Aeration performance of triangular planform labyrinth weirs

A high level of dissolved oxygen is vital for the maintenance of healthy streams and rivers. Structures in rivers can increase dissolved oxygen levels by creating turbulent conditions where small air bubbles are carried into the bulk of the flow. Plunging overfall jets from weirs are a particular instance of this, and the aeration properties of such structures have been studied widely in the laboratory and field over a number of years. On the other hand, labyrinth weirs, where the weir sill is cranked in planform thus increasing their length, have received little or no attention in this context. They have a proven hydraulic advantage over straight weirs of increased discharge at the same head for design conditions. However, they also serve to modify the combined overfall jet as individual jets from adjacent sections of the weir collide. This paper describes an experimental investigation into the nature of these jets and how they affect the aeration performance of a triangular plan labyrinth weir. It is demonstrated that the aeration efficiency of these labyrinth weirs generally is better than their equivalent-length linear weir and that this advantage becomes more pronounced as the weir included angle becomes smaller and also at lower overfall drop heights and higher discharges. These results point to the possible advantage of these type of weir in situations where both hydraulic and aeration performance needs to be optimized.