Les aménagements du gard : nouvelle gestion des retenues

New water management objectives for the Gard dams

COYNE et BELLIER

Abstract

After disastrous floods in 1958, the County Council of the Gard Department in France embarked on a programme for building nine flood control dams for the rivers from the Cevennes mountains. They were designed to delay moderate floods through ungated sluices, with the 100-year floods and above spilling over an ungated sill. Except for Mialet dam which is partly for irrigation water supply and Conqueyrac on a highly karstic site, all the Gard schemes have always had a small permanent reservoir to boost summer flows, but the capacities involved, ranging initially from a minimum of 150,000 m³ to a maximum of 1,500,000 m³ did not justify multi-purpose development. Four of these dams have been completed : Sainte Cécile d'Andorge (1967), Ceyrac (1968), Rouvière (1970) and Sénéchas (1976). Now, slightly less than half-way through the programme, the Council, which will be responsible for operating the dams after completion, is becoming interested in drawing more benefit from them by making them true multi-purpose schemes. The remaining dams will therefore incorporate these other objectives, and some of the completed four will very probably be modified to the same end. The reasons for this move towards enlarging the scope of the projects are mainly : a) The ungated sluices were made large enough to handle fairly large floods, meaning that the dams do not have any spectacular effect on the more frequent moderate river floods. b) The authorities now require flood control dams to be fitted with gates anyway for testing purposes, and as they must be capable of reliably controlling the upstream level and regulating discharge, they are fully equivalent to permanent gates. It is therefore quite legitimate to wish to make the most use of the dams, since no extra expense is required, except for a water management study ; and flood control is provided anyway. The method is described in the second part ; it was used to draw up the water management plan for Saint Paul Lacoste dam, one of the next to be built. In addition to be above factors, there is another consideration which is more imperative : c) There is a growing need for water to meet domestic, farm and industrial needs. Examples of this are Sénéchas, where the reservoir capacity was doubled in the course of the design work by raising the level of the sluices, Saint Paul Lacoste where the permanent reservoir was increased from 1,500,000 m³ to 10,000,000 m³ during design, and Sainte Cécile d'Andorge, at which the permanent reservoir was doubled by fitting gates in the sluices after the dam had been operational for about ten years. d) Some dams might be capable of producing electricity, a topical factor since the energy crisis appeared. The third part describes the economics of installing a turbine at Sénéchas.