Haut-Rhône à l'amont de Lyon : étude hydraulique de l'ile de Miribel-Jonage

Abstract

Two arms of the Rhône above Lyons form the Miribel and Jonage canals (connected with the Electricité de France power scheme at Cusset), with an 18 km long by 2-5 km wide island between them, which very readily floods at the present time, though the highest flow velocities concentrate in the Miriel canal and an old arm of the river, the "Vieux Rhône". During the latter half of the last century, the Miribel canal underwent a drastic change in its hydraulic conditions, due to the concentrated solids in the water it carried, with the result that today, the downstream part of this canal has taken the form of an alluvial cone with a substantial gravel bank whose stability it is most important to preserve. Possible ways of putting part or the whole of the Miribel-Jonage island permanently above water are either by extensive dyking, or by deepening the Miribel canal downstream from a drop structure, though provided neither navigability of the Jonage canal nor the downstream part of the Miribel canal are affected and this part of the river does not lose any of its present sediment- conveying capacity. With such a scheme, considerable areas could be cleared in the immediate vicinity of the city as building land or for extensive artificial lakes for its population. In view of the proximity of this built-up area of over one million inhabitants, it was most important to check that the proposed reclamation work would not adversely affect the following : (i) The highest flood discharge level through Lyons ; (ii) The rate of arrivaI of the flood peak (i.e. acceleration due to removal of part of the island, and hence some of its attenuating action). In answer to a request by the Rhône and Saône Navigation Section of the 'Ponts et Chaussées' Administration, the Chatou Hydraulics Laboratory studied the flow of the 1957 flood past the Miribel-Jonage island on a mathematical model working on the Barré de Saint-Venant equation. This model successfully cleared up the previously obscure question of how the flood flow divides between the Miribel canal, the 'Vieux Rhône', the Tonage canal and the flood plain as a whole. From its simplified initial stage with steady flow, the mathematical model stndy was then carried further with varying flow conditions representative of the flood flows. The results of this study are most promissing, as they show partial raising of the island to be feasible wilhout any adverse effects.