And then it must be remembered that there are at all times suspended over the plains and valleys which lie beneath the Alpine ranges enormous masses of water in the form of snow and ice. Although in general these suffer no changes but those due to the partial melting which takes place in summer, and the renewed accumulation which takes place in winter, yet when heavy rains fall upon the less elevated portions of the Alpine snow, they not only melt that snow much more rapidly than the summer sun would do, but they wash down large masses, which add largely to the destructive power of the descending waters.
The most destructive floods which have occurred in Switzerland have usually been those which take place in early summer. The floods which inundated the plains of Martigny in 1818 were a remarkable instance of the effects which result from the natural damming up of large volumes of water in the upper parts of the Alpine hill-country. The whole of the valley of Bagnes, one of the largest of the lateral branches of the main valley of the Rhone above Geneva, was converted into a lake, in the spring of 1818, by the damming up of a narrow pass into which avalanches of snow and ice had been precipitated from a lofty glacier overhanging the bed of the river Dranse. The ice barrier enclosed a lake no less than half a league in length and an eighth of a mile wide, and in places two hundred feet deep. The inhabitants of the neighbouring villages were terrified by the danger which was to be apprehended from the bursting of the barrier. They cut a gallery seven hundred feet long through the ice, while the waters had as yet risen to but a moderate height; and when the waters began to flow through this channel, its course was deepened by the melting of the ice, and at length nearly half the contents of the lake were safely carried off. It was hoped that the process would continue, and the country be saved from the danger which had been so long impending over it. But as the heat of the weather increased, the central part of the barrier slowly melted away, until it became too weak to bear the enormous weight of water which was pressing against it. At length it gave way, so suddenly and completely that all the water which remained in the lake rushed out in half an hour. The downward passage of the water illustrated, in a very remarkable way, the fact that the chief mischief of floods is occasioned where water is checked in its outflow. For it is related that, ‘in the course of their descent the waters encountered several narrow gorges, and at each of these they rose to a great height, and then burst with new violence into the next basin, sweeping along forests, houses, bridges, and cultivated land.’ Along the greater part of its course the flood resembled rather a moving mass of rock and mud than a stream of water. Enormous masses of granite were torn out of the sides of the valleys and whirled for hundreds of yards along the course of the flood. M. Escher relates that one of the fragments thus swept along was no less than sixty yards in circumference. At first the water rushed onwards at a rate of more than a mile in three minutes, and the whole distance (forty-five miles) which separates the valley of Bagnes from the Lake of Geneva was traversed in little more than six hours. The bodies of persons who had been drowned in Martigny were found floating on the farther side of the lake of Geneva, near Vevey. Thousands of trees were torn up by the roots, and the ruins of buildings which had been overthrown by the flood were carried down beyond Martigny. In fact, the flood at this point was so high that some of the houses in Martigny were filled with mud up to the second storey.‘ Beyond Martigny the flood did but little damage, as it here expanded over the plain, and was reduced both in depth and velocity.
(From the Daily News for October 20, 1868.)
A GREAT TIDAL WAVE.
During the last few days anxious questionings have been heard respecting the next spring tides. A certain naval officer, who conceives that he can trace in the relative positions of the sun and moon the secret of every important change of weather, has described in the columns of a contemporary the threatening significance of the approaching conjunction of the sun and moon. He predicts violent atmospheric disturbances; though in another place he tells us merely that the conjunction is to cause ‘unsettled weather,’ a state of matters to which we in England have become tolerably well accustomed.
But people are asking what is the actual relation which is to bring about such terrible events. The matter is very simple. On October 5, the moon will be new—in other words, if it were not for the brightness of the sun, we should see the moon close by that luminary on the heavens. Thus the sun and moon will pull with combined effect upon the waters of the earth, and so cause what are called spring tides. This, of course, happens at the time of every new moon, but sometimes the moon exerts a more effective pull than at other times; and the same happens also in the case of the sun; and on October 5, it happens that both the sun and the moon will give a particularly vigorous haul upon the earth’s waters. As regards the sun, there is nothing unusual. Every October his pull on the ocean is much the same as in preceding Octobers. But October is a month of high solar tides—and for these reasons:—In September, as everyone knows, the sun crosses the equinoctial; and, other things being equal, it would be when on the equinoctial that his power to raise a tidal wave would be greatest. But other things are not equal; for the sun is not always at the same distance from the earth. He is nearest in January; so that he would exert more power in that month than in any other, if his force depended solely on distance. As matters actually stand, it will be obvious that at some time between September and January the sun’s tidal power would have a maximum value. Thus it is that October is a month of high solar tidal waves.
But it is the lunar wave which will be most effectively strengthened at the next spring tide. If we could watch the lunar tidal wave alone (instead of always finding it combined with the solar wave) we should find it gradually increasing, and then gradually diminishing, in a period of about a lunar month. And we should find that it was always largest when the moon looked largest, and vice versâ. In other words, when the moon is in perigee the lunar wave is largest. But then there is another consideration. The lunar wave would vary according to the moon’s proximity to the equinoctial; and (other things being equal) would be largest when the moon is exactly opposite the earth’s equator. If the two effects are combined, that is, if the moon happens to be in perigee and on the equinoctial at the same time, then of course we get the largest lunar tidal wave we can possibly have.
Now this ‘largest lunar wave’ occurs at somewhat long intervals, because the relation on which it depends is one which is, so to speak, exceptional. Still the relation does recur, and with a certain degree of regularity. When it happens, however, it by no means follows that we have a very high tide; because it may occur when the tides are near ‘neap’; in other words, when the sun and moon exert opposing effects. The largest lunar wave cannot stand the drain which the solar wave exerts upon it at the time of neap tides. Nor would the large lunar tidal wave produce an exceptionally high tide, even though it were not the time of ‘neap,’ or were tolerably near the time of ‘spring’ tides. Only when it happens that a large lunar wave combines fully with the solar wave, do we get very high tides. And when, in addition to this relation, we have the solar wave nearly at a maximum, we get the highest of all possible tides. This is what will happen, or all but happen, on October 5 next. The combination of circumstances is almost the most effective that can possibly exist.
But, after all, high tides depend very importantly on other considerations than astronomical ones. Most of us remember how a predicted high tide some two years ago turned out to be a very moderate, or, if we may use the expression, a very ‘one-horse’ affair indeed, because the winds had not been consulted, and exerted their influence against the astronomers. A long succession of winds blowing off-shore would reduce a spring tide to a height scarcely exceeding the ordinary neap. On the other hand, if we should have a long succession of westerly winds from the Atlantic before the approaching high tide, it is certain that a large amount of mischief may be done in some of our riverside regions.[10]