Experimental investigation of surface and sub-surface flow interaction in a middle bar

Hyporheic exchange has a crucial effect in ecology and hydrology cycle. Hyporheic flows are highly influenced by stream morphologies like Middle-gravel bars. In this study, flows around a three dimensional symmetric middle gravel bar) 2 m length, 0.64 m width and 0.1 meter height) were investigated. To consider the submergence effect, the experiments were conducted in partial and fully submerged cases. Flow and pressure patterns around middle bar were simulated by a CFD model, then the computed pressure along the bed were assigned as a top boundary condition to simulate the three dimensional subsurface flow using groundwater model. The results showed that dimensionless index of exchange flow decreased by increasing surface flow and have a good linear correlation with Reynolds number in porous media of the bar. The ratio of exchange flow to surface flow was 3.5 to 7.5 percents. Spatial expansion of flow paths increased by increasing discharge and in partial submerged cases, flow paths extended like two dimensional pool-riffles and dunes laterally. Residence time and length of Path lines were log normally distributed which tends to symmetric form by increasing discharge. Dimensionless median residence time is decreased by increasing discharge and ranged from 0.004 to 0.01.