Investigation of the effect of step penetration depth in porous medium on the hyporheic fluxes

From environmental aspects, the exchange of surface and subsurface flows in riverbeds due to river morphology and in stream structures is very important. The flow path structures especially control structures have a more effective role than the river morphology in the formation of these exchanges. In this study, the effect of penetration depth of such structures in the porous bed on the characteristics of exchange flows was investigated both experimentally and numerically. The experiments were performed in a flume with a length of 10 m, width of 20 cm, depth of 30 cm and a slope of 0.01, at three penetration depths of 9, 11 and 13 cm respectively. Potassium permanganate tracer was used for tracking the flow. In addition, to obtain the characteristics of the exchange flow; the mainstream and the exchange pattern were simulated by particle tracking method using Flow 3D software. The results showed that in the Reynolds range of 1020 to 3450, increasing the penetration depth of the structure from 9 to 13 cm, would increase the residence time but decreases the exchange rate. However, increasing the flow rate would decrease the exchange rate and increase the retention time. Accordingly, the use of a step with a higher penetration depth is recommended to create a longer residence time. However, a step with a smaller penetration depth may be used to produce a higher exchange rate.