Investigation of hydraulic changes in flow in meander compound channel with flexible submerged vegetation

Introduction: Estimating hydraulic characteristics in the channels and floodplains, in the case of vegetation is very difficult. Individual factors and their effects can be determined with acceptable accuracy, such as for a meandering channel with the presence of vegetation. In view of the need for further research on overbank flow in a meandering channel with the presence of vegetation, work will be carried out to fill the gaps to provide the required information. The key purposes of the present research are to enhance our knowledge of the flow resistance caused by vegetation and to report the results of laboratory investigations into the physical processes involved in the flow structure as well as to understand the flow characteristics and flow mechanisms in compound meandering channel with different arrangements of non-vegetated and vegetated floodplains. This study is focused on the influence of vegetation on overbank flow characteristics. In this research, the effect of submerged flexible artificial vegetation in the floodplain on two relative depths of 0.35 and 0.55 has been studied in the laboratory.
Methodology: All the experiments reported here were conducted in one flume at the ّFerdowsi University of Mashhad. The flume is built on a number of rigid steel structures to support its weight, achieve maximum stability and maintain its longitudinal gradient. It was constructed include to tanks, sumps and pipeworks. Both sidewalls of the flume built were using glass to ease visibility during the setting-up of the instruments used.
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. Artificial grass with an average height of 2.5 cm 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.
Results and discussion: The results showed that the presence of flexible vegetation in the floodplain for a constant relative depth has reduced the flow capacity. 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. By examining the velocity contour lines in the presence of vegetation, the core of the maximum velocity in the main channel is increased. Also, the longitudinal velocity above the submerged vegetation has been significantly increased. The values in the transverse and vertical velocity components in floodplains with vegetation are much higher than in uncovered conditions. The directional secondary vectors of the flow in section S1 indicate a counter-clockwise flow and in section S3 indicate a round clock flow in the main channel. The presence of vegetation disturbed the secondary flow pattern, and larger vectors were observed at the junction of the two channels in the presence of vegetation. It seems that the presence of vegetation, 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. 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. So that, the presence of vegetation has increased the transmission flow of the main canal compared to the simple state. So that if you calculate the average of sections, the rate of flow through the main channel compared to the total flow of compound meandering channel for relative depth of 0.35 and 0.55 is equal to 54 and 36%, which shows a 19% and 6% increase compared to the control mode of transmission through the main channel, respectively.
Conclusion: In this study, using a laboratory model, the effect of submerged flexible 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 floodplains of the uncovered state is higher than in the case with the cover. Although, flow velocities in and around the main channel seem to be close to those measured in uncovered channel which indicates the diversion of flow to the main channel.