Investigating the effect of rigid non- submerged artificial vegetation on flow characteristics in Compound Meander Channel

Rivers can be classified as straight, meandering and braiding, Meandering is the most common plan-form acquired by natural rivers. The meandering channel flow is considerably more complex than the straight channel. Compound meandering channels are abundant in nature that including the main channel for the flow in normal times and one or two floodplain (usually covered with plants) at the sides during floods. Vegetation is an important property of many rivers, enhancing amenity values for people and providing habitat for other organisms. Vegetation stabilizes stream banks, provides shade that performs an essential role in nutrient cycling and water quality and supports wildlife. Therefore, it is necessary to study this issue in order to better understand the flow during floods. This study is focused on the influence of vegetation on overbank flow characteristics. In this research, the effect of non-submerged rigid artificial vegetation in the floodplain on two relative depths of 0.35 and 0.55 has been studied in the laboratory.

The experimental research was carried out in a non-mobile bed meandering channel constructed in a 10 m long and 0.78 m wide flume which included the main channel and two floodplains on its sides. The channel wavelength and meander belt width were one meter and 0.58 m, respectively with the sinuosity of 1.3. The geometrical parameters for the main channel were: width, Bmc=0.2 m and depth, Hmc= 0.1 m. Plastic cylinders of 9mm diameter are used to simulate the emergent floodplain vegetation. A movable weir located at downstream of flume controlled water level. Velocity data were extracted and analyzed using Acoustic Doppler Velocimetry. The minimum recording time for each point velocity was 60s. ADV measures the 3D velocities of water particles located 5 cm below its probe. The measurement sections located 6 m downstream of channel inlet, with the names of S1 to S5.

The results showed that the presence of vegetation in the plain flood for a constant relative depth has reduced the flow capacity. The decrease in discharge for depths of 0.35 and 0.55 is equal to 23 and 12 percent, respectively, for a density of 0.77 percent per unit area of one square meter. The pattern of contour lines of the longitudinal velocity in the main channel in the presence of vegetation changes at both relative flow depths relative to the uncovered state. The absolute values of velocity in the main channel in the uncovered state are greater than in the covered state. Also, the values in the transverse and vertical velocity components in plain floods with vegetation are much higher than in uncovered conditions. The directional secondary vectors of the flow in section S1 indicate a counter-clockwise flow in the main channel. The presence of vegetation at a relative depth of 0.55 has reduced the size and values of these vectors in both the main channel and the floodplain. It seems that the presence of coverage, as observed during the experiments, has changed the patterns and directions of vectors on the floodplain. These changes are also observed at the relative depth of 0.35, but specifically the presence of vegetation at this relative depth has caused the flow transfer from the right floodplain to the left floodplain. For all sections, the average values of longitudinal velocity on both sides of the floodplain are greater than the uncovered state and increase by moving away from the main channel to the glass wall of the channel. Although the capacity of covered flow is less than the uncovered one, flow velocities in and around the main channel seem to be close to those measured in uncovered channel. This indicates the high impact of floodplain vegetation on the hydraulics of the flow in the compound meandering channels. Also, the presence of cylinders has increased turbulence and consequently increased shear stress. The values of shear stress at the bottom of the main channel and the convex coast of the floodplain are higher than other areas. In addition, the cover has increased shear stress in the floodplain and reduced the kinetic energy of turbulence (TKE) in the floodplain.

In this study, using a laboratory model, the effect of non-submerged rigid artificial vegetation on the floodplain of a compound meandering channel was investigated. The following is a summary of the results of this study. The presence of vegetation reduced the water transfer capacity, due to the increased resistance to flow. The average longitudinal velocity of the flow in the cross section of the channel in the uncovered state is higher than in the case with the cover. Due to the non-submerged rigid artificial vegetation used, the shear stress in the floodplain has increased. Keywords: Flood, Flow Pattern, Meander Channel, Relative Depth, Vegetation