Conception des ouvrages d'évacuation des crues et de vidange du barrage de Souapiti (Guinée)

Design of spillways and outlet works of Souapiti dam (Guinea)

Bureau d'Ingénieurs-Conseils Coyne et Bellier, Paris

Abstract

The Souapiti dam project on the Konkoure in Guinea is a 130 m high earthfill dam. The development includes a free sill spillway and two provisional diversion tunnels to be used on completion of the dam as outlets for the reservoir. 1. The spillway The facility will have a maximum discharge of 4500 m3/sec under a total head of 120 m. It is called on to operate on average two months a year. These conditions have made the development of the appropriate construction measures necessary, in order to avoid the risk of occurence of cavitation phenomena on the concrete, especially : - large 80 m chute, to reduce the specific flows and thicknesses of throughflows to low values and facilitate fast self aeration of the latter. - two deflector systems associated with air levels aerating the throughflows in the neighbourhood of the apron are installed on the chute, in a zone in which the average rate of through flow is of the order of 30 m/sec. Two hydraulic scale-models have been used to design the layout of the facility : - a 1/100 scale-model to study the spillway as a unit. - a 1/20 scak-model to define the deflector systems for the aeration of throughflows. 2. Drainage-levels of the reservoir The two lower provisional diversion tunnels, coated diameter 10.6 m, are conveneniently equiped for ultimate use as bottom outlets. Each outlet, which can absorb a maximum rate of flow of 500 m3/sec, has the following layout : - upstream, the guard and regulating gate control tower. As the gates are top articulated, their maximum working head is limited to 83 m, - the lower diversion tunnel is connected via a 6 m diameter gallery and then by a torus of the same diameter as the diversion tunnels. Flows are well aerated in these sections. Two narrows of 40 m2 cross section installed in each diversion tunnel ensure that they are fed by formation of a jump upstream. This layout ensures low speeds, of the order of 7 m/sec, at the downstream outlet ; - two chambers immediately upstream from each of the narrows deacrate the throughflows. The principle of this design is based on earlier experimental results obtained using a 3: 200 scale-model in very similar operating conditions. A new model will nevertheless be necessary to check and perfect the new layouts envisaged.