Local surface aeration at hydraulic structures

In open channel flows, local surface aeration processes occur mainly at weirs and drop structures and in hydraulic jumps. Air entrainment in the wakes of bluff bodies is observed only in high speed flows. In water pollution abatement, the natural gas exchange at the free surface is sometimes not sufficient to balance the oxygen consumption due to pollutants. In such cases, local aeration is needed in order to increase the oxygen transfer into the water body. In this context, hydraulic structures leading to local surface aeration provide the increased oxygen transfer by way of the entrained air bubbles. Thus the air-entrainment and oxygen-transfer characteristics have become an important design criterion for many hydraulic structures. Contrary to the structural design considerations, which would seek to avoid air entrainment, water quality considerations are geared towards increasing the air entrainment rates. Local surface aeration processes are a complex phenomena which depend sensitively upon many parameters. The boundary geometry and the absolute scales of geometry and flow rates are extremely important. Therefore, no generally applicable design criteria can be formulated. Each of the flow configurations described in Chapter 1 and each of the various limitations of the entraining process have to be studied in detail. The following material is a summary of the available information about local air entrainment and oxygen transfer, and it is intended to serve as an orientation for preliminary design considerations. Limited results can be presented for a small number of flow geometries. These results can only be used for similar geometries; the flow configurations must really be comparable. Application of these results to other flow configurations could lead to erroneous estimates. The majority of results presented here stems from laboratory investigations at small scales. These data can only be safely applied within the range of scales and · discharges in which they have been obtained. This range is therefore quoted for all