364 HYDRAULICS AND ITS APPLICATIONS 



range of fluctuation. Usually the data are embodied in the form of a 

 rating curve, showing graphically the relationship between discharge 

 and height of surface level. The length of time during which such a curve 

 can be safely applied depends on the class of channel. Where the channel 

 is constantly shifting it cannot be used for many months unless the 

 soundings are frequently checked with reference to the original datum 

 level. 



ART. 103. GAUGING OF ICE-COVERED STREAMS. 



When a river is ice bound its flow becomes somewhat similar to that in 

 a closed flume, the water now flowing under pressure. A series of 

 measurements of such streams by members of the U.S.Geol. Survey 1 lead 

 to the following conclusions : 



(1) The maximum velocity occurs at a point between 35 per cent, and 

 40 per cent, of the depth measured from the underside of the ice. The 

 ratio of mean to maximum velocity ranges from about '80 with a depth of 

 3 feet to '92 with a depth of 16 feet, having a mean value of *85. 



(2) There are two points of mean velocity on a vertical, the first lying 

 between '08 and '013 of the depth, and the second between *68 and '74 of 

 the depth. 



(3) The vertical velocity curve becomes more concave as the river rises, 

 owing to the increased head. 



(4) In making gaugings of such streams the vertical velocity curve 

 method, or the integration method, should be adopted in preference to 

 any of the single-point and co -efficient methods. 



EXAMPLES. 



(1) A canal whose depth is 4 feet, having slopes 2 to 1, has a bottom 

 width of 10 feet. The bed is of earth (Kutter's N = '025), and the 

 gradient is 1 foot per mile. Determine the discharge in cubic feet per 

 second. 



Hydraulic mean depth = 2' 58 feet. 



Answer 



C = 68-8. 



.Discharge = 109*5 cubic feet per second. 



(2) A rectangular flume 4 feet wide and 2 feet deep is roughly con- 

 structed of unplaned timber, and is required to deliver 80 cubic feet of 

 water per second. Determine the necessary gradient, and assuming it to 

 supply water to a power station distant 5 miles from the supply reservoir, 



i Water Supply and Irrigation Paper. No. 95, p. 158. 



