THE IRRIGATION AGE. 



13 



In the vertical velocity curve method a series of vel- 

 ocity determinations is made in each vertical at regular 

 intervals, usually about 10 to 20 per cent of the depth 

 apart. By plotting these velocities as abscissas and their 

 depths as ordinates and drawing a smooth curve among 

 the resulting points, the vertical velocity curve is devel- 

 oped. This curve shows graphically Jhe magnitude and 



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Plate II. The Price Current Meter. 



changes in velocity from the surface to the bottom of the 

 stream. The mean velocity in the vertical is then obtained 

 by dividing the area bounded by this velocity curve and 

 its axis by the depth. This method of obtaining the mean 

 velocity in the vertical is probably the best known, but on 

 account of the length of time required to make a com- 

 plete measurement its use is largely limited to the deter- 



mination of coefficients for purposes of comparison and to 

 measurements under ice. 



In the second multiple-point method the meter is held 

 successively at 0.2 and 0.8 depth, and the mean of the 

 velocities at these two points is taken as the mean velocity 

 tor that vertical. (See PI. I, A.) On the assumption that 

 the vertical velocity curve is a common parabola with 

 horizontal axis, the mean of the veloci- 

 ties at 0.22 and 0.79 depth will give 

 (closely) the mean velocity in the 

 vertical. Actual observations under a 

 wide range of conditions show that 

 this multiple-point method gives the mean 

 velocity very closely for open-water con- 

 ditions and that in a complete measure- 

 ment it seldom varies as much as 1 per 

 cent from the value given by the vertical 

 velocity curve method. Moreover, the in- 

 dications are that it holds nearly as well 

 for ice-covered rivers. It is very exten- 

 sively used in the regular practice of the 

 United States Geological Survey. 



The single-point method consists in 

 holding the meter either at the depth of 

 the thread of mean velocity or at an arbi- 

 trary depth for which the coefficient for 

 reducing to mean velocity has been deter- 

 mined or must be assumed. 



Extensive experiments by means of 

 vertical velocity curves show that the thread 

 of mean velocity generally occurs between 

 0.5 and 0.7 total depth. In general prac- 

 tice the thread of mean velocity is con- 

 sidered to be at 0.6 depth, and at this 

 point the meter is held in most of the 

 measurements made by the single-point 

 method. A large number of vertical 

 curve measurements, taken on many 

 streams and under varying conditions, show 

 at the average coefficient for reducing the 

 velocity obtained at 0.6 depth to mean 

 velocity is practically unity. The variation 

 of the coefficient from unity in individual 

 cases is, however, greater than in the 0.2 

 and 0.8 method and the general results 

 are not as satisfactory. 



In the other principal single-point 

 method the meter is held near the sur- 

 face, usually 1 foot below, or low enough 

 to be out of the effect of the wind or other 

 disturbing influences. This is known as 

 the subsurface method. The coefficient for 

 reducing the velocity taken at the subsur- 

 face to the mean has been found to be in 

 general from about 0.85 to 0.95, depending 

 on the stage, velocity, and channel con- 

 ditions. The higher the stage the larger 

 the coefficient. This method is especially 

 adapted for flood measurements, or when 

 the velocity is so great that the meter can 

 not be kept in the correct position for the 

 other methods. 



The vertical integration method con- 

 sists in moving the meter at a slow, uni- 

 form speed from the surface to the bot- 

 tom and back again to the surface and 

 noting the number of revolutions and the 

 time taken in the operation. This method 

 has the advantage that the velocity at each 

 point of the vertical is measured twice. It 

 is useful as a check on the point methods. 

 In using the Price meter great care should 

 be taken that the vertical movement of the 

 meter is not rapid enough to vitiate the 

 accuracy of the resulting velocity. 



The determination of the flow of an ice-covered stream is 

 difficult, owing to diversity and instability of conditions dur- 

 ing the winter period and also to lack of definite infor- 

 mation in regard to the laws of flow of water under ice. 

 The method now employed is to make frequent discharge 

 rreasnrements during the frozen periods by the 0.2 and 

 0.8 and the vertical velocity curve methods. 



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