Photo d'un radier - 1922
Par Alexandre Modesto le samedi 22 août 2015, 22:05 - Lien permanent
Photo du livre :
"Impressions of European forestry : letters written during a six months' visit to England and to the continent " Année 1922
Voici un lien vers le livre : https://archive.org/stream/cu319240...
Le texte associé (Anglais) :
EXAMPLES OF STREAM CONTROL WORK Perhaps as interesting examples of the control of torrential streams through engi- neering works as can be found anywhere in as near as 100 feet. On less abrupt gradients the dams are farther apart though still num- erous. The length of the stream bed that must be so treated often runs up to several miles. It will thus be seen that the construc- tion of dams, or barrages, is an expensive pro- cedure, even with the lower prices for labor that obtained before the war. But this is not all. In the case of the worst torrents the streams have to be walled in, wing dams constructed to deflect the over- flow in times of unusual freshets, and in some instances the entire bed of the stream between its retaining walls, has to be substantially paved with stone blocks to withstand the ero- sive pounding of the rocks and other waste that is carried along by the rushing waters. A typical stream is the torrent of Saint Julien in this valley. Up to 1913 over $129,- 000 had been expended on engineering and tree planting work in connection with itsPrance are to be seen in the valley of Saint Jean de Maurienne, east of Chambery, in the Department of Savoie. It is through this valley that one of the main continental rail- roads winds its way, finally to plunge through the Mt. Cenis tunnel and emerge in Italy. It is a narrow valley. On either side rise high mountains with steeply sloping sides. The many branched ravines that run well back toward the summits of the ridges form ideal channels for concentrating the run-off from the heavy precipitation. The result is that immense volumes of water rush down the stream courses and, if not controlled, work havoc to everything along their path. To meet this situation the French forest engineers construct elaborate masonry dams across the stream bed to break the force of the waters. The height of the dams and their distance apart depend on the gradient of the stream, the object being to secure a series of nearly level steps. Where the grade is steep the dams must be close together, sometimes ( correction. And this was for a single stream, one of many in the valley. The longer cor- rection work is put off the more it costs when it is done. The old adage about the ounce of prevention was never truer than when applied to the work of reboisement. Of similar character, though less elabor- ate, are the corrective works farther up- stream. Here dams made of timbers are used. Higher up still, logs laid lengthwise with the stream are fixed firmly in place across the bed, but always with the idea of making a series of steps that shall break the force of the torrent. On the upper reaches of the streams, too, wattle work, consisting of small poles woven in and out between up- right stakes, replaces the masonry side walls. But even so it is work that quickly runs into money and that would not be un- dertaken were it not absolutely essential. In Switzerland, as well as in France, in the region of the High Alps,
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