Page 55


Page 55
Colorado River problem
Is Part Of,8
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360 ALLISON ON THE COLORADO RIVER PROBLEM American Canal to samples taken elsewhere in the United States, and the mechanical analysis of the sands of the Lybian Deserts and the regions of the Sahara, justify the assumption that the phenomena governing the movement of sands in the older and more carefully studied regions, will be repeated in the case at hand. In Fig. 17 is shown the positions from which the speaker's samples on the All-American Canal were taken. [Fig. 17. WIND ACTION ON SAND DUNES OR "BARCHANS"] The scouring action of fluids varies theoretically as the square of the velocity and so does the diameter of the particle transported. If the velocity is doubled, the diameter of the particles transported may be increased four times. The transporting power of wind varies approximately with its erosive force, that is, as the square of the velocity, as in fluids, whereas its lifting capacity changes according to the sixth power. The range of velocity of dune-making winds certainly exceeds twice the normal, and hence it might be expected that the bulk of sand, in some places at least, would consist of grains many times as large as in others, were it not for scarcity of the larger material. Size alone may make a dune a permanent hill even if it is composed of loose sand throughout. Given a constant climate, a large desert dune might easily outlast the highest mountains, for the denuding agency continually renews the surfaces. On the other hand, there is necessarily a limitation of the process whereby dunes grow, which prevents their attaining heights equal to those of mountains formed by erosion. Winds have greater power at considerable elevation than at the surface so that more sand is removed from the summits than is replaced by the wind and thus the lowering of the summits is not offset by the deepening of the trough; action on the summits of dunes is hastened, and in the trough hindered, by gravity, and should a trough be created by excavation through the sand dunes, the phenomena mentioned would continue until the point was reached by the filling of the trough where any deepening would be compensated by the lowering of the summits. Obstacles, such as a wall, affect the distribution of sand by wind in two ways, directly as obstacles and indirectly by affecting the motions of the air. Wind evokes eddies on each side of an obstacle. These eddies on the weather side pick up the finer grains which those on the lee side capture, throwing away only the finest of them. Sand, therefore, deposits against both sides of the wall. A high canal spoil bank, even though wetted, oiled, and grown with shrubs, acts merely as a wall, the sand being deposited both on the outside and on the canal side. If the sand is fine and the wind strong, the principal deposit is on the lee side or in the canal; if the wall or embankment is high, nothing at first will get over it, except dust wafted in the air. In time,

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