Break in 20-Inch Main Submerges Automobiles
Accident to Catskill Main in New York City Covers Pleasure Vehicle and Truck with Water—Illustrates Destructive Possibilities of Great City Water Supply
Deputy Chief Engineer
Department of Water Supply, Gas and Electricity. New York City
A RECENT break in a 20-inch main supplied from a nearby Catskill shaft released such a large volume of water that two automobiles passing through the street were trapped and submerged, one disappearing entirely from sight under the flood waters. Such an occurrence is of more than passing interest in graphically picturing the destructive possibilities of the modern water supply system with its enormous aqueducts and high pressures. In the following description of this incident attention is directed to the interesting details.
Where the Break Occurred
Central Park in New York is crossed by several transverse roads for east and west bound traffic, these roads being
carried generally at a level several feet below the park surface, so as not to interfere with park traffic or landscape treatment. Water mains are laid in all of these streets, and it was in what is known as the 79th Street Transverse Road that the break in the 20 inch main occurred. I here is also a 48-inch main in this street, which was unharmed.
Supply from the Catskill Shaft
The 20-inch main is used as a feeding main for the intermediate service area lying east of the park, but is under substantially full Catskill pressure to give a maximum delivery, the intermediate service gradient being about 200 feet above tide level, while the Catskill gradient at this point is about 275 feet above tide level. I he pressure in the main was about 70 pounds per square inch. The Catskill supply is delivered in New York through a concrete lined rock tunnel driven here about 200 feet below the surface and having a diameter of 14 feet. At the shafts, which are about 4,000 feet apart, there are either one or two large pipes, generally 48 inches in diameter, extending from the tunnel to the chamber at the surface, where connection is made with the street mains. One of these shafts was in this transverse road, and a single 48-inch rising main was installed to carry the water from the aqueduct to the 20-inch main.
How the Automobiles were Trapped
The break occurred to the east of a stone arch park roadway bridge spanning the transverse road, the lower road sloping down on each side at a moderate grade to pass under this bridge. The Catskill shaft chamber is on the southerly side of the road and about 100 feet west and down grade from the break. The water thus had only a short flow from the shaft to the opening in the pipe, and the conditions were most favorable for a record discharge to take place. While it is not known what was the exact rate of discharge, it was equivalent to tens of millions of gallons daily. The water rushed down the street grade and rapidly filled the street under the bridge, the level to which the water rose in the half hour required to make the shut down being shown in the photograph marked Fig. 1. At the time of the break there were two automobiles going east, one a Ford truck, and the other a limousine. The truck reached a point east of the bridge about where the second man in the picture is standing and then tried to turn around. It got halfway around and then stalled and was abandoned by the driver on account of the rising water. This car was submerged above the driver’s seat. The limousine tried to turn under the bridge and got around headed west when it too was abandoned, it being reported that a woman
passenger was rescued by a horse and wagon. This car was completely covered by the flood.
Flow Shut off in Street and at Catskill Shaft
The repair gangs had to shut off the head gates on the 20-inch main to the east and to the west of the break, and also close the valve on the connection in the Catskill chamber. There was no special condition governing the closing of the gates in the street, but at the chamber the water was pouring in through the doorway, which is at the sidewalk level. The chamber which is about 20 feet by 25 feet and 10 feet deep below the street level was partially filled with water, but the men were able to get to the gates and close them. If they had not been able to enter the chamber the flow would have been automatically cut off by the operation of a. valve which is placed a hundred feet or more below the surface at the bottom of the 48-inch main leading from the tunnel to the chamber. This valve was designed to meet flood conditions of an unusual character and has already proven
its value. It is operated by water pressure, and a very ingenious mechanism permits of automatic closing by a float or by a control which may be set for a given veloocity of flow through the rising main. At the time of the break the float was in commission but the velocity control was not. The raising of the float arm at the time showed that this would have closed the valve if the chamber had been flooded to the float level.
Damaged Caused by the Flood
The damage from the flood was confined to the two automobiles and the pavement. Figure 2 shows the hole under the pavement after the surface stone and concrete had been broken away. The water did not break through the pavement at the break in the main, the strength of the concrete bed and granite blocks laid in cement being sufficient to withstand the water impact. The presure was sufficient to raise the entire roadway and sidewalk, as shown in Figure 1, and for several hundred feet this pavement has been entirely removed and will have to be relaid. The cost of this work will be several thousand dollars.
It is difficult for those who are not connected with water supply work to realize what may occur if water under good pressure is released by a failure of any part of the system where a large volume of water can be delivered to the broken section. The experience herein described is both unusual and impressive.