Abstract

Introduction: Rivers are complex systems in which different chemical, biological, and physical processes occur in it. When the flow moves along the river, there is an exchange between the surface flow and the subsurface flow. The hyporheic zone is a saturation zone below the riverbed, which plays an important role in many biological and chemical processes. Residence time is the most important characteristic of the hyporheic zone. Because the chemical and biological reactions that occur inside the sediments depend on the time at which flow paths remain in the bed for a while and then return back to the surface flow. Hyporheic exchange is the mixing of surface and subsurface flow just beneath the river bed. Such exchanges can be caused by the presence of different bedforms in the river. Dunes are one type of river bed that can be observed in straight, meander, and braided rivers. The pressure gradient between the upstream and downstream of a dune leads to hyporheic exchanges. The wavelength, amplitude, and slope of the lee side and stoss side can affect the rate of exchanges. In the present study, the effect of the dune lee side slope at angles of 10, 20, and 30 degrees on the characteristic of the hyporheic zone (i.e., residence time, exchange flow, and hyporheic depth) has been investigated numerically.
Methodology: The FLOW3D software is used for the numerical simulation of surface flow. The simulation domain consists of a flume with 2.7m length, 0.1m width, and 0.3m height. The model running time was 120 seconds for surface flow simulation, which, with the passing of this time, the flow in the channel becomes stable. The pressures along dunes are introduced as a Dirichlet boundary condition on top of the groundwater model, i.e., MODFLOW. Then, the effect of the dune lee side slope at angles of 10, 20, and 30 degrees on the characteristic of the hyporheic zone (i.e., residence time, exchange flow, and hyporheic depth) has been investigated.
Results and Discussion: The results show that the maximum and minimum pressure occurred on the stoss side and the crest of the dune, respectively. By increasing the dune lee side slope, the distance between the maximum and minimum pressure is reduced, the depth of hyporheic exchange decreases, and the exchange rate and residence time increase. Also, for all three angles, with a constant ratio of the subsurface to surface flow, the depth of hyporheic exchange increases with the increase of the hydraulic conductivity to the dune length ratio (K/A). Increasing the velocity of the subsurface flow causes the subsurface flow to dominate the surface flow and the flow in the subsurface flow moves towards the surface flow. As a result, by increasing the ratio of subsurface flow velocity to surface flow velocity, the exchange flow increases, and the depth of hyporheic exchange decreases.
Conclusion: The results show that as the lee side slope increases, the residence time, and exchange flow increase, and hyporheic depth decreases. Also, by increasing the hydraulic conductivity, the hyporheic exchange depth increases, but by increasing the subsurface flow velocity and the porous media thickness, the hyporheic exchange depth decreases.