199.  The Bursting of Dams and Reservoirs.  The construction of a safe reservoir is one of the most important problems of engineers.  In October, 1911, a town in Pennsylvania was virtually wiped out of existence because of the bursting of a dam whose structure was of insufficient strength to resist the strain of the vast quantity of water held by it.  A similar breakage was the cause of the fatal Johnstown flood in 1889, which destroyed no less than seven towns, and in which approximately 2000 persons are said to have lost their lives.

Water presses not only on the bottom of a vessel, but upon the sides as well; a bucket leaks whether the hole is in its side or its bottom, showing that water presses not only downward but outward.  Usually a leak in a dam or reservoir occurs near the bottom.  Weak spots at the top are rare and easily repaired, but a leak near the bottom is usually fatal, and in the case of a large reservoir the outflowing water carries death and destruction to everything in its path.

If the leak is near the surface, as at a (Fig. 155), the water issues as a feeble stream, because the pressure against the sides at that level is due solely to the relatively small height 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.

[Illustration:  FIG. 155.—­The flow from an opening depends upon the height of water above the opening.]

The dam at Johnstown was nearly 1/2 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 destroyed 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 1-1/2 tons.  At a depth of 75 feet, the pressure on the sides would be three quarters that on the bottom, or 2-1/4 tons.  As the bottom of the reservoir is approached, the pressure against the sides increases, and more powerful construction becomes necessary.