Page 11


Page 11
Colorado River problem
Is Part Of,8
Full text
THE COLORADO RIVER PROBLEM 315 It is not possible from the data at band to segregate the expenditures for rock revetment and maintenance prior to 1918 into amounts for current maintenance and for permanent improvements. Some of the rock has been used to slow down the erosion of natural banks at dangerous bends that were rapidly approaching the levee, and much that has been placed in concentrated quantities on the levee at temporary danger points should not be classed as permanent improvement. Of the rock, 1,650,000 cu. yd. have been placed at a cost of approximately $1600,000, or about $1 per cu. yd. Of this amount, about three-fourths, or $1200,000, should be considered, it is believed, as chargeable to permanent betterments. This leaves $888,578 as the current maintenance cost for the period, 1909-23, and an average of about $67,000 per year for the 13 years of active defense of the entire system. Flood Relief Required The difficulties in maintaining the levees in the Lower Colorado Basin have resulted in an effort to seek relief by providing storage to reduce the flood flow. There is considerable difference of opinion as to what flow can be handled with safety in the Lower Colorado. This is not surprising because of the wide difference in flood height for any given flow. The Yuma gauge has recorded a stage of 25 ft., with flows ranging from 35,000 to 121,000 sec-ft. In a river with a stable channel, flood heights can be predicted for any given flow, but the Colorado is not such a river. It flows in a deep bed of silt and the channel enlarges as the flow increases, so that the effect of reducing the flow by storage is uncertain. The best information available on the Lower Colorado is contained in records and reports of the U. S. Bureau of Reclamation. The most accurate data regarding flow, velocity, silt content, and scour come from the Yuma gauging station and cannot be generally applied to the entire river. They do apply with fair accuracy, however, to the river from Laguna to the delta cone. E. C. Bebb, Assoc. M. Am. Soc. C. E., has collected and compiled most of the information on which the following conclusions are based. The ability of the river to pass its flood flow without damage to the Lower Basin depends on three factors, namely, (a) the quantity of water that has to be passed and the rate at which it passes; (b) the quantity of silt carried through and deposited in the lower reaches; and (c) the amount of scour performed in the bed of silt through which the river flows. The danger of breaching the levees is rarely from pressure of high water, but from undercutting due to meandering of the stream. The tendency to meander is present at all stages, but is particularly serious during a falling stage when the banks are saturated and sloughing is accelerated by reduction of hydrostatic pressure from the river. During this stage, also, meandering is accentuated because, with the checking of velocity, the heavier silt deposits rapidly in the slower water near the convex bank at bends, thus contracting the channel and deflecting the main current more and more toward the concave bank. If at this stage a small quick rise occurs that rapidly increases the velocity without giving time for the removal of the newly formed bar, the sat-

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