Irrigation Engineering by Arthur Powell Davis Herbert Michael Wilson

Author:Arthur Powell Davis , Herbert Michael Wilson
Language: eng
Format: epub, pdf
Publisher: John Wiley & sons, inc.
Published: 1919-03-25T05:00:00+00:00

Fig. 177.—Trap for Measuring Sand Rolling on Bottom of Stream.

results can never be too large. On the other hand some sand may pass under the edge of the lid, when the bottom of the river at this point is marred with local inequalities. This is supposed to be one cause of the small results on certain days, when other observations immediately before or after, give large results. Another persistent source of erro^ of unknown magnitude is the washing out of the sediment by the current over the weir formed by the back of the pan. To test the importance of this theoretical possibility, a temporary partition was placed in the pan, perpendicular to the current, and nearly as high as the

sides of the pan, the theory being that if all sediment were stopped by the partition and deposited in front of it, that would be good evidence that in the absence of the partition all would be stopped by the back of the pan, and none lost. In the first experiment more sediment was deposited behind than in front of the partition, and the quantity that passed out of the pan is unknown. This result was essentially repeated for most of the experiments, showing conclusively that more or less sediment is carried out over the back of the pan by the scour which it occasions. It is important to bear this fact in mind, when stud3dng the results, for it is certain that the results are quantitatively too small, and should be regarded as showing that large quantities of sediment are traveling on the bed of the stream, and as roughly indicating the relative amount.

3. Removal of Silt from Reservoirs. —The great Assuan Dam in Egypt, forms a reservoir on the Nile, which has a very large discharge and carries great volumes of silt. The large capacity of the reservoir is formed mainly by the very slight slope of the river, so that the dam causes slack water for a distance of over 40 miles up the river, making a long narrow reservoir, with no very wide valley submerged. The dam is provided with 180 large sluices, and during the rising flood each year, when the river is carrying most sediment, the sluices are left open, and the rushing torrent carries its load through the reservoir and also scours out a portion of the sediment deposited the previous year. As the flood declines in volmne it carries less sediment, and the sluice gates are then closed and the waters stored. The sediment deposited during the storage period is largely washed out by the first part of the next year's flood.

This program seems to be effective in preserving sufficient storage capacity for present needs of irrigation, but it requires the waste of the major portion of the water supply in normal years, and also depends upon the extremely long narrow reservoir, in which a swiftly flowing stream is very effective in cutting out the deposited sediment. The variations of flow of

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