Effect of different hydraulic conditions on the hyporheic flow charachteristics around gabion weir structures

Gabion weir at the longitudinal variation of hydraulic head causes that surface flow enters the porous medium and mix with the subsurface flow and return to the surface water flow again. This mixing takes place in the hyporheic zone play an important role in the river ecology. In this study, the flow pattern around the gabion weir structure was investigated by conducting laboratory studies. The particle pathes were traced by injection of dye and the hydraulic head profiles were measured under different hydraulic conditions. Downstram hydraulic conditions were also adjusted in order to create three flow conditions, including through flow gabion weir (TFGW), overflow gabion weir with no hydraulic jump (NFGW) and overflow gabion weir with hydraulic jump (PNGW).In order to investigate the characteristics of the hyporheic zone, the measured hydraulic head on the sediment-water interphase was introduced as a drichlet boundary to the groundwater model and flow modeling was performed in a porous media. The results show that there is a good agreement between the observed and computational flow pattern in the hyporheic zone.The observations showed that in TFGW and NFGW flow conditions, the particle pathlines are downstream directed upwelling (DDU) and in PNGW flow condition, there are both upstream directed upwelling (UDU) and downstream directed upwelling (DDU).Also the hyporheic exchange flow (Qex) was ranged between 1.63 and 2.80 l/s.The residence time in all flow conditions increases with decreasing Reynolds number. In this study, the relationships with high accuracy were presented for estimating residence time and hyporheic exchange flow.