Page 56


Page 56
Colorado River problem
Is Part Of,8
Full text
ALLISON ON THE COLORADO RIVER PROBLEM 361 however, the mechanical obstacle offered by the bank results in building up a sloping platform on the weather side and when the platform reaches sufficiently near the top of the embankment, the winnowing action described commences. The deposit on the lee side being formed by sedimentation takes its angle of repose, usually quite steep. If, however, by favoring conditions, the sloping platform on the windward side should in time surmount the bank or wall, so that the large grains could roll over, a proper dune or "barchan" profile is produced (Figs. 14 and 17), and the movement of the dune or bar-chan into the canal corresponds with the movement of the dune of similar character without the area of the canal prism. In the observations made by Mr. H. J. L. Beadnell, formerly of the Geological Survey of Egypt, on the dunes of the Lybian Desert, the following results were obtained with an air meter under different conditions of wind in the dune belts (from personal observation, corresponding results are obtained in the sand-dune regions of the All-American Canal): 1.—On the dunes, the sand commences to move when the winds attain the velocity of a light breeze, or 13 miles per hour. 2.—On the open plains, the wind becomes visibly charged with sand as soon as the velocity of a moderate breeze (23 miles per hour) is obtained. 3.—Sand-storm conditions obtained when the wind exceeded a moderate breeze, the maximum velocity actually recorded being 34 miles per hour. The following experiments correspond in general to those made in an attempt to trace the annual movement of a sand dune through the section of the proposed All-American Canal: 1.—According to Dr. Franz Czerny, dunes on the Baltic Coast in the Province of Courland moved at the rate of 18 ft. per year. 2.—Bromontier measured the movement of the dunes of Gascogne at the rate of 25. m. per year. 3.—Cornish found the average rate of advance of a crest of a dune near Ismailia to be about 3/4 in. per hour. 4.—Mr. C. E. Enock estimated the movement of barchans in South America to be as much as 2 ft. per hour in a brisk breeze. 5.—On the Lybian Desert, the movement of barchans is as much as 25 m. per year. 6.—In the sand dunes along the route of the All-American Canal, move-ment of barchans as great as 85 ft. per year has been recorded. Fig. 18 shows a profile of that section of the proposed All-American Canal through the sand-dune region. It is reasonable to expect from the experiments mentioned that the sand dunes through which the canal is cut will shift an average of 80 ft. per year. The cross-section of the canal (Fig. 16), indicates as nearly as possible, what effect this shifting will have on the canal prism. A calculation of the quantity of sand shifted per year into the canal, as reflected in the cross-section, gives 3,136,000 cu. yd. The mechanical analysis of the sands shifted into the canal corresponds to that of materials dragged along the bottom of Colorado River and like

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