Observations et mesures effectuées sur les coins salés du grand et petit Rhône

¹ Directeur-adjoint à la Compagne Nationale du Rhône
² Chef du Service des Etudes Générales à la Compagnie Nationale du Rhône
³ Chef de la Division Hydraulique, Hydrologie, Statistique à la Compagnie Nationale du Rhône

Résumé

As part of its hydrological research programme, the Compagnie Nationale du Rhône has undertaken measurements on salt-water penetration from the Mediterranean into the two branches of the Rhone delta, the so-called Petit-Rhône and Grand-Rhône. Experimental apparatus includes fixed level-recorders used to determine levels corresponding to given densities (Fig. 1, 2, 3 and 4) and an ultrasonic probe which gives directly the lengthwise interface level variation (Fig. 7, 8 and 9). Density is also measured directly by grab sampling. Figures 5, 6, 8 and 9 give an idea of salt-wedge configuration. It can be seen that the interfacial thickness varies within wide limits and that it is affected by holes and sills. Results point up the factors which affect salt-wedge behaviour. These are : (i) river discharge-effect is as predicted by theory (Fig. 10 and 11) ; (ii) tidal effects in the Mediterranean, though weak, are sufficient to generate progressive waves, the amplitude of which is greater at the interface than the free surface (fourfold amplification) (Fig. 12); (iii) wind action, which cannot be neglected, is complicated since it affects both the level of the sea and the fresh-water surface (where a shear stress is set up). It appears that a moderate wind blowing off the sea (south wind) has a tendency to drive the salt wedge towards the sea, whereas moderate winds off the land (north winds) will drive it inland. Strong winds, however, seem to have precisely the opposite effect. Figures 13 and 14 have been used in an endeavour to analyse the combined action of the various parameters, which of course rarely operate singly. Difficulties encountered and Jack of precision in the results would seem to be due to the fact that steady-state conditions are never really reached. This is why it is thought that future progress is more likely to come from continued study of the mathematical salt-wedge model sep up a few years back by the Laboratoire d'Hydraulique at Chatou.