Aeration aux ouvrages hydrauliques.

The design and management of dams must, first of all, form part of the management framework for hydrographic networks which must meet a wide range of objectives. Nowadays, criteria associated with the maintenance of river quality and general living standards are linked to more traditional criteria regarding improved conditions for river navigation and a reduction in the risk of floods. Dams are privileged sites along watercourses where oxygen transfer is fairly intense in the liquid phase, thereby improving the quality of the river. This is especially true when the dam is situated on a stretch of water where dissolved oxygen content is not very high. These structures, therefore, significantly contribute to efforts at river improvement, which is, moreover, carried out to maintain or improve the quality of the surface water. However, the design of new dams allowing for bottom flow and other new uses, such as electricity generation (hydroelectric turbines), may seriously reduce this positive aspect. It is, thus, vital to know the levels of oxygen transfer associated with the different types of riverflow, in order to quantify the expected effects of any modifications, thus optimizing dam management. For example, a cheap and original way of improving the river's dissolved oxygen content would be to pass the underflow with part of the liquid flowing vertically (free overfall) over the hydraulic jump. Moreover, various other structures are at present under study, which may either increase the overflowing discharge air intake or improve the downstream rate of oxygen exchange. Comparison of the specific oxygen contribution made by dams with those made by artificial aeration facilities shows that the efficiency of dams is at least comparable if not higher. It would, therefore, be advisable to make better use of this capacity. This does not, however, mean that we suggest building dams to aerate rivers, although this is really very effective and thus their impact ought to be the subject of a more general study. However, when a dam is planned or has been built for other reasons, it is obviously a good idea to make the maximum use of the possibilities provided by this structure.