Flow behavior in Non-Prismatic Convergent Compound Channel with Submerged Vegetation on Floodplains

Flow behavior in Non-Prismatic Convergent Compound Channel with Submerged Vegetation on Floodplains

Introduction
Vegetation in compound channels by increasing factors such as roughness in the floodplains rather than main channel, velocity difference and momentum exchange between the sub sections, leads the transverse velocity gradient and apparent shear stress of interface to increase. In natural rivers by changing the cross section, uniform flow converts to non-uniform. In prismatic channels, shear stress in the interface between main channel and floodplains, influences the transfer capacity and velocity distribution pattern significantly. This effect in non-prismatic channels, due to the extra momentum exchange between the sub sections is more intense. In such condition identifying the flow hydraulic is very complex. Although forming the vegetated and non-prismatic floodplains at the same time in natural rivers is highly probable, but there are no specialized studies yet to investigate the hydraulic of flow in such conditions. Therefore in the present study, experimental measurements were conducted in a compound channel with non-prismatic and vegetated floodplains simultaneously and flow behavior is investigated on it.

Methodology
The experiments were conducted in a 10 m long, 2 m wide compound channel located at the National Laboratory for Civil Engineering (LNEC) in Lisbon, Portugal. The channel cross section consists of two equal rectangular floodplains (floodplain width Bfp=0.7 m) and one trapezoidal main channel (bank full height, hb=0.1 m, bottom width bmc = 0.4 m, bank full width Bmc = 0.6 m and side slope of 45°, sy= 1). The channel bed is made of polished concrete and its longitudinal slope is S0= 0.0011 m/m. The vegetated floodplains were obtained by covering their bottoms with a 5 mm hight synthetic grass. For the polished concrete, n= 0.0092 m-1/3s and ks=0.15 mm and for the synthetic grass, n=0.0172 m-1/3s and ks=6.8 mm. Measurements were performed for relative depths of 0.21 and 0.31. The experiments in the non-prismatic channel performed at two convergent angles of floodplains (θ=7.25◦ and θ=11.3◦). In this cases, the mentioned relative depths were set up in the middle sections of convergences by changing the downstream tailgate. The velocity measurements performed for Entrance, Middle and End sections of convergent angles.

Results and discussion
high velocity distribution pattern’s gradients are observed in non-prismatic rather than prismatic channel for a given relative depths. Comparisons between similar sections indicates by increasing relative depth, the interaction intensity through the main channel and floodplain decreases. Because, presence of vegetation on the floodplain leads to channel transfer capacity decrease and in high values of relative depth, this effect decreases. Except the areas close to wall and interface, the flow on floodplain is two-dimensional while in main channel and especially in low relative depths is three-dimensional. This issue has also been affected by different convergence angles. The maximum velocity generally occurs near the outer wall of the main channel. But, by increasing the relative depth, position of the maximum velocity moves to the floodplain. In the lower relative depth, in vicinity of interface, a bulge is visible in isovel lines that is already reported in previous works. Due to the mass transfer from the floodplain towards the main channel, this bulge occurred more intensively in the middle sections in both the convergent angels. In non-prismatic channel, for all the flow cases, at the interface, the intensity of the secondary flows is more apparent and in the down part main channel flow, a vortex is formed that by increasing the relative depth from 0.21 to 0.31 and convergent angels from 7.25◦ to 11.3◦, moves from the outer wall of the main channel towards the floodplain. Also at the beginning of the floodplain, one vortex is formed that becomes more apparent by increasing the convergence angles. Due to converging floodplains, in the upper layers, a transverse current is directed from the floodplains to the main channel. This transverse current enters the main channel from both sides and, due to symmetry of flow, plunges to the channel bed and as a result, two helical secondary flows are generated in the main channel, rotating in the channel length which is very important in terms of sediment and pollutant transport. By increasing relative depth, velocity gradient between the main channel and floodplains decreases.

Conclusion
In present study, using an experimental model, flow behavior in a prismatic and non-prismatic compound channel is investigated. Non-prismatic channel consists of two convergence angles; 7.25° and 11.3°. All the experiments are conducted at two relative depths of 0.21 and 0.31. In order to investigate vegetation cover effects, floodplains are covered with synthetic grass and a vecterino (ADV) is used to measure the fluctuations of instantaneous flow velocity. Variations in the streamwise velocity distribution, secondary currents and Reynolds stresses based on proportions of vegetation and non-prismaticity in the flow hydraulic are investigated. Results show for high relative depth, by increasing convergent angles, the floodplains are less involved in discharge carrying and transferring. The maximum velocity values which occur at the main channel center, by increseang the relative depth and extending secondary currents, towards to the floodplains. By increasing the convergent angle, the roughness values in the main channel and floodplains increases. Distribution of flow mean kinetic energy shows that by increasing relative depth, its values in the middle section decreases for both the convergent angles.


Key words: Non-prismatic, Compound Channel, Vegetation, Convergent floodplains