Riprap sizing for scour protection at river confluence

Document Type: Research Paper


1 Department of Water Science Engineering, Shahrkord University, Shahrkord, Iran

2 Department of Water Science Engineering, Shahid Chamran University, Ahvaz, Iran

3 Department of Water Science Engineering, Lorestan University, Khoram Abad, Iran


River confluence is a common feature of most irrigation and drainage channels and river systems, where tributary conflicts the main channel. In these section, rapid changes in velocity and discharge, sediment distribution and flow turbulent result in a deep confluence scour, a bar point in the separation zone at downstream of junction corner  and finally vortex flow. Thus, the main goal of this study is to conduct a series of experimental tests to investigate the required size of rocks to control the scour hole. The results show that for a constant ratio discharge, Qr, the size of riprap in the incipient motion increases with decreasing in tailwarer depth. In other words, for any rock size the tailwater depth required for incipient motion increases with increasing the ratio of discharge, Qr. For each constant ratio discharge, or, the size of riprap in the incipient motion increases with increasing in tailwater velocity, Vt. Finally, some equations are presented for predicting the size of rocks and the proposed equations has been compared with existing ones.


  1. N.B. Webber, and C.A. Greated, 1966. An investigation of flow behavior at the junction of rectangular channel. Proc. Instn. Civ. Engres., 34: 321-334.
  2. J.D. Lin and H.K. Soong, 1979. Junction losses in open-channel flows. Water Resour. Res., 15: 414-418.
  3. J.L. Best and I. Reid, 1984. Separation zone at open-channel junctions. J. Hydraulic Eng., 110: 1588-1594.
  4. A.S. Ramamurthy, L.B. Carballada and D.M. Tran, 1988. Combining open channel flow at right angled junctions. J. Hydraulic Eng., 114: 1449-1460.
  5. W.H. Hager, 1989. Transition flow in channel junctions. J. Hydraulic Eng., 115: 243-259.
  6. S.K. Gurram, K.S. Karki and W.H. Hager, 1997. Subcritical junction folw. J. Hydraulic Eng. ASCE, 123: 447-455.
  7. C.C. Hsu, F.S. Wu and W.L. Lee, 1998. Flow at 90 equal-width open-channel junction. J. Hydraulic Eng. ASCE, 124: 186-191.
  8. C.C. Hsu, W.J. Lee and C.H. Chang, 1998. Subcritical open-channel junction flow. J. Hydraulic Eng. ASCE, 124: 847-855.
  9. K.F. Bradbrook, P.M. Biron, S.N. Lane, K.S. Richards and A.G. Roy, 1998. Investigation of controls on secondary circulation in a simple confluence geometry using a three-dimensional numerical model. Hydrol. Processes, 12: 1371-1396.
  10. S.N. Lane, K.F. Bradbrook, K.S. Richards, P.A. Biron and A.G. Roy, 1999. The application of computational fluid dynamics to natural river channels: Three-dimensional versus two-dimensional approaches. Geomorphology, 29: 1-20.
  11. S.B. Weerakoon, Y. Kawahara and N. Tamai, 1991. Three-dimensional flow structure in channel confluences of rectangular section. Proceedings of 24th Congress of International Association for Hydraulic Research, September 1991, Madrid, Spain, pp: 373-380.
  12. P.M. Biron, A. Richer, D.A. Kirkbride, G.A. Roy and S. Han, 2002. Spatial patterns of topography at a river confluence. Earth Surf. Processes Land Forms, 28: 913-928.
  13. L.J. Weber, E.D. Schumate and N. Mawer, 2001. Experimentals on flow at a 90° open-channel Junction. J. Hydraulic Eng. ASCE, 127: 340-350.
  14. J.L. Huang, L.J. Weber and G.L. Yong, 2002. Three-Dimensional numerical study of flows in open-channel junctions flow. J. Hydr. Engrg., 128: 268-280.
  15. M.P. Mosley, 1976. An experimental study of channel confluences. J. Geol., 85: 535-562.
  16. J.L. Best, 1988. Sediment transport and bed morphology at river channel confluences. Sedimentology, 35: 481-498.
  17. A.G. Roy, R. Roy and N. Bergeron, 1988. Hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material. Earth Surface Processes Land forms, 13: 583-598.
  18. B.L. Rhoads and S.T. Kenworthy, 1998. Time-averaged flow structure in the central region of a stream confluence. Earth Surface Processes Landforms, 23: 171-191.
  19. B.L. Rhoads and A.N. Sukhodolov, 2001. Field investigation of three-dimensional flow structure at stream confluences: 1.Thermal mixing and time-averaged velocities. Water Resour. Res., 37: 2393-2410.
  20. R.B. Bryan, and N.J. Kuhn, 2002. Hydraulic conditions in experimental rill confluences and scour in erodible soils. Water Resour. Res., 38: 1-22.
  21. C. Boyer, A.G. Roy and J.L. Best, 2006. Dynamics of a river channel confluence with discordant beds: Flow turbulence, bed load sediment transport and bed morphology. J. Geophys. Res., 111: F04-F04.
  22. D.R. Parsons, J.L. Best, S.N. Lane, O. Orfeo, J.R. Hrady and R. Kostaschuk, 2007. Form roughness and the absence of secondary flow in a large confluence-diffluence rio parana, argentina. Earth Surface Processes Landforms, 32: 155-162.
  23. R. Ghobadian, and M. Shafai Bejestan, 2007. Investigation of sediment patterns at river confluence. J. Applied Sci., 7: 1372-1380.
  24. R. Ghobadian, 2007. Investigation of flow, scouring and sedimentation at river-channel confluences. Ph.D. Thesis, Department of Hydraulic Structures, Shahid Chamran University, Ahwaz, Iran.
  25. E. Ghanbari Adivi, M. Shafai Bajestan and M. Saghi, 2013. Laboratory study of stability in the riprap materials of bed at the confluence of rivers. J. Water and soil Sci. 24: 43-58.