La chambre d'eau de l'usine d'Oraison

The forebay at the Oraison plant

Electricité de France.

Résumé

The forebay of a hydroelectric plant is usually designed to connect the supply canal to the penstocks and to limit the extent of any waves caused by changes in plant operation. It is a bulky structure taking up a lot of room on the ground. Due to its position on a hillside, the Oraison forebay is narrow in shape, with an unusual transition section from the canal supplying it. The forebay also feeds a relief bypass system of a special type serving a special purpose. The supply canal to the forebay is 22 kilometres long, with flat banks, a water level range of 6.5 metres at its downstream end, 110w velocities of up to 3 metres/sec., and two opposite bends just before where it leads into the forebay. No special arrangements are required to deal with waves caused by changes in plant operation, as the water level range provides an adequate margin. Because of its unsymmetrical flow distribution, the forebay has unsymmetrical shape and is provided on the bottom with three flow guides only 90 centimetres in height. The flow velocity through the intake screen has been reduced to 1 metre/sec. by lowering the apron level, and it has thus been possible to keep the width of the forebay down to acceptable limits for the steeply sloping surrounding ground. As the major axis of the underground power house runs roughly parallel to the Slipply canal, the forebay flow has to turrn round through almost a right angle, which is brought about by two 110w guides forming the three sluiceways leading to the penstocks. There are no valves at the foot of the penstocks ; protection for each power unit is afforded by a valve at the head of its penstock and a watertight manifold. Rapid filling arrangements are essential for safety wirth an unusual layout of this type, and those provided enable the requisite 2000 cubic metres of water to be fed into the penstocks in as litlle as 4 minutes. There is a slight break in the centreline of the horizontal part of the pressure pipe, which causes the flow to adhere to the duct wall, leaving the centre clear for air to escape. The now energy is absorbed by a cushion of water which forms initially at the foot of the duct. Emulsified air is released via a bubble trap on completion of filling. The relief bypass system comprises a 'vortex shaft' 7 metres in diameter, trough which the flow can be diverted in the event of a power set failure. It must therefore be able to operate for prolonged periods without interfering with the power units still running. It is supplied through a suitably faired central honow pier at the downstream end of the forebay, with an intake orifice and flat control gate on its front face. Some energy is dissipated in a hydraulic jump forming inside the pier and stabilised by transverse bars. The floor inside the pier has a number of square holes in it through which the water spurts out, and the energy of these jets is dissipated in a basin underneath. The transition section to the shaft is a free-now tunnel leading to the spiral chamber in which the swirling flow is generated. The flow through the pier is thus controlled by two orifices in series, one being the pier gate and the other the holes in the floors at a rate praportional to √h, i.e. to the square root of the difference between the forebay water level and the perforated floor level. This ensures a stable rate of flaw and prevents excessive flows in the tailrace. This system is giving very satisfactory service.