PREVENTION OF TORRENTS AMONG THE ALPS. 355 



occnrred, which, though less fatal to human life, destroyed the vine 

 harvest, valued at 8,000,000 francs. The losses in one department by 

 by four Hoods, coming in three successive years, was 15,669,000 francs.* 



THE PREVENTION OF TORRENTS AMONG THE ALPS. 



A statement of the methods employed, and the diflBculties to be over- 

 come, in preventing the erosion of torrents, and in repairing the dam- 

 ages they may have done, will be of interest wherever there may be 

 occasion to restrain or repair such injuries. These methods embrace 

 the various dams and other barriers and devices constructed by engi- 

 neers, as well as the planting done by foresters, and both classes of 

 remedies must often be employed for securing these results.^ 



The disasters caused by inundations consist chiefly in the nature 

 and quantity of the materials borne down by the flood, which they 

 without doubt augment in volume and force. When these transported 

 materials consists of mud the banks may not suffer, and the temporary 

 disorder is always reparable, and sometimes this mud improves the soil, 

 and increases its power for vegetable growth. But when these ma- 

 terials are stones and gravel, they carry ruin along their whole course. 



The real damage from floods consists, therefore, in the degradation 

 and transportation of gravel from the flanks of the mountains into the 

 valleys, and the end in view is, therefore, to prevent these materials 

 from moving. The means actually employed for this end are simple, 

 but before stating them it will be proper, in order to make them under- 

 stood, that we should indicate how this degradation takes place. It is 

 in one of two ways, viz : By erosion at the foot of the mountains — 

 this is the most dangerous; by atmospheric influences under the tread- 

 ing of sheep and. goats — this is the more common manner. Being un- 

 dermined at the foot, the mountains, composed of incoherent materials, 

 loose their hold and slide ; or being loosened by the frost, softened by 

 the water, and trodden by flocks, the soil on the slope is washed down 

 by the storms. 



1 Scientific inquiry since directed to the cause, shows the origin to have been in the 

 naked valleys of the Pyrenees, and speecially that of the Aude, a torrent rising 2,130 

 meters above sea-level, and having most of its descent near the ujjper part of its course. 

 It is about 141 miles long, and a considerable part once a fertile plain. The basin 

 drained by this torrent has an area of 460,000 hectares, of which but 60,000 admits of 

 the infiltration of water, the rest allowing a speedy delivery of the rain-fall, which, in 

 the June flood of 1675, amounted to 691,000,000 cubic meters in fifty-two hours. The 

 result of the inquiries made with reference to future prevention is, that besides t-trong 

 dams for retaining the waters, the basin should be planted, and especially the waste- 

 lands, with evergreen trees, which at all seasons intercept a considerable part of the 

 rain. From thiee years' observation in this region, M. Rousseau found the rain-fall as 

 follows : 



Millimeters. Millimeters. 



In winter, in the open air 179.:?; underwoods 55.1 



In spring, in the open air 162.7; underwoods 84.4 



In summer, in the open air 302.3; underwoods 169.8 



la autumn, in the open air 323.1; underwoods 162.1 



967. 4 471. 4 



In the heavy rain-fall of June, 1875, the amount was 162.2 millimeters in the open 

 grounds, and 108 under the shelter of trees ; but this was a steady rain, and the leaves, 

 once wet, only served to break the fall of rain, and the evaporation was little or none. 

 Yet even this difference would have reduced the volume of the flood 35 per cent. The 

 estimated cost of reboisement of 140,000 hectares, under peculiar ditBculties, is 16,000,000 

 francs. — (Extract from Atlas Meidorologique dn V Observatoire de Paris, in the Eevue dee 

 Eaxix €t Fortts, July, 1877, p. 290.) 



*Thi8 statement Is translated from the Eevue des Eaux et Forets, 1872, p. 84. 



