HYDROGRAPHIC SURVEYS. 



Submerged Discharge. Where the weir is submerged and velocity of approach is 

 to be considered formula (2) becomes 



in which v = approximate mean velocity of approach = 



^ X 



and A' area of water-section above lower pool at the place where d is measured. 



Discharge in Open Channels. On large rivers and in places where a weir cannot 

 be erected for the purpose, the gauging is done by ascertaining the velocity and using 

 it in combination with the cross-section. The surface velocity may be found by floats, 

 and the velocity at any point in a vertical plane by rod- or tube-floats, gauge-tubes, 

 or hydrometers, or current-meters. The velocities change with the depth and width, 

 and those at the surface also vary greatly as the water is deep or shallow. The obser- 

 vations should be made at times of comparative quiet, as the effect of wind upon the 

 floats often leads to inaccurate results. 



The mean velocity of a stream in feet per second is obtained by dividing the dis- 

 charge in cubic feet per second by the sectional area in square feet, and is the average 

 of all the elements of the current. The motion of each particle of water in open channels 

 depends upon the inclination of the surface, two forces being at work, that of acceleration 

 and that of resistance. The former is gravity, the latter friction, which may occur 

 either between the particles of water or on the bed and banks. 



The quantity of water passing a given point in a given time can only be found 

 with an approximate degree of accuracy. It is also desirable sometimes to ascertain 

 the quantity which a proposed section of channel having a known slope will pass, and 

 many attempts have been made to find a fixed relation between the slope of a stream 

 and its cross-section, but so far without success. The formulas in use are, of necessity, 

 to a greater or less extent empirical, having been deduced from assumed laws or experi- 

 ments. It has not been practicable to obtain a formula which can be applied to all 



