GAUGING OF FLOW IN STREAMS 



347 



f- 



high to prevent overflow in times of flood, and the section should be 

 outside the sphere of influence of bridge piers or mill dams. 



Where a high degree of accuracy is demanded, as may be required in 

 determining the flow of compensation water from the supply reservoir of 

 a waterworks, the best method is to deflect the stream and 

 catch the whole discharge for a given time in a graduated 

 tank. 



This can, however, only be carried out in small streams 

 where a measuring tank of sufficient capacity to hold the 

 discharge for about two minutes is available. In this case 

 the error should not exceed 1 per cent. 



For larger streams, the most accurate method is that of 

 gauging the flow by placing a weir across the stream and 

 allowing the whole flow to take place over this or over one 

 or more triangular or rectangular notches, the former being 

 used for small and the latter for fairly large streams. Where 

 every precaution is taken as explained in Art. 59 this method 

 gives results which may be relied upon as being accurate 

 within about 5 per cent. As a temporary measuring con- 

 trivance, however, the weir is too cumbrous and costly to be 

 applied to a river of even moderate dimensions, and where 

 the discharge is great the only method of obtaining the dis- 

 charge is to obtain as nearly as possible the mean velocity 

 (v feet per second) of the stream, to multiply this by the 

 cross sectional area (A square feet), and to get the discharge 

 Q by the relation. 



Q = v A cubic feet per second. 



The value of v may be approximated to in many ways. 



(a) By using one or other of the formulae given in Art. 

 85, a suitable coefficient being applied to take into account 

 the state of the channel. The slope of the stream must 

 then be obtained by field observations. To do this a 'long 

 straight reach of the river should be selected where possible, 

 and the reduced levels of bench marks placed at each end 

 of the reach should be obtained by levelling. The level of 

 each bench mark should then be transferred to a hook gauge 

 (Fig. 153) or measuring staff placed in a gauge pit communicating with 

 the bed of the stream through a pipe which opens out at a point away 

 from any disturbing influences likely to lead to eddy formation. The 

 difference of surface level" at each end of the reach can then be obtained. 



FIG. 153. 



Hook Gauge. 



