Streamwise Velocity Distributions in The Presence of Submerged Vegetation that is Tilted or Uneven in Height

Riparian vegetation has been recognized to exert significant effects on flood carrying capacity and geomorphology of rivers as well as the river environment including wildlife habitat. Hence the management of riparian vegetation has attracted much attention of river engineers. Many studies have been carried out to clarify the key issue, the interaction mechanism between flow and submerged vegetation, experimentally and numerically. Some fundamental phenomena, however, still remain unclear. In riparian zones vegetation shows different height and density and it shows tilting under high water condition. The effects of such properties of vegetation on the flow resistance and velocity distributions are not well understood and need to be explored in depth.

This study aims to provide fundamental and reliable experimental data on the velocity distributions in the presence of submerged rigid cylinders that are tilted or whose heights are uneven. The measurements are performed in a flume that is 27.0 m long, 0.8 m wide with a bed slope of 1/1,000 for a constant discharge of 23.2 l/s. The channel bed is fully covered by a regular array of rigid cylinder whose diameter is 1.0 cm and the stem-to-stem centreline spacing of 5.0 cm. Three cases are discussed. In Case 1 the cylinders are installed vertically to the bed with the same height of 4.0 cm, while in Case 2 the cylinders are slanted by 30 degrees from the vertical line with the same height of 4.0 cm. In Case 3 vertical cylinders have the mean height of 4.0 cm with a height of 3.0 cm or 5.0 cm in a staggered manner. A micro-ADP is deployed to measure three velocity components inside and just above the vegetation layer and a two-component electro-magnetic velocimetry is used above the vegetation layer.

The comparisons between Case 1 and Case 2 show that the tilting of the cylinders changes the flow in the direction normal to the cylinder, resulting in more uniform distribution of the streamwise velocity in the vegetation layer, and smaller flow resistance. The comparisons between Case 1 and Case 3 demonstrate that the difference in water depth is small, hence, the increase of flow resistance due to the variation of vegetation height is small even though the streamwise velocity distribution shows a large difference. It is found that the dispersive shear stress has the same order of magnitude to the volume averaged Reynolds shear stress in the vegetation layer.