348 HYDKAULICS AND ITS APPLICATIONS 



The distance from end to end of the reach may be obtained by chaining 

 a line running as nearly as possible parallel to the centre line of the 

 river. Soundings should then be taken at short distances apart at several 

 cross sections of the stream, and these cross sections plotted. From these 

 a mean value of the wetted perimeter and of the sectional area, and thus 

 of the hydraulic mean depth, may be obtained, and the formula may 

 then be applied. During the whole observation period the stream should 

 be in a state of steady flow, and neither rising nor falling. 



The method has the disadvantage that it is extremely difficult to 

 measure the slope of a river accurately. Captain Cunningham as a result 

 of some hundreds of slope measurements on the Ganges Canal, 1 found 

 that the slope was very different at different points of a reach from one to 

 two miles long, and varied by as much as 50 per cent, at different sides of 

 the stream. An examination of the Mississippi 2 showed that with the 

 main body of water flowing south with a velocity of four to five miles 

 per hour, the water near the shore may be moving north at a speed of 

 one or two miles per hour. It was in fact not unusual to find a slope 

 towards the south on one bank and towards the north on the opposite 

 bank. The slope then is so uncertain an element that no great accuracy 

 is to be expected for any such formula, except possibly in the case of an 

 artificial channel of uniform section. Under any other circumstances the 

 results cannot be relied upon as being accurate within 25 per cent., 

 and may under specially unfavourable circumstances, even with the most 

 skilful observers, be in error by as much as 100 per cent. 



Wherever possible, then, the mean velocity should be obtained in some 

 other manner. This may be done 



(&) By using a current meter or Pilot tube to give the velocity at a 

 point or series of points in a cross section, and by deducing the mean 

 velocity from such observations ; 



(c) By using one or other type of float, and by measuring the time 

 necessary for a series of these to traverse a given length of the 

 channel. 



Before considering these methods in detail, a few general observations 

 as to their relative advantages and disadvantages may be made. Experi- 

 ment shows that the motion at any point in an open channel is never 

 steady and uniform, but suffers a series of pulsations, the periodic time 

 of which may vary from a few seconds to two or three minutes. These 

 are due to a variety of causes. Eddies formed at the sides and bottom 



1 "Proc. Inst. C.'E.," vol. 71, p. 11. 



2 Report on the Mississippi, Humphreys and Abbot, p. 218. 



