum