Experimental investigation of floodplain vegetation density effect on flow hydraulic ‎in divergent compound channels

Document Type : Research Article

Authors

1 Dep. of Water Eng. Lorestan University

2 water Eng. , faculty of agriculture , Lorestan university

3 Dep. of Water , Lorestan University

4 Dep. Water Eng. Lorestan University

Abstract

Introduction:
The velocity difference in the main channel with higher velocity and floodplain with lower velocity creates a strong shear layer in their junction, causing the production of additional turbulence structures, especially large-scale vertical vortices in this interface. In addition, because of turbulence anisotropy in the bottom and wall of the channel, secondary currents occur around the longitudinal axis and in a spiral shape. On the other hand, in most cases, due to the existence of vegetation on floodplains, investigation of the flow mechanism is far more complicated. There are usually three methods to explain the flow field and shear stress with the existence of vegetation on floodplains: 1) field measurements, 2) hydraulic models, and 3) analytical and numerical models. In natural rivers, since the flow cross-section changes along the river and the cross-section shape changes from prismatic to non-prismatic, with these conditions causing more mass and momentum exchange from the floodplain to the main channel and vice versa. this study has explored the effects of divergence angle and vegetation density on the flow structures in a non-prismatic compound channel.
Methodology:
The experiments of this study were performed in an asymmetric compound channel made of Plexiglas with a length of 12 m, width of 0.6 m with a bed slope (So) of 0.8810-3. In order to model the vegetation on the floodplain, rigid cylindrical plastic rods with a diameter of (D) 10 mm were used. The spacing ratio (Sr = ly / D) for the three vegetation densities will be equal to 5, 7.5, and 10. Three divergence angles () equal to 3.8, 5.7, and 11.3 ͦ were created on the floodplain. Due to the formation of non-uniform flow in non-prismatic sections, the relative depths (Dr=yf/H) of 0.15, 0.25, 0.35, and 0.45 were set in the middle of the divergence region for all experiments. The longitudinal, transverse, and vertical components of the instantaneous flow velocity were measured by a 3D Vectrino profiler velocimeter at three sections: entrance, middle, and end of the divergence region. Using the transverse distribution of depth-averaged velocity, contribution of each section to the conveyance capacity was calculated. Due to the interaction between the rods, the flow structures are very different from the behavior of a single rod; thus, this should be considered in calculating the drag coefficient of an element set on the floodplain. For determine f, the Keulegan (1938) equation for smooth surfaces was modified. Jafari et al. (2011) proposed the an equation to calculate Strouhal number in a row arrangement.
Results and Discussion:
Because of outbreak the Kelvin–Helmholtz instability due to the existence of vegetation on the floodplain, in the interface between the main channel and the floodplain, coherent vortices and intense momentum exchange were formed from the main channel to the floodplain. Since the flow momentum prepared a shear layer around the vegetation stems, which causes inflection points in the velocity profile, which is consistent with Sanjou et al. (2010), Mulahasan et al. (2017), and Ahmad et al. (2020) results. At Sr = 7.5, the distance of the elements well forms Von Kàrmàn vortex streets and increases the flow resistance. At all relative depths, increasing vegetation density has reduced the Ufp / Umc ratio. The discharge rate through floodplain with vegetation has reduced by an average of 58.6 and 69.3% compared to non-prismatic channel without vegetation in the middle and end of the divergence reach, respectively. The results indicate that with increasing Dr, zonal roughness coefficient in the floodplain has increased nonlinearly and is linear in the main channel. This result is consistent with the Musleh and Cruise (2006) research. Drag coefficient has decreased nonlinearly with increasing the rod Reynolds number. In addition, it can be found that the drag coefficient caused by floodplain vegetation is directly related to the vegetation density. The results show that with increasing the vegetation density from Sr = 10 to Sr = 5 on the floodplain in the middle and end of the divergence, the bed shear stress has decreased by 44.2 and 54.6%, respectively. The vortex frequency is a linear function of Rerod and the increasing rate of vortex frequency versus Rerod in the middle of the divergence is higher than the end. In the zone close to the vertical interface between the main channel and the floodplain, the secondary currents have suddenly reached their maximum and minimum values.
Conclusion:
The results showed that with emergent vegetation, Kelvin-Helmholtz instability caused the generation of primitive Von Kàrmàn vortex streets in downstream of the elements. The existence of vegetation in the floodplain caused a sharp reduction in the bed shear stress in this region and increased it in the main channel. As the vegetation density increased, so did the drag coefficient and flow friction factor significantly. The flow passing through the vegetation was controlled by coherent vortices whose maximum size was in the interface between the main channel and the floodplain.

Keywords


Ahmad, M., Ghani, U., Anjum, N., Pasha, G.A., Ullah, M.K. and Ahmed, A. (2020). Investigating the flow hydrodynamics in a compound channel with layered vegetated floodplains. Civil Engineering Journal, 6(5), 860-876.
Barrios-Piña, H., Ramírez-León, H., Rodríguez-Cuevas, C. and Couder-Castañeda, C. (2014). Multilayer numerical modeling of flows through vegetation using a mixing-length turbulence model. Water, 6(7), 2084-2103.
Bousmar, D. and Zech, Y. (1999). Momentum transfer for practical flow computation in compound channels. Journal of hydraulic engineering, 125(7), 696-706.
Bousmar, D. and Zech, Y. (2004). Velocity distribution in non-prismatic compound channels. In Proceedings of the Institution of Civil Engineers-Water Management, 157(2), 99-108, Thomas Telford Ltd.
Bousmar, D., Wilkin, N., Jacquemart, J.H. and Zech, Y. (2004). Overbank flow in symmetrically narrowing floodplains. Journal of hydraulic engineering, 130(4), 305-312.
Das, B.S. and Khatua, K.K. (2018). Flow resistance in a compound channel with diverging and converging floodplains. Journal of Hydraulic Engineering, 144(8), 04018051.
Das, B.S. and Khatua, K.K. (2019). Water surface profile computation for compound channel having diverging floodplains. ISH Journal of Hydraulic Engineering, 25(3), 336-349.
Das, B.S., Khatua, K.K. and Devi, K. (2017). Numerical solution of depth-averaged velocity and boundary shear stress distribution in converging compound channels. Arabian Journal for Science and Engineering, 42(3), 1305-1319.
Dean, R.G. and Dalrymple, R.A. (1984). Water wave mechanics for engineers and scientists. In Unknown Host Publication Title. Prentice-Hall Inc, 368 p.
Devi, K., Das, B.S., Khuntia, J.R. and Khatua, K.K. (2018). Analytical solution of non-uniform flow in compound channel. In E3S Web of Conferences, 40, p. 06041, EDP Sciences.
Dupuis, V., Proust, S., Berni, C., Paquier, A. and Thollet, F. (2015, June). Open-channel flow over longitudinal roughness transition from highly submerged to emergent vegetation. E-proceedings of the 36th IAHR World Congress,28 June – 3 July, 2015, The Hague, the Netherlands.
Ghisalberti, M. and Nepf, H.M. (2004). The limited growth of vegetated shear layers. Water Resources Research, 40(7), W07502, doi:10.1029/2003WR 002776
Hamidifar, H., Omid, M.H. and Keshavarzi, A. (2016). Kinetic energy and momentum correction coefficients in straight compound channels with vegetated floodplain. Journal of Hydrology, 537, 10-17.
Hu, C., Ji, Z. and Guo, Q. (2010). Flow movement and sediment transport in compound channels. Journal of Hydraulic Research, 48(1), 23-32.
Jafari, A., Ghomeshi, M., Bina, M. and Kashefipour, S.M. (2011). A new equation for simulating strouhal number of wave frequency due to flow passing through cylinder obstacles. Irrigation Sciences and Engineering (JISE), 34(1), 45-54, (in Persian).
James, C.S., Birkhead, A.L., Jordanova, A.A. and O'sullivan, J.J. (2004). Flow resistance of emergent vegetation. Journal of Hydraulic Research, 42(4), 390-398.
Jing, H., Li, C., Guo, Y. and Xu, W. (2011). Numerical simulation of turbulent flows in trapezoidal meandering compound open channels. International journal for numerical methods in fluids, 65(9), 1071-1083.
Keulegan, G.H. (1938). Laws of turbulent flow in open channels. US: National Bureau of Standards, (21), 707-741.
Knight, D.W. and Demetriou, J.D. (1983). Flood plain and main channel flow interaction. Journal of Hydraulic Engineering, 109(8), 1073-1092.
Knight, D.W. and Hamed, M.E. (1984). Boundary shear in symmetrical compound channels. Journal of Hydraulic Engineering, 110(10), 1412-1430.
Knight, D.W. and Shiono, K. (1990). Turbulence measurements in a shear layer region of a compound channel. Journal of hydraulic research, 28(2), 175-196.
Koftis, T. and Prinos, P. (2018). Reynolds stress modelling of flow in compound channels with vegetated floodplains. Journal of Applied Water Engineering and Research, 6(1), 17-27.
Kothyari, U.C., Hayashi, K. and Hashimoto, H. (2009). Drag coefficient of unsubmerged rigid vegetation stems in open channel flows. Journal of Hydraulic Research, 47(6), 691-699.
Lu, S. and Chen, J. (2014). Effects of Rigid Vegetation on the Turbulence Characteristics in Sediment-Laden Flows. Journal of Applied Mathematics and Physics, 2(12), 1091-1098.
Mulahasan, S., Stoesser, T. and McSherry, R. (2017). Effect of floodplain obstructions on the discharge conveyance capacity of compound channels. Journal of Irrigation and Drainage Engineering, 143(11), 04017045.
Musleh, F.A. and Cruise, J.F. (2006). Functional relationships of resistance in wide flood plains with rigid unsubmerged vegetation. Journal of hydraulic engineering, 132(2), 163-171.
Naik, B., Khatua, K.K., Padhi, E. and Singh, P. (2018). Loss of energy in the converging compound open channels. Arabian Journal for Science and Engineering, 43(10), 5119-5127.
Pasche, E. (1984). Turbulence mechanism in natural streams and the possibility of  its mechanical representation. Mitteilungen Institut für Wasserbau and Wasserwirtschaft, (52).
Pasche, E. and Rouvé, G. (1985). Overbank flow with vegetatively roughened flood plains. Journal of Hydraulic Engineering, 111(9), 1262-1278.
Proust, S., Riviere, N., Bousmar, D., Paquier, A., Zech, Y. and Morel, R. (2006). Flow in compound channel with abrupt floodplain contraction. Journal of hydraulic engineering, 132(9), 958-970.
Rameshwaran, P. and Shiono, K. (2007). Quasi two-dimensional model for straight overbank flows through emergent. Journal of Hydraulic Research, 45(3), 302-315.
Rezaei, B. and Knight, D.W. (2009). Application of the Shiono and Knight Method in compound channels with non-prismatic floodplains. Journal of Hydraulic Research, 47(6), 716-726.
Rezaei, B. and Knight, D.W. (2011). Overbank flow in compound channels with nonprismatic floodplains. Journal of Hydraulic Engineering, 137(8), 815-824.
Roshko, A. (1954). A new hodograph for free-streamline theory, https://digital.library.unt.edu /ark:/67531/metadc57027/.
Sanjou, M. and Nezu, I. (2011). Turbulence structure and concentration exchange property in compound open-channel flows with emergent trees on the floodplain edge. International journal of river basin management, 9(3-4), 181-193.
Sanjou, M., Nezu, I., Suzuki, S. and Itai, K. (2010). Turbulence structure of compound open-channel flows with one-line emergent vegetation. Journal of Hydrodynamics, 22(1), 560-564.
Sarkar, A. (2012). Vortex-excited transverse surface waves in an array of randomly placed circular cylinders. Journal of Hydraulic Engineering, 138(7), 610-618.
Schlichting, H. (1968). Boundary layer theory. McGraw-Hill, New York, 817 p.
Sellin, R.H.J. (1964). A laboratory investigation into the interaction between the flow in the channel of a river and that over its flood plain. La Houille Blanche, (7), 793-802.
Shiono, K. and Knight, D.W. (1991). Turbulent open-channel flows with variable depth across the channel. Journal of Fluid Mechanics, 222, 617-646.
Sonnenwald, F., Stovin, V. and Guymer, I. (2019). Estimating drag coefficient for arrays of rigid cylinders representing emergent vegetation. Journal of Hydraulic Research, 57(4), 591-597.
