Page 18


Page 18
Colorado River problem
Is Part Of,8
Full text
THE COLORADO RIVER PROBLEM 321 If the increase in discharge from 20,000 to 40,000 sec-ft. could be made gradual and extend over a period of, say, 20 or 30 days, there probably would be less trouble due to instability of channel. After 40,000 sec-ft. is reached, the discharge can be increased more rapidly without much risk of starting new meanders. Small peaks in the flow are likely to cause trouble because they are accompanied by temporary rises in gauge heights which mean temporary rises in velocity. Examination of the operation and maintenance reports of the U. S. Bureau of Reclamation shows that such peaks are particularly objectionable on a falling river. Such peaks on a falling river have been present nearly every time the undercutting of levees has gotten beyond control. On a falling river, the discharge should be decreased slowly until the flow is about 20,000 sec-ft., in order to decrease the effect of sloughing banks and to avoid undue silting of the channel. [Fig. 10. SILT PERCENTAGE CURVE - COLORADO RIVER AT YUMA, ARIZONA, May 1, 1916] A study of the velocity curve, Fig. 7, the scour of channel curve, Fig. 5, and cross-sections on Fig. 11,* shows that the river will have the power to meander and cut its banks whenever the discharge is more than 10,000 sec-ft. It is impossible to reduce the flow to that quantity, so storage of flood water will not eliminate the necessity for bank protection and levee maintenance. The records of past flood troubles show that the river has never gone beyond control sufficiently to damage property behind the levees, except when the discharge was more than 100,000 sec-ft. If the flow is reduced to a maximum of 80,000 sec-ft. and abrupt variations in discharge are eliminated, the river will be within its banks so that danger of property damage due to flooding will be reasonably removed for the Yuma and Imperial District Projects. It is doubtful whether reducing the maximum discharge below this point will produce much saving in cost of upkeep of flood-protection works on the Yuma and Imperial District Projects and there is small chance that it will reduce the cost of property damage. Gila floods have occurred at various times between December 1 and March 1, and they come without much warning. As any storage dam on the Colorado * In Fig. 11,100 ft. was added to the gauge readings to avoid the use of negative heights. in the other diagrams, straight gauge heights were used. Zero of the gauge is approximately 102 ft. above sea level.

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