Oxygen transfer variation along steps of a prototype-scale stepped spillway in nappe flow regime

Based on long-term data, oxygen levels have decreased in temperate natural lakes because of thermal stratification and
eutrophication. Similar conditions can be observed in newly filled reservoirs, with consequences on the environment. Air-water
flows taking place on hydraulic structures are helpful to increase the dissolved oxygen concentration. However, due to the complex
nature of the air-water flows and scale effects distorting results from scale physical models, investigations are still needed to better
characterize the effect of flow past hydraulic structures on dissolved oxygen concentration. In this paper, laboratory experiments
conducted at the Engineering hydraulics laboratory (HECE) at Liege University on a prototype scale 15° stepped spillway made
of 6 steps 50 cm high and operated in nappe flow conditions are presented. The objective of these tests was to document the oxygen
transfer profile along different steps by means of direct dissolved oxygen concentration measures. The concentration of dissolved
oxygen was continuously measured using optical DO sensors and validated by Winkler tests. The optical sensors yield reliable
results when not placed in bubbly flows or under jet impact. The results showed that, on individual steps, aeration efficiency
decreases with an increase in flow rate. The filling of the cavity below the nappe accelerates this reduction. Aeration efficiency
from the upstream edge to the cavity is slightly higher than from upstream edge to downstream edge of a step when the cavity is
not full of water.