62 



HYDROLOGY 



scale of the gauge being previously set at zero w f th the vernier, 

 the base is fastened to the box above the water in a vertical posi- 

 tion and at such a height that the point of the hook is at the same 

 level as the crest of the weir, the precise point being secured 

 by moving the hook in the tube. The point of the hook 

 will, of course, be under water and level with the crest of the 

 weir. 



The depth of water flowing over the weir is the distance between 

 the point of the hook in the position named and the exact surface 

 of the water. To ascertain this, the hook is raised by turning the 

 milled head nut until the point of the hook, appearing a little 



FIG. 21. -Typical Gauging Station with Automatic Gauge. 



above the surface, causes a distortion in the reflection of the light 

 from the surface of the water. A slight movement of the hook in 

 the opposite direction will cause the distortion to disappear, and 

 will indicate the surface with precision. The reading of the scale 

 will then give the depth of water passing over the weir, in thou- 

 sandths of a foot. 



Where measurement by weir is impracticable the amount of 

 water can be calculated by ascertaining the average velocity of 

 the water and the cross-section of the stream, the quantity being 

 the product of these two factors. The mean velocity is the func- 

 tion of the cross-section, surface slope, wetted perimeter, and 

 roughness of the bed, while the cross-sectional area depends on 



