Étude sur modèle réduit de la sédimentation dans l'estuaire de la vilaine après construction du barrage d'Arzal

A scale model study of silting in the river Vilaine due the Arzal dam

Ingénieur en Chef au Laboratoire Central d'Hydraulique de France.

Résumé

After a brief review of the previous state of the Vilaine and the Arzal scheme in course of construction, certain aspects of the sediment studies carried out by the Laboratoire Central d'Hydraulique de France for the dam project design are discussed at greater length. The most important information obtained from the field observations carried out during the project study and official statistical data are considered first, which are connected with the following: (i) Precipitation affecting the eatchment area, for which a statistical distribution law is put forward; (ii) River flow regimena; (iii) The origin of the river mue! and how it is conveyed; (iv) Changes in the river bed. Emphasis is laid upon the distinction one should make between very gradual "warping of the ri ver by sediment from outside sources and the cyclic annual river bed changes which are associated with internal exchanges between different parts of the fluvial and maritime river course. A few special points are then mentioned regarding the model tests, notably the simulation of the river mud with mud of a similar composition and the chemical checking of the propcrties of the suspensions fed into the model. It is pointed out that treatment of the water in the model procluces similar effects to those observee! in the Vilaine in winter, when fresh water inflows from upstream reduce the salinity of the river water to less than 2 or 3 per 1 000, even when these inflows are so small as to be without any appreciable effect upon the river flow velocities. The final point considered is the effect of the dam on silting between it and the sea, as determined from the model tests. It was found that the clam does not accelerate silting but that it causes the biggest deposits to forin far ther downstream than before and the sill at the river mouth to extend farther up the estuary, but without gaining in heigth. Sluicing operations during the mean water period (about four months for this river) are an effective means of maintaining adequate depth.