Model Experimental Study on the Driving Mechanism of Hyporheic Exchange Induced by Weir Structures

Weir‐induced hyporheic exchange plays a critical role in regulating nutrient cycling, contaminant transport and thermal regimes within riverine ecosystems, with significant implications for watershed management. To elucidate the underlying mechanisms, this study integrates controlled laboratory flume experiments with a coupled surface–subsurface numerical model to quantitatively assess alterations in the groundwater temperature field following weir installation and to identify the dominant factors governing exchange dynamics. Results indicate that near‐bed velocity variations exhibit a positive correlation with surface water velocity ( v ) and weir height ( h ), and a negative correlation with weir width ( d ) and water depth ( h w ). Specifically, elevating the weir height from 0.1 to 0.2 m amplified the maximum cross‐weir pressure gradient by 102%, with the value rising from 52.9 to 106.9 Pa. Concurrently, the maximum streamwise Darcy velocity exhibited an order‐of‐magnitude increase from 1.58 × 10 −5 to 2.99 × 10 −4 m/s. Furthermore, the spatial extent of the hyporheic thermal front expands significantly with increases in v and h . Numerical simulations reveal that a distinct low‐pressure zone develops immediately downstream of the weir structure, which drives the formation of upward flow fields upstream and downward flow fields downstream. These findings establish a quantitative, pressure‐driven framework linking weir geometry directly to hyporheic exchange intensity—a linkage previously unexplored in systematic flume experiments. The results provide a mechanistic basis for optimising weir designs to enhance biogeochemical processing and thermal regulation in engineered river corridors.