A coupled model of surface water-groundwater interaction with the effect of riverbed deformation in the alluvial channel

• A coupled surface-subsurface model for hyporheic exchange process was proposed. • Higher weir heights led to an increase in local scour volume, hyporheic exchange flux, and temperature response area. • Local bed deformation downstream of the weir modified the subsurface pressure distribution and enhanced the hyporheic exchange flux. Hyporheic exchange process is a common mode of surface water-groundwater interaction, involving the transport of substances, nutrients and energy, which is an important foundation for river ecology. However, the effect of riverbed deformation driven by weir in natural rivers remains unclear. In this study, the effect of local scour pit created by low-head weirs on hyporheic exchange process was studied by the coupled model of surface water-groundwater. While many studies assume static riverbed conditions, this study integrates a dynamic surface water model that accounts for sediment transport and evolving scour pits, while employing a steady-state representation for subsurface flow to reflect the gradual adjustment of hyporheic exchange. The variation of the local scour depth, hyporheic exchange flux, hyporheic zone pressure field and temperature response area were investigated under different weir height and flow discharge. The results showed that: (a) the increasing weir height significantly enhanced hydrodynamic effect, with the morphological changes in scour pits altering the distribution of hyporheic zone pressure field and further intensifying surface water-groundwater exchange; (b) when the weir height increased from 15 cm to 25 cm, the scour pit depth increased from 9 cm to 25 cm, the volume of the local scour pit increased by approximately 3.3 times, and the hyporheic exchange flux increased to a maximum value of 0.011 m 2 /s; (c) the peak downwelling Darcy velocity in the hyporheic zone increased from 0.0039 m/s to 0.0067 m/s as the flow discharge increased from 0.0621 m 3 /s to 0.1200 m 3 /s; (d) the scour pit formation altered the local bed topography, then influencing the hyporheic exchange pathways and rates; the peak downwelling Darcy velocity with the riverbed scouring effect was 2.5 times higher than that of the others, with the hyporheic exchange flux increasing by 1.5 times. This study can provide theoretical insights for optimizing low-head weir designs and water management in river ecological restoration.