Nature-Based Aeration at In-Stream Weir: Influence of Hydraulic Jump Configuration on Air Entrainment and Water Quality

Self-aeration in open-channel flows is an important mechanism for improving the water quality of polluted rivers. In polluted rivers, natural oxygen exchange across the free surface is often inadequate, and reoxygenation takes place after several kilometers of downstream flow. Hydraulic structures, such as weirs, induce air entrainment, enhancing oxygen transfer by entraining air bubbles into highly turbulent flows over short river reaches.  In this study, experiments were conducted to examine air–water flow characteristics and oxygen transfer downstream of a rectangular weir under various hydraulic jump configurations. Three distinct flow regimes were investigated for the same drop height: (i) a hydraulic jump formed when the deflected jet became parallel to the channel bed, (ii) a hydraulic jump formed when the deflected jet was non-parallel to the bed, and (iii) a submerged hydraulic jump beneath a plunging jet characterized by a wavy flow with breaking waves. The results indicate longitudinal and vertical variations in air concentration for all cases. Maximum air entrainment occurred near the toe of the hydraulic jump and decreased downstream due to bubble rise, coalescence, and release at the free surface, while vertical profiles showed increasing air concentration toward the free surface due to buoyancy effects. The average air concentration was highest for the non-parallel deflected jet (0.43), followed by the submerged jump (0.22) and the parallel deflected jet (0.18). Correspondingly, the oxygen transfer efficiency was 0.11, 0.13, and 0.08 for the parallel, non-parallel, and submerged hydraulic jump cases, respectively, indicating maximum aeration performance for the non-parallel deflected jet condition and the least for the submerged jump.