L'ouverture du nouveau chenal d'accès au port de Rouen dans l'estuaire de la Seine

Abstract

1. The improvement of navigation conditions in a tidal river frequently requires major changes to be made to its channel position and characteristics, and the less the natural conditions lend themselves to the maintaining of a deep channel, the more radical the changes required. Generally speaking, these can be achieved either by the progressive construction of training works or by a cutting made to rapidly connect up two bodies of water separated by banks or dykes. Both methods were resorted to in opening up the new Rouen harbour shipping channel, by first cutting openings in a dyke and a sand bank and then following up with progressive construction work. 2. The most delicate part of the operation was to change the channel between Ronfleur and the Risle confluence, which had to be done without interrupting navigation on the river. This was achieved by excavating a thirty-million cubic metre capacity basin out of an emergent sand bank and by connecting this basin to the estuary, first by the piercing of an opening in its southern end, and then by gradually opening up its northern end as well. The embankment marking the future channel ran along the southern edge of the basin. 3. The diversion of increasing proportions of the tidal flow into the new channel was made easier not only by the progressive closure of the old channel, but also because the new channel layout backed up against a concave embankment, and because of the increased flow cross-section and lower flow resistance resulting from dredging operations carried on at the time. 4. Considerable unwelcome deep scouring is likely to occure where flows are concentrated in narrow cross-sections ; as the banks this forms are a shipping hazard, such conditions should only be allowed to remain for the minimum possible length of time. The sudden method scores to some extent in this respect, and was already used successfully during the last century to open the Tancarville bend in the tidal part of the Seine. A more recent example was the opening of a new channel at Ronfleur. It should not be used, however, unless it has been confirmed - for instance by tests on a loose-bed model - that the flow will have a natural tendency to follow the new channel, and that the available dredding facilities provide an adequate means of correcting the channel irregularities that are bound to occure until it has finally settled down. 5. Generally speaking, any major modification of the alignment and dimensions of a river with a wide tidal range is necessarily a difficult undertaking. Its success depends on long painstaking study, a plan of operations prepared down to the last detail, and every possible use of natural forces, over which full control must be maintained at all times.