Modeling depth-averaged velocity and bed shear stress in compound channels with emergent and submerged vegetation

Representing plants as rigid cylinders in experiments and models

Experimental research was undertaken to investigate the changes in spatial turbulence intensity in a compound channel and the influence of rigid, emergent floodplain vegetation on turbulence intensity. Five tests for two various roughness values of floodplains were realized. In the first series of experiments (three tests), the surface of the main channel bed was smooth and made of concrete, whereas the floodplains and sloping banks were covered by cement mortar composed of terrazzo. In the second set of experiments (two tests), emergent vegetation (trees) on the floodplains, modeled by aluminum pipes, was added. Instantaneous velocities were measured with the use of a three-component acoustic Doppler velocity meter. The influence of floodplain trees, on the distributions of relative turbulence intensity (u′/U, v′/U, w′/U) in the main channel and on the floodplains was presented. It was found that the longitudinal (u′/U) and transverse (v′/U) turbulence values decreased from the bottom upward to t...

Three-Dimensional Turbulence Intensity in a Compound Channel

A comparative study of longitudinal dispersion models in rigid vegetated compound meandering channels.

Experimental research was undertaken to investigate the changes in spatial turbulence intensity, water turbulent kinetic energy, the time and spatial macro-scale, scales of turbulent eddies (macro- and microeddies) in a compound channel. Three tests for two various roughness values were realized. The surface of the main channel bed was smooth and made of concrete, whereas the floodplains and sloping banks were covered by cement mortar composed of terrazzo. Instantaneous velocities were measured with the use of a three-component Acoustic Doppler Velocimeter (ADV). The distributions of relative turbulence intensity (u′/U, v′/U, w′/U) in the main channel and on the floodplains were presented. It was found that the longitudinal (u′/U) and transverse (v′/U) turbulence values decreased from the bottom upwards to the floodplain elevation (z/h = 0.56) in the main channel, but remained constant above the floodplain level. Vertical relative turbulence intensity (w′/U) increased going up from the bottom until z/h = 0.15, decreased until about z/h = 0.7, and then increased again upwards to the water surface. The distributions of relative turbulence intensities were described with regression equations. The distributions of turbulent kinetic energy at different water depths were described by regression equations. Vertical distributions of turbulent kinetic energy on the floodplains and over the banks of the main channel were divided into three zones. Over the bottom of the main channel, four zones were determined, containing the middle zone of the flow field divided into two zones of different trends. Measurements of instantaneous velocities are used to investigations of the longitudinal sizes of the smallest eddies (Kolmogorov’s microscale). Presented analyses were based on the already published results.

Measurements of Turbulence Structure in a Compound Channel

Vegetation such as trees and shrubs are often observed at the floodplain edge in natural rivers. Spanwise profiles of streamwise velocity component are influenced significantly by the drag force of trees. That is to say, the streamwise velocity decreases locally behind trees and near the junction between the main-channel and the floodplain. This property is quite different from that observed in typical compound open-channel flows, in which the velocity profile has a single inflection point and forms a mixing layer related to large-scale horizontal vortices. In contrast, in compound open-channel flow with a one-line emergent vegetation, a “V”-shaped velocity profile appears with twin inflection points. It is thus very important to investigate these hydrodynamic properties and turbulence structure considering the emergent vegetation effects in river engineering and eco hydraulics. So, in the present study, turbulence measurements by 3-D acoustic Doppler anemometer (ADV) were conducted in 150cm wide laboratory flume, in which 2cm diameter and 25cm height cylinders are placed as vegetation models with 10cm span along the junction edge.

Turbulence structure of compound open-channel flows with one-line emergent vegetation

PIV and LIF techniques were conducted to reveal relationship between sedimentation and mass exchange property in a groyne field. It was found from the PIV measurements that different bed-form conditions, that is to say, with and without bed slope, yielded corresponding gyre formations. The other experiment, the LIF results allowed us reasonably to evaluate transfer velocity of dye through boundary between mainstream and dead water zone, They suggest that mass exchange properties have significant relations with horizontal gyres, thereby, with the conditions of bed slope. The results normalized by turbulence intensity and length scale of gyre were in good agreement with a large-eddy model.

Sohei Suzuki

Papers

Turbulence structure of compound open-channel flows with one-line emergent vegetation

Effects of slope-changed bed in a groyne field on its flow structures and mass-exchangeing

Hydrodynamic effects of bed configuration in embayment on flow patterns and related turbulence transport in open-channel flows with side cavity zone