Stoesser, T., Kim, S.J. and Diplas, P. (2010). Turbulent flow through idealized emergent vegetation. Journal of Hydraulic Engineering, 136(12), 1003–1017.
Stoesser, T., Kim, S.J. and Diplas, P. (2010). Turbulent flow through idealized emergent vegetation. Journal of Hydraulic Engineering, 136(12), 1003-1017.
Sun, X. and Shiono, K. (2009). Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel. Advances in Water Resources, 32(3), 430-438.
Sun, X., Shiono, K., Fu, X.Y., Yang, K.J. and Huang, T.L. (2013). Application of Shiono and Knight method to compound open channel flow with one-line emergent vegetation. In Advanced Materials Research. Trans Tech Publications Ltd. 663, 930-935.
Takemura, T. and Tanaka, N. (2007). Flow structures and drag characteristics of a colony-type emergent roughness model mounted on a flat plate in uniform flow. Fluid dynamics research, 39(9-10), 694.
Tang, X. and Knight, D.W. (2009). Lateral distributions of streamwise velocity in compound channels with partially vegetated floodplains. Science in China Series E: Technological Sciences, 52(11), 3357-3362.
Tang, X., Knight, D.W. and Sterling, M. (2011). Analytical model for streamwise velocity in vegetated channels. Proceedings of the Institution of Civil Engineers-Engineering and Computational Mechanics, 164(2), 91-102.
Terrier, B. (2010). Flow characteristics in straight compound channels with vegetation along the main channel. Ph.D Thesis, Loughborough University, 353p.
Terrier, B., Robinson, S., Shiono, K., Paquier, A. and Ishigaki, T. (2010). Influence of vegetation to boundary shear stress in open channel for overbank flow. River Flow 2010, 285-292.
Västilä, K., Järvelä, J. and Koivusalo, H. (2016). Flow–vegetation–sediment interaction in a cohesive compound channel. Journal of Hydraulic Engineering, 142(1), 04015034.
Wang, W.J., Peng, W.Q., Huai, W.X., Katul, G.G., Liu, X.B., Qu, X.D. and Dong, F. (2019). Friction factor for turbulent open channel flow covered by vegetation. Scientific reports, 9(1), 1-16.
Yang, J.Q. and Nepf, H.M. (2019). Impact of vegetation on bed load transport rate and bedform characteristics. Water Resources Research, 55(7), 6109-6124.
Yang, K., Cao, S. and Knight, D.W. (2007). Flow patterns in compound channels with vegetated floodplains. Journal of Hydraulic Engineering, 133(2), 148-159.
Yang, K., Nie, R., Liu, X. and Cao, S. (2013). Modeling depth-averaged velocity and boundary shear stress in rectangular compound channels with secondary flows. Journal of Hydraulic Engineering, 139(1), 76-83.
Yonesi, H.A., Omid, M.H. and Ayyoubzadeh, S.A. (2013). The hydraulics of flow in non-prismatic compound channels. J Civil Eng Urban, 3(6), 342-356.
Zdravkovich, M.M. (1987). The effects of interference between circular cylinders in cross flow. Journal of fluids and structures, 1(2), 239-261.
Zhang, M., Jiang, C., Huang, H., Nanson, G.C., Chen, Z. and Yao, W. (2017). Analytical models for velocity distributions in compound channels with emerged and submerged vegetated floodplains. Chinese Geographical Science, 27(4), 577-588.
Zhao, K., Cheng, N.S. and Huang, Z. (2014). Experimental study of free-surface fluctuations in open-channel flow in the presence of periodic cylinder arrays. Journal of Hydraulic Research, 52(4), 465-475.
Zong, L. and Nepf, H. (2010). Flow and deposition in and around a finite patch of vegetation. Geomorphology, 116(3-4), 363-372.
 
  • Receive Date: 07 January 2021
  • Revise Date: 17 January 2021
  • Accept Date: 21 January 2021
  • First Publish Date: 21 January 2021