Restructuration du déversoir de Djorf-Torba

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There is a reason Djorf-Torba dam, built in 1968 on the Guir, impounds the largest man-made reservoir in Algeria, and contols a sub-Saharancatchment of 22 000 km2. It is a concrete gravity structure, 35 m in height, with a 650 million m3 flood attenuation capacity above the 250 million m3 operating capacity, providing a considerable degree of flood control. Records acquired since the construction of the dam however, have led to an upward increase in the estimate of the flood risk, with a greater likelihood of spilling 1 000-3 500 m3/s. The essential part of the dam has tiered sills, and behind the dam there is a 100 m wide apron. A proper hydraulic-jump stilling pool would have been set 5 to 10 m lower, and would probably have been much stronger. The 2 m thick concrete apron has no reinforcement and is not pinned down to the rock, and there are no seals in the construction joints. One of the joints at the upstream end even seems to be dangerous because it slopes towards upstream, so that it might channel the water pressure under the slab. The first phase of the remedial works consisted of pinning the apron to the limestone foundation rock by means of unstressed vertical rods, to provide extra weight and combat uplift forces from the velocity hear or turbulence in the hydraulicjump. This consists of 32 mm TOR. bar on 1.5 m centres both ways, i.e. 1 per 2.25 m2. They are alternately 6 m to 12 m long, and concentrated at the upstream edges of the slabs. This makes a total of 20 000 m of anchor, to be instal1ed in the next 18 months. A hydraulic scale model was built to determine the stability limits of the hydraulic-jump on the apron, evaluate the underscour risk, and find the best means of channeling water back into the river. From the outset, it was decided to keep the apron at its present level, because the dam is operational and spillage may occur at any time. The 1/50 th scale model, built by the Laboratoire Central d'Hydraulique de France was invaluable in that it was able to handle the three-dimensional features of the jump caused by the tiered central and side sills. It was also an excellent tool in producing a design that would prevent the slabs lifting off the foundation. The main steps in the test programme are described. After a few moths testing, it was clear that it would not be necessary to lengthen the apron or deepen the existing cut-off. But in its present condition, there is an unacceptable risk of the floor slabs loosening. The solution will be to build a plain continuous reinforced concrete wier 2 m high at the downstream edge, and raise the existing training walls. Construction work has just commenced, and will be a new step forward in modern hydraulic design.