A Comparative Study of Oxygen Transfer Over Contracted and Uncontracted Weirs

Rapid population growth and urbanization in India have significantly increased the discharge of untreated effluents into rivers, resulting in deteriorating water quality. Hydraulic structures, such as weirs, offer an effective solution to improve water quality in urban drains and streams by enhancing oxygen transfer and minimizing pollution loads. While natural streams often require several kilometers to re-oxygenate, weirs can achieve substantial oxygenation within shorter distances. This study investigates the oxygen transfer efficiency of contracted and uncontracted rectangular weirs through laboratory experiments conducted in a flume measuring 15 m in length, 0.5 m in width, and 0.75 m in depth. Dissolved oxygen concentrations were measured upstream and downstream of the weirs to evaluate oxygen transfer efficiency. The results show that contracted rectangular weirs outperform uncontracted weirs in oxygen transfer efficiency, with increased contraction leading to higher efficiency. Furthermore, the efficiency was found to increase with head loss over the weir and with the downstream Froude number. Hydraulic jumps formed downstream of the weir further contributed to oxygen transfer by entraining air bubbles into the flow. Using a phase detection probe, air concentration measurements revealed that the highest air concentration occurred near the jump toe, with depth-averaged air concentration decreasing with distance from the toe.