THE BURSTING OF DAMS AND RESERVOIRS 21$ 



FIG. 155. The flow from an 

 opening depends upon the 

 height of water above the open- 

 ing. 



of water above a (Section 195). If the leak is lower, as at b, 



the issuing stream is stronger and swifter, because at that 



level the outward pressure is much greater than at a, the 



increase being due to the fact that 



the height of the water above b is 



greater than that above a. If the 



leak is quite low, as at c, the issuing 



stream has a still greater speed and 



strength, and gushes forth with a 



force determined by the height of 



the water above c. 



The dam at Johnstown~was nearly 

 \ mile wide, and 40 feet high, and so 

 great was the force and speed of 

 the escaping stream that within an 

 hour after the break had occurred, 

 the water had traveled a distance of 18 miles, and had de- 

 stroyed property to the value of millions of dollars. 



If a reservoir has a depth of 100 feet, the pressure exerted 

 upon each square foot of its floor is 62.5 x 100, or 6250 

 pounds ; the weight therefore to be sustained by every 

 square foot of the reservoir floor is somewhat more than 

 3 tons, and hence strong foundations are essential. The 

 outward lateral pressure at a depth of 25 feet would be only 

 one fourth as great as that on the bottom hence the strain 

 on the sides at that depth would be relatively slight, and a 

 less powerful construction would suffice. But at a depth of 

 50 feet the pressure on the sides would be one half that of 

 the floor pressure, or i \ tons. At a depth of 75 feet, the 

 pressure on the sides would be three quarters that on the 

 bottom, or 2\ tons. As the bottom of the reservoir is ap- 

 proached, the pressure against the sides increases, and more 

 powerful construction becomes necessary. 



