Three-dimensional hyporheic exchange driven by local riverbed scouring below the in-stream structure

In-stream structures influence water flow, sediment transport, bed deformation, and hyporheic exchange, aiding river ecological restoration. However, the impact of local riverbed scouring (LBS) on hyporheic exchange remains unclear. This study develops a coupled surface–subsurface model incorporating LBS, validated with field data (scouring depth, temperature variations) and achieving high accuracy (NSE = 0.75–0.93). The results showed that: (i) the pressure gradient of the sediment-water interface increased by 2.3–2.7 times as the weir height increased from 15 to 25 cm, and it increased by 1.1–1.3 times as the inlet discharge increased from 0.062 to 0.12 m3/s; (ii) the pressure gradient, temperature response area, and hyporheic exchange flux of the hyporheic zone, respectively, increased by 2.5, 2.3, and 3.1 times as the weir height increased from 15 to 25 cm; (iii) the pressure gradient, temperature response area, and Darcy velocity of the hyporheic zone can be greatly improved when the LBS effect was considered in the coupled surface–subsurface model, i.e., the hyporheic exchange flux (q) increased by 1.28 times; (iv) the hyporheic exchange characteristics in the horizontal section and vertical direction were different under certain conditions. Therefore, the LBS effect on the hyporheic exchange driven by the in-stream structure needs to be considered for higher simulation accuracy, and it can also provide recommendations for river ecological restoration.