HYDROGRAPH1C SURVEYS. 



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(2) DISCHARGE. 



By the discharge of a stream is meant the amount of water passing a given point 

 in a given time. It is usually reckoned in cubic feet per second. The purpose of ascer- 

 taining the discharge of a stream is to be able to determine its value in the study of 

 the improvements necessary to control it or utilize it for navigation. It varies greatly 

 with local conditions, slope, etc., and reliable results can only be obtained by numerous 

 gaugings covering all the varying conditions of the river, and extending over a consid- 

 erable period of time. 



Observations for ascertaining the discharge of a small river can be made with a 

 fair degree of accuracy, but for large streams with swift currents, irregular cross-sections, 

 and unstable banks, the undertaking is one fraught with unsatisfying estimates to 

 those who would have nothing but precise results. Thus it has never yet been pos- 

 sible to accurately ascertain the extreme high-water discharge of the Mississippi because 

 of the indirectness of its currents, both horizontal and vertical, its numerous changes, 

 and its extraordinary width. Attempts have been made to compute the discharge 

 by means of percentages of rainfall, but the results are not reliable because of the un- 

 certainty as to the percentage which reaches the river. 



Discharge over Weir. A satisfactory manner of gauging a stream, with results 

 probably within five per cent of the truth, consists in concentrating all the water so 

 it will flow over a weir. It is easy then to get the length and depth of overflow and 

 thereby arrive at the discharge. Weirs for this purpose should be placed at points 

 of impermeable bottom in order that no leakage or percolation may take place: other- 

 wise the discharge measured will be less than the actual volume. 



Free Discharge. The latest experiments for determining discharges by this method 

 were made in 1899 at Cornell University, New York State, by Mr. Geo. W. Rafter and 

 Professor Gardner S. Williams,* and are the only ones up to the present time in which 

 a considerable depth of water was used on the crest. In the experiments of Bazin, 

 made a few years previously, a maximum depth of about 18 inches was allowed, while 

 in those at Cornell University the depth in some of the experiments was over 4^ feet. 

 From the latter the accompanying diagram of discharge curves was prepared, and sum- 

 marizes conveniently the results of the investigations, combining them at the same 

 time with the formula deduced by Bazin. This formula, which is for free discharge, 

 that is, where there is no water below to check the flow over the weir and no contrac- 

 tions at its ends, and neglecting the velocity of approach, is as follows: 



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* Proceedings of the American Society of Civil Engineers, March, May, August, 1900. The accom- 

 panying diagrams are reproduced from the May " Proceedings," from the discussion by Professor Williams. 



