58 HYDROLOGY 



The diagrams 1 , shown in Fig. 18, represent the ideal regula- 

 tion of the Hudson River, and was based on a proposed extensive 

 reservoir system and the stream-flows for the years 1908-09. 

 Other stream-flow records would, of course, modify the result, 

 while, on the other hand, such ideal flow can seldom be obtained 

 at a cost which would be commercially possible. 



From the above it can readily be seen that usually very careful 

 measurements of stream-flow extending over many years' time are 

 necessary to enable good. estimates of available power to be made, 

 particularly where the contemplated development has no storage 

 facilities. 



Measurements of Stream-flow. The methods by which the 

 records of stream discharge are made differ according to the nature 

 and importance of the work. The simplest and most accurate 

 method for a small stream is by means of a weir. This consists 

 of a dam extending across and at right angles to the stream, and 

 having a rectangular notch cut in the top plank, with both side 

 edges and bottom sharply beveled toward the intake, as shown in 

 Fig. 19. The bottom of the notch, which is called the " crest " 

 of the weir, should be perfectly level and the sides vertical. 



There are certain proportions which must be observed in the 

 dimensions of this notch. Its length, or width, should be between 

 four and eight times the depth of water flowing over the crest of 

 the weir. The pond back of the weir should be at least 50 

 per cent wider than the notch and of sufficient width and 

 depth that the velocity of flow or approach be not over 1 foot per 

 second. 



On the up-stream side in the pond a stake is then driven down 

 in the bottom near the bank, so that its top is level with the bot- 

 tom edge of the notch, this level being easily found when the water 

 is beginning to spill over the crest. The stake should be placed 

 several feet from the board and at least not nearer than the length 

 of the notch. 



By means of a rule, as shown in the illustration, the depth of 

 water over the top of the submerged stake is measured, allow- 

 ance being made for the capillary attraction of the water against 

 the sides of the weir. Having ascertained this depth, the 

 amount of water flowing the weir may be readily found from 

 Table XXIV. 



1 D. W. Mead, " Flow of Streams." 



