Laboratory and Numerical Investigation of the Effect of Riffle-Pool Bed-Form Areas on Pollution Transmission in Gravel-Bed Rivers

Document Type : Research Paper


1 Department of Water Engineering, Sari Agricultural Sciences and Natural Resources University, Sari, Iran.

2 Department of Water Engineering, Sari Agricultural Sciences and Natural Resources University,sari,iran

3 Associated Professor, Dep. of Water Engineering, Faculty of Water and Soil, Gorgan University of Agricultural Sciences and Natural Resources, Golestan.

4 Department of Water Engineering, Sari Agricultural Sciences and Natural Resources University ,Sari ,Iran


In the present study, the efficiency of the advection-dispersion equation in simulation of the pollution transport through the Gravel-Bed Rivers with Riffle-Pool bed-form was investigated. Experiments of tracer material (NaCl) were performed in a flume with a length of 11 m, width of 0.5 m and height of 0.7 m and with a longitudinal slope of 0.006 in three flow discharges (7.5, 10 and 12.5 lit/s). Four bed-forms of Riffle-Pool with different heights and wavelengths were considered to simulate hyporheic exchanges. The laboratory results were also simulated by the OTIS numerical model. The laboratory results showed that in bedless-form flow, increasing the flow rate increases the longitudinal dispersion coefficient. The opposite of this trend was observed at the presence of bed-form due to hyporheic exchanges. Increasing the height of the bed-form increases the Reynolds number in the hyporheic zone and consequently hyporheic exchanges increase and the longitudinal dispersion coefficient increases. Simultaneous increase of flow rate and the bed-form height causes excessive increase of hyporheic exchanges. Therefore, the residence time of the pollution in the sedimentary bed area is reduced and as a result, the pollution returns to the main flow area with less temporary storage in the storage zones. Therefore, increasing the longitudinal dispersion coefficient with increasing the bed-form height in the range of high flow rates is not significant. Increasing the wavelength of the bed-form also increases the residence time of contamination in the hyporheic zone, which increases the longitudinal dispersion coefficient. Increasing the flow rate reduces the role of hyporheic exchanges so that the main volume of pollution is transferred to the main flow area. Therefore, the effect of increasing the wavelength of the bed-form on the longitudinal dispersion coefficient decreases with increasing the flow rate. The comparison of laboratory results with numerical solution of OTIS model shows the high accuracy of this model in predicting the transmission of contamination.


Azhdan, Y., Emadi, A., Chabokpour, J. and Daneshfaraz, R. (2018, a). Experimental investigation and evaluation results of numerical simulation and analytical solution of classical ADE for conservative solutes. Journal of Water and Soil Resources Conservation, 8(2), 39-55.
Azhdan, Y., Emadi, A., Chabokpour, J. and Daneshfaraz, R. (2018, b). Experimental and Numerical Study of Advection- Dispersion of Pollutant in a Gravel Bed Rivers. Journal of Water and Soil Sciences, 28(4), 127-140.
Biddulph, M. )2015(. Hyporheic Zone: In Situ Sampling, Geomorphological Techniques. Chapter 3, Section 11.1.
Boano, F., Harvey, J. W., Marion, A., Packman, A. I., Revelli, R., Ridolfi, L. and Wörman, A. (2014). Hyporheic flow and transport processes: Mechanisms, models, and biogeochemical implications. Reviews of Geophysics, 52(4), 603-679.
Cardenas, M. B. and Wilson, J. L. (2006). The influence of ambient groundwater discharge on hyporheic zones induced by current-bedform interactions. Journal of Hydrology, 331(1-2), 103–109.
Carling, P. and Orr, H. G.  (2000). Morphology of riffle-pool sequences in the River Severn, England. Earth Surf. Processes Landforms, 25(4), 369 – 384.
Chanson, H. (2004). Environmental hydraulics of open channel flows. Elsevier Butterworth-Heinemann Linacre House, Jordan Hill, Oxford.
Kashefipour, S. M. and Falconer, R. A. (2002). Longitudinal dispersion coefficients in natural channels. Water Research, 36(6), 1596–1608.
Keller, E. A. and Melhorn, W. N. (1978). Rhythmic spacing and origin of pools and riffles. Geol. Soc. Am. Bull., 89(5), 723 – 730.
Leopold, L. B., Wolman. M. G. and Miller, J.P. (1964). Fluvial Processes in Geomorphology. W. H. Freeman, New York, 522 pp.
Mahmoodian-Shooshtari, M. (2009). Principles of flow in open channels (2nd ed). Iran, Ahwaz: Shahid Chamran University. (In Farsi)
Meddah,  S.,  Saidane,  A.,  Hadjel,  M.  and  Hireche,  O.  (2015).  Pollutant  dispersion modeling in natural streams using the transmission line matrix method.  Journal of Water, 7(12), 4932-4950.
Montgomery, D. R., Buffington, J. M., Smith, R. D., Schmidt, K. M. and Pess, G. (1995). Pool spacing in forest channels. Water Resource. Res., 31(4), 1097–1105.
Rodríguez, J. F, García, C. M. and García, M. H. (2013). Three‐ dimensional flow in centered pool‐riffle sequences. Water Resources Research, 49(1), 202-215.
Seo, I. W. and Cheong, T. S. (2001). Moment-based  calculation of  parameters  for the storage zone model for river dispersion. Journal of Hydraulic Engineering, 127(6), 453-465.
Sokác, M. (2017). Determination of the longitudinal dispersion coefficient in lowland streams with occurrence of dead zones. Environmental Engineering 10th  International Conference, Vilnius Gediminas Technical University Lithuania, 27-28 April.
Stonedahl, S. H. (2011). Investigation of the Effect Multiple Scales of Topography on Hyporheic Exchange. PhD Dissertation, Northwestern University.
Thibodeaux, L. J. and Boyle, J. D. (1987). Bedform-Generated convective transport in bottom sediment. Nature: 325(6102), 341-343.
Tonina, D. and Buffington, J. M. (2007). Hyporheic exchange in gravel bed rivers with pool‐riffle morphology: Laboratory experiments and three‐dimensional modeling. Water Resources Research, 43(1), 1-16.
Trauth, N., Schmidt, C., Maier, U., Vieweg, M. and Fleckenstein, J. H. (2013). Coupled 3‐D stream flow and hyporheic flow model under varying stream and ambient  groundwater flow conditions in a pool‐riffle system. Water Resources Research, 49(9), 5834-5850.
Zeng,  Y.  and  Huai,  W.  (2014).  Estimation  of  longitudinal  dispersion  coefficient  in rivers. Journal of Hydro- Environment Research, 8(1), 2-8.
Zhou, T. and Endreny, T. A. (2013). Reshaping of the hyporheic zone beneath river restoration structures: Flume and hydrodynamic experiments. Water Resources Research, 49(8), 5009-5020.