Fermetures fluviales sur le Rhône

\n The "Compagnie Nationale du Rhône" has dammed the river at nine points during the last twenty-five years. Four of these cases are described, also a few ways of cutting costs and limiting failures. Damming methods can be considered to fall within the three following main groups : -a) "Instantaneous" damming (similar to natural damming) by the toppling of a block into the river, as in the Saguenay River scheme. b) "Mechanical" damming by closing the openings in a structure with sheet piling or metal screens. This method is mainly used in marine projects. c) "Dumped rubble" damming in which the material is either dumped in successive horizontal layers, or forming dykes gradually extending further out into the river. Description of four cases of damming on the Rhône. a) GÉNISSIAT (April 1939). - In order to build the Génissiat dam, it was necessary to divert the Rhône's average flow of 400 cu.m/sec. into two tunnels, which required raising the water level for a flood flaw of 1,300 cu.m/sec by 7 metres. As the river ran through a comparatively inaccessible gorge at this point, it was decided ta build the diversion structure up gradually in horizontal layers of rock fill, assuming a maximum permissible overflow of 9 cu.m/sec per metre length of structure, i.e. a total overflow of 300 cu.m/sec. Tests by SOGREAH showed that it was necessary to build two structures and to steepen the face of the downstream one with metal tetrahedrons in order to reduce its volume. The damming operation was completed in a month ; requirements for both structures included 40,000 cu.m of rock fill up to 400 kgs in weight and 500 metal tetrahedrons weighing 130 kgs each and held in position by cables. b) SEYSSEL (Compensation reservoir for Génissiat, December 1994). - This closure was made between the piers of the third sluiceway in the dam, covering an overall width of 43 metres. The Seyssel dam stands on an 18 metres deep alluvium layer, with a substantial metal gate hoist bridge running above the considered sluice. The upstream side of the dam apron is protected by sheet piling taken down to bedrock level. In view of these particular conditions, it was finally decided after model tests at the "Compagnie Nationale du Rhône" laboratory at Lyons to use the upstream cut off curtain to close the gap. As the difference between upstream and downstream water levels after closure was 2 metres, and the river bed was 8.50 metres below the sluice floor levels, the following were necessary : (i) To support the sheet piling on a horizontal beam, itself supported by the main sluiceway piers and a small intermediate one. - (ii) To sink two layers of gabions 56 cu.m in volume in order to reduce scour. - (iii) To make elaborate guiding arrangements when ramming sheet piling in. Larssen VI sheet piling reinforced over a quarter of its length was used for this work. The closure took only one month to complete, in spite of a 700 cu.m/sec. flood. e) ROCHEMAURE (June 1957). - This dam half-way between Lyons and the sea features six sluiceways 26 metres in width. At the time of closure, four of these had been completed, also the left-bank abutment pier. It was proposer to close a 56-metre gap featuring a bed of loose material by gradually filling out into the river. Tests were initially carried out at the "Laboratoire National" at Chatou with a flow of 2,000 cu.m/sec., but as one of 2,400 cu.m/sec was experienced in the meantime, it was decided to repeat them with conditions as they were when the working procedure improvised during the flood flow was being applied at the site. The model results were found to agree perfectly with the actual site conditions. The closure was completed in 90 hours ; it required 16,000 cu.m of material, including 20 % quarry run up to 70 kgs in weight and 5 % rock fill weighning up to 400 kgs for the final stage. The difference between upstream and dowstream water levels on completion of the work was 1 metre. d) CHARMES (August 1963). - This site resembled Rochemaure, except that the river had a firm bed and a steeper gradient. The gap to be closed was 70 metres wide, resulting in a level difference of 1.4 metres after closure. The operation was carried out at flows varying between 2,300 cu.m/see and 2,800 cu.m/sec. From tests carried out at the "Laboratoire National" at Chatou, it was anticipated that 32,000 cu.m material would require placing in 80 hours, but it eventually turned out that only 28,500 cu.m were in fact required, taking 74 hours to place, including 3,800 cu.m of material in the 20 kg-70 kg range. Material sizes were as predicted by the model, also the proportions in the damming structure and the bank parallel along the pier, i.e. 70 % and 30 % respectively. Conclusions : WAYS OF REDUCING COSTS AND LIMITING FAILURES : Model tests appear to the an essential requirement for flows of any size and where final differences between upstream and downstream water levels exceed 0.5 m. The damming method used will depend on site conditions and the type and cost of the material obtainable near the site, also the type of site equipment available. The ultimate purpose of the structure must be considered, and whether or not it is to remain as a permanent feature of the dam. All these were considered to be decisive cost-reducing factors in the Rhône and many other projects as well. At Fort Randall on the Missouri for instance, the availability of a 1,000 cu.m/hour capacity suction dredger enabled the material in the closure to be built up in successive horizontal layers ; at Genissiat, on the other hand, the material was dumped from a bridge, which was the only way making use of the railway facilities available on the site. A number of different and sometimes inexpensive procedures can be used to reduce the cost of a flow-damming structure, or of other project works, such as the following for instance : (i) The use of metal tetrahedrons, as at Génissiat, or of concrete tetrahedrons such as the 12-ton units used in the Columbia river, so as to reduce the volume and coverage of the damming structure. (ii) The practice of "pushing" the material into the water in batches of 10 cn.m or 20 cu.m at a time, as was done in the Lower Rhône, for which purpose smaller-sized materials can be used than were simply dumped into position ; alternatively, two structures can be built at the same time as at Albany Falls for instance, where this procedure enabled stone weights to be reduced from 10 tons to 2 tons. The same results can be delieved bv the method used for the Oahe dam on the Missouri, which involved the direct placing of a structure of the same thickness as the length of the" bund" forming along the river bank due to material dumped from the end of the closing dyke. In conclusion, therefore, the damming of a river can certainly be considered to be a delicate operation requiring much careful thought, and it should always be carried out as rapidly as possible.\n

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