Hydrothermal regulation in cold-region river restoration: Coupled effects of ice-cover and in-stream structures

In river ecological restoration projects within cold regions, in-stream structures are commonly implemented to enhance habitats, while seasonal ice covers significantly modify hydrodynamic and thermal conditions. Their complex effects on key processes in the hyporheic zone, such as hydraulic exchange and heat transport, require urgent clarification. Therefore, we propose a three-dimensional surface water and two-dimensional groundwater coupled numerical model, and systematically investigate the influence of in-stream structures combined with ice covers of varying roughness on the pressure distributions, thermal regimes, and hyporheic exchange processes in river systems. The results indicate that (i) the synergistic effect of ice cover roughness and in-stream structures significantly alters bed pressure distribution; the peak pressure difference across the log increases from 61.9 to 105.9 Pa with the increasing roughness, thereby enhancing the intensity of both downwelling and upwelling flows; (ii) with the effect of ice covers, a semi-elliptical relatively colder zone forms beneath the in-stream structures, with its area expanding by 20%–30% as ice roughness increases; however, the insulating effect of the ice layer reduces the overall thermal gradient; (iii) thermal heterogeneity in the hyporheic zone is markedly enhanced; under high-roughness ice cover, the peak Darcy velocity increases by approximately 68.6%, from 0.015 to 0.0253 m/s, accelerating heat exchange between surface water and the hyporheic zone; (iv) the sensitivity ranking of the factors influencing the hyporheic exchange process is as follows: inflow discharge ice cover roughness permeability streambed length ≈ hyporheic zone depth. This research establishes a theoretical foundation for optimizing instream structure parameters in cold-region river restoration projects.