L'aménagement de la vallée de l'Authion

Abstract

The Authion river valley runs alongside the river Loire for 60 kilometres above Angers and is separated from the latter river by a dyking system. High water in the Loire causes very severe flooding in spite of isolation gates designed to prevent back-flow into the Authion, since the former river is then at a higher level than the latter which, therefore, no longer has an outlet for its flow. As a result of these conditions, activities in this region have suffered a severe setback ; in an attempt to remedy the situation, a development plan has been prepared which is mainly based on resettlement of horticulturists displaced by growing urban development in the Anjou region. Adequate control of water levels was an absolute prerequisite for implementation of this plan, which meant tint major river engineering works would have to be provided for. In addition, action was required under the following headings: a) Land management, mainly in the form of highly-specialised new farms and improvement of existing farms. b) Assistance for resettled horticulturists. c) Professional training schemes. d) Overall rural development planning for the region. The main hydraulic structure is a pump station transferring water from the Authion to the Loire, which is located just above the confluence of the two rivers and is designed to operate when flood flow in the Loire prevents gravity outflow from the Authion. Rated capacity of the plant is 60.3 m3/sec for a geometric lift of 4.5 m. It comprises four identical bays each equipped with a vertical pump driven by an electric motor. Leading particulars of this equipment are as follows : a) Bergeron single-cell centrifugal pumps with concrete volutes and 2.7 m diameter cast-iron impellers, performance data as follows : Speed : ... 74.5 rpm ; ... 93 rpm Discharge at 4 m lift : 7.25 m3/sec ; 12.65 m3/sec b) Rheinstahl 8:1 planetary reduction gear unit. c) CEM 5.5 kV, 800 kW two-speed motor. d) Self-priming siphon transferring flow over the dyke between the Authion and Loire. The pump station is fully automatic in operation, being controlled by stage data in the uppermost flooding area of the Authion 9 kilometres upstream. It is designed for an average 52 days' operation in the year, i.e. a total flow output of 128 million cubic metres and a total power input of 1.1 million kWh. It was built in several stages from 1972 to 1974 at a total cost of 14.2 million French Francs